Adipokines and the complications of obesity

Inflammatory molecules are likely candidates exerting a molecular link between the adipose tissue and the metabolic, cardiovascular, hepatic and thrombotic complications, and certain cancer types occurring in conjunction with or as a consequence of human obesity. A myriad of candidate adipokines are proposed to play this role⁽Reference Matsuzawa^98–Reference Chen¹⁰⁰⁾. In the cardiovascular field, they can be considered as risk factors, and even directly play a pathophysiological role favouring the initiation and progression of atherosclerosis. Relationships between abnormalities of cardiac function in obese subjects, the accumulation of abdominal fat and low-grade inflammation have been suggested⁽Reference Malavazos, Cereda and Morricone^101, Reference Malavazos, Corsi and Ermetici¹⁰²⁾. Among the candidates secreted by the adipose tissue, the increase in IL-6, IL-8 and MCP-1 and the decrease in adiponectin are considered to be particularly important⁽Reference Malavazos, Cereda and Morricone^101, Reference Malavazos, Corsi and Ermetici¹⁰²⁾. The studies of the pathophysiological links between adipokines and cardiovascular health can be illustrated by the analysis of the effects of adiponectin in rodents. Overexpression of adiponectin results in diminished size of the lesions observed following acute ischaemic myocardial infarction, increased angiogenic properties and reduced size of atheromatous plaques in the genetically predisposed apoE^− / − mouse⁽Reference Ouchi, Kihara and Funahashi¹⁰³⁾.

Several inflammatory mediators produced by adipose tissue, such as CCL5, IL-1β and IL-8, as well as markers of oxidative stress, are increased in diabetic or glucose-intolerant patients, and the amelioration of hyperglycaemia by insulin therapy reduces circulating concentrations of these molecules⁽Reference Herder, Haastert and Muller-Scholze^104, Reference Stentz, Umpierrez and Cuervo¹⁰⁵⁾. The increase in the concentrations of TNF-α, IL-6, IL-1β, IL-8, resistin and many other factors produced by macrophage activation could contribute to the deterioration of insulin sensitivity (Fig. 6)⁽Reference de Luca and Olefsky¹⁰⁶⁾. The precise relationship between the importance of macrophage and T-cell accumulation in adipose tissue depots, adipokine secretion and the modifications of insulin sensitivity needs to be further addressed in humans.

Fig. 6 Schematic representation of the role of adipose tissue inflammation in the initiation and maintenance of systemic insulin resistance. Reproduced with permission from de Luca & Olefsky⁽Reference de Luca and Olefsky¹⁰⁶⁾.

Macrophage accumulation is more abundant in the abdominal adipose tissue⁽Reference Cancello, Tordjman and Poitou⁶⁵⁾ than in the subcutaneous tissue, and this could explain some of the risks associated with the accumulation of intra-abdominal fat. For example, a relationship between the increase in macrophages in the abdominal adipose tissue and hepatic inflammation and fibrosis has been reported⁽Reference Cancello, Tordjman and Poitou⁶⁵⁾. In another study, the expression of the MCP-1 and colony-stimulating factor-1 genes and proteins was associated with macrophage accumulation in obese subjects⁽Reference Harman-Boehm, Bluher and Redel¹⁰⁷⁾. Since the abdominal adipose tissue is partly drained by the portal system, it cannot be excluded that some adipokines, together with high NEFA fluxes and hormones delivered by the adipose tissue, could contribute to the alteration of hepatic function observed in obese subjects, the mechanism of which needs to be better dissected.

Adipokines and weight loss

Even modest weight reduction improves the metabolic and cardiovascular risks associated with human obesity. Measures of endothelial activation also improve after weight reduction⁽Reference Berg and Scherer^108–Reference Ziccardi, Nappo and Giugliano¹¹⁰⁾. Many studies have shown that weight loss induced by a decrease in energy intake, and sometimes an increase in exercise, reduces systemic inflammation. A reduction in concentrations of numerous inflammatory molecules and endothelial risk factors, and an increase in adiponectin concentration have been observed during weight-loss programmes, and these are sometimes associated with improvement of insulin sensitivity⁽Reference Esposito, Pontillo and Di Palo¹¹¹⁾. Such changes have been described for CRP⁽Reference Heilbronn and Clifton¹¹²⁾, IL-6⁽Reference Bastard, Jardel and Bruckert¹¹³⁾, IL-18⁽Reference Esposito, Pontillo and Ciotola¹¹⁴⁾, IL-1ra⁽Reference Meier, Bobbioni and Gabay²⁶⁾, PAI-1⁽Reference Bastard, Vidal and Jardel¹¹⁵⁾, SAA⁽Reference Poitou, Viguerie and Cancello^23, Reference Gomez-Ambrosi, Salvador and Rotellar¹¹⁶⁾, cathepsin S⁽Reference Taleb, Cancello and Poitou¹¹⁷⁾, matrix metalloproteinase-9⁽Reference Laimer, Kaser and Kranebitter¹¹⁸⁾, soluble adhesion molecules (soluble intercellular adhesion molecule-1 (sICAM-1), soluble vascular cell adhesion molecule-1 (sVCAM-1))⁽Reference Ziccardi, Nappo and Giugliano¹¹⁰⁾, tissue factor⁽Reference Kopp, Kopp and Steiner¹¹⁹⁾, MIF⁽Reference Church, Willis and Priest¹²⁰⁾, MCP-1⁽Reference Christiansen, Richelsen and Bruun¹²¹⁾, soluble receptors of TNF (sTNFR) and for eotaxin, an inflammatory factor implicated in asthma, another complication of obesity⁽Reference Vasudevan, Wu and Xydakis¹²²⁾. Weight loss induced by gastric bypass reduced the circulating concentrations of visfatin⁽Reference Krzyzanowska, Mittermayer and Krugluger¹²³⁾ and TNF-α⁽Reference Bastard, Hainque and Dusserre^124, Reference Kopp, Kopp and Festa¹²⁵⁾. One study followed sixty obese patients during the course of weight loss induced by bariatric surgery and reported a reduction of 30 % of initial weight, a decrease in CRP, SAA, orosomucoid protein, IL-6, TNF-α and fibrinogen concentrations, and an increase in adiponectin concentration⁽Reference Poitou, Coussieu and Rouault¹²⁶⁾. After the surgery, the concentration of IL-6 dropped slowly while the concentrations of SAA and CRP dropped more quickly⁽Reference Poitou, Coussieu and Rouault¹²⁶⁾.

There was a significant modification in the expression of inflammatory genes in the subcutaneous adipose tissue of obese women following a hypoenergetic diet⁽Reference Clement, Viguerie and Poitou¹²⁷⁾. The expression of 100 genes linked to inflammatory processes was modified after 4 weeks (41 % increased and 59 % decreased). These genes belonged to at least twelve functional families including cytokines, the complement factor cascade, acute-phase proteins of inflammation, and molecules involved in cellular adhesion and in the remodelling of the extracellular matrix. The improvement of the inflammatory profile (at the level of gene expression) involved both the decreased expression of pro-inflammatory factors and the increased expression of anti-inflammatory factors such as IL-10 and IL-1ra. The modification of the inflammatory gene expression profile was very similar in subjects following bariatric surgery and was associated with a reduction of macrophage infiltration. In this study, the protein expression of IL-10 increased, suggesting a possible M1 to M2 (pro-inflammatory to anti-inflammatory) switch of macrophage phenotypes⁽Reference Cancello, Henegar and Viguerie³¹⁾. Overall, the mitigation of the systemic inflammatory profile observed during weight loss is associated with modifications of adipose tissue inflammatory gene expression, and this may be linked with altered profiles of inflammatory protein secretion. The eventual consequence of this phenomenon on the local biology of the adipose tissue remains to be identified.

Body weight

Obesity is considered an important determinant of the magnitude of the postprandial inflammatory response⁽Reference Manning, Sutherland and McGrath¹⁶³⁾, perhaps being more important than any specific component of a meal inducing the response. The exaggerated postprandial inflammatory response of the obese is reversible upon reduction of body weight⁽Reference Plat, Jellema and Ramakers^155, Reference Jellema, Plat and Mensink¹⁶⁴⁾.

Hyperglycaemia and type 2 diabetes

Patients with type 2 diabetes exhibit a higher postprandial inflammatory response than non-diabetics, irrespective of their body weight⁽Reference Esposito, Ciotola and Sasso^165, Reference Nappo, Esposito and Cioffi¹⁶⁶⁾. The magnitude of the postprandial inflammatory response appears to correlate with the degree of insulin resistance⁽Reference Esposito, Ciotola and Sasso¹⁶⁵⁾.

Drugs

Certain medications including statins and angiotensin II receptor antagonists ameliorate the postprandial inflammatory response in obese patients⁽Reference Ceriello, Assaloni and Da¹⁶⁷⁾.

Acute v. regular exercise

IL-6 and other cytokines that are produced and released by skeletal muscles have been suggested to be involved in mediating the health-beneficial effects of exercise and to play important roles in the protection against diseases associated with low-grade inflammation. The following chain of events is based on observations by Pedersen and colleagues and has been excellently reviewed elsewhere⁽Reference Pedersen^206–Reference Fischer, Berntsen and Perstrup²⁰⁸⁾:

1. (1) Contracting skeletal muscle is a major source of circulating IL-6 in response to acute exercise. Plasma IL-6 increases in an exponential fashion with exercise and is related to exercise intensity, duration, the mass of muscle recruited and endurance capacity. During heavy exercise, such as a marathon, there is up to a 60-fold increase in plasma IL-6 concentration⁽Reference Ostrowski, Rohde and Zacho²⁰⁹⁾, with the duration of the event explaining more than 50 % of the variation in concentration⁽Reference Fischer²¹⁰⁾. Interestingly, IL-6 shows a markedly lower response to acute exercise in trained subjects.

2. (2) Physiological concentrations of IL-6 stimulate the appearance in the circulation of the anti-inflammatory cytokines IL-1ra and IL-10 and inhibit the production of the pro-inflammatory cytokine TNF-α. The health benefits of long-term regular exercise are ascribed to the anti-inflammatory response elicited by an acute bout of exercise, which is partly mediated by muscle-derived IL-6.

3. (3) The anti-inflammatory effects of exercise may therefore offer protection against TNF-induced insulin resistance. Moreover, IL-6 stimulates lipolysis as well as fat oxidation. The increase in IL-6 at the end of exercise is responsible for the increased CRP levels during late recovery.

In response to regular physical activity, basal as well as post-exercise plasma concentrations of IL-6 will decrease by mechanisms that might include increased glycogen content, improved antioxidant capacity and improved insulin sensitivity. The lower concentrations of IL-6 in the circulation will subsequently result in lower CRP levels.

Few studies have prospectively examined the effect of exercise training on low-grade inflammatory status, and the data obtained from intervention studies are less consistent when compared with cross-sectional population studies. A lower number of subjects or a good physical condition in the start of some intervention studies may explain a part of this inconsistency. Nevertheless, two longitudinal studies in athletes show that regular training induces a reduction in CRP concentration⁽Reference Fallon, Fallon and Boston^211, Reference Mattusch, Dufaux and Heine²¹²⁾. Conflicting findings exist in clinical trials that have involved exercise only. Several training interventions have not produced changes in basal IL-6 or CRP concentrations⁽Reference Hammett, Prapavessis and Baldi^213–Reference Fischer, Plomgaard and Hansen²¹⁸⁾, while significant reductions in inflammatory markers have been observed following training without changes in BMI or body fat in elderly participants⁽Reference Kohut, McCann and Russell^219, Reference Stewart, Flynn and Campbell²²⁰⁾. The largest trial was performed in 652 sedentary healthy, young and middle-aged, white and black women and men in the HEalth, RIsk factors, exercise Training And Genetics (HERITAGE) Family Study⁽Reference Lakka, Lakka and Rankinen²²¹⁾. They were subjected to a 20-week standardised exercise training programme; there was no control group, and each subject served as its own control. A non-significant reduction in CRP concentration was consistent across all groups and varied between 1·2 and 2·2 mg/l. Considering that the over-time variation in CRP in healthy individuals with stable lifestyle is small⁽Reference Pearson, Mensah and Alexander²²²⁾, the reduction, although not significant, could nevertheless reflect the true effect of exercise training. Further stratification according to basal CRP levels showed a reduction by about 1·3 mg/l in subjects with initial CRP levels above 3·0 mg/l.

Effects of exercise in elderly people

Elderly people have higher basal levels of inflammation independently of disease status, and a considerable number of studies have been carried out in this population to assess associations between physical activity and inflammatory markers⁽Reference Bruunsgaard, Ladelund and Pedersen^190, Reference Elosua, Bartali and Ordovas^223–Reference Jankord and Jemiolo²³⁰⁾. Rather consistent inverse, BMI-independent, associations are found and the associations are suggested to be dose-dependent; the more physically active the person, the lower the inflammatory markers⁽Reference Fischer, Berntsen and Perstrup^208, Reference Colbert, Visser and Simonsick²²⁴⁾. Also subjects over 80 years of age show consistent inverse associations between inflammation and physical activity⁽Reference Bruunsgaard, Ladelund and Pedersen¹⁹⁰⁾. Functional fitness was inversely associated with IL-6 and IL-1ra concentrations (but not with CRP, TNF-α, IL-10 or IL-1β) in a prospective population-based study of 1020 participants aged 65 years and older⁽Reference Elosua, Bartali and Ordovas^223, Reference Cesari, Penninx and Pahor²³¹⁾. Muscle strength was also evaluated in this study and low hand-grip strength was associated with high levels of CRP and IL-6⁽Reference Cesari, Penninx and Pahor²³¹⁾. Other studies have also shown a negative association of CRP, IL-6 and TNF-α with muscle strength⁽Reference Taaffe, Harris and Ferrucci^228, Reference Visser, Pahor and Taaffe²³²⁾.

Exercise intervention in elderly people or in patients with CVD shows consistent anti-inflammatory effects. After a 6-month individualised, supervised exercise programme for forty-three subjects at high risk of IHD, a 35 %, albeit non-significant, reduction in CRP concentration was observed. The subjects exercised for a mean of 2·5 (range 0·3–7·4) h/week⁽Reference Smith, Dykes and Douglas²³³⁾. One reason for the lack of a significant effect despite the fairly large reduction in CRP concentration is the small size of the study. Another study reported a decrease in basal plasma IL-6 concentration after aerobic training in patients with coronary artery disease⁽Reference Goldhammer, Tanchilevitch and Maor²³⁴⁾. A randomised trial of thirty-nine patients with intermittent claudication demonstrated that both serum CRP and SAA concentrations were significantly reduced after 3 and 6 months of supervised exercise compared with controls⁽Reference Tisi, Hulse and Chulakadabba²³⁵⁾. In a relatively large intervention study of exercise training on cardiac rehabilitation patients, the median CRP concentration was reduced by 41 %, while CRP concentrations did not change in subjects who did not exercise⁽Reference Milani, Lavie and Mehra²³⁶⁾. Again, the exercise training seemed to be more effective in those with the highest initial CRP concentrations, independently of changes in body weight or percentage of body fat, indicating that baseline levels of low-grade inflammation may be an important factor.

Studies in patients with diabetes⁽Reference McGavock, Mandic and Vonder Muhll²³⁷⁾ or the metabolic syndrome⁽Reference Aronson, Sella and Sheikh-Ahmad^238, Reference Aronson, Sheikh-Ahmad and Avizohar²³⁹⁾ have consistently demonstrated inverse associations between fitness and inflammation, independently of fatness. In one study, the independent associations of fitness were in fact more prominent among metabolic syndrome patients compared with healthy participants⁽Reference Aronson, Sella and Sheikh-Ahmad^238, Reference Aronson, Sheikh-Ahmad and Avizohar²³⁹⁾.

Effects of exercise in middle-aged and younger adults and in children

Several studies of large population cohorts, such as the British Regional Heart Study⁽Reference Wannamethee, Lowe and Whincup²²⁵⁾, the Greek ATTICA study⁽Reference Panagiotakos, Pitsavos and Chrysohoou²⁴⁰⁾, the Third National Health and Nutrition Examination Survey (NHANES)⁽Reference Abramson and Vaccarino^241, Reference King, Carek and Mainous²⁴²⁾, the Men's Health Professionals’ Follow-Up Study⁽Reference Pischon, Hankinson and Hotamisligil²⁴³⁾, the Nurses’ Health Study⁽Reference Pischon, Hankinson and Hotamisligil²⁴³⁾ and the Women's Health Study⁽Reference Mora, Lee and Buring²⁴⁴⁾, provide evidence for an inverse, independent dose–response relationship between plasma CRP concentration and level of physical activity in both men and women, but the consistency is less than in elderly subjects, or in disease states. In contrast, the associations found between self-reported physical activity and TNFR1, TNFR2, IL-6 and CRP concentrations in a study including healthy men from the Men's Health Professionals’ Follow-Up Study and healthy women from the Nurses’ Health Study⁽Reference Pischon, Hankinson and Hotamisligil²⁴³⁾ were no longer significant when adjusting for BMI and leptin. Thus, the effect of physical activity on circulating markers of low-grade inflammation appears to be mediated by weight loss. In another study in healthy men and women, BMI, but not previous year or current physical activity, predicted CRP concentration⁽Reference Rawson, Freedson and Osganian²⁴⁵⁾. Similarly, a cross-sectional study⁽Reference Verdaet, Dendale and De²⁴⁶⁾ in men found no relationship between leisure-time physical activity and CRP, fibrinogen and SAA concentrations, after correction for BMI and current smoking status. CRP levels in 2120 Finns were associated with obesity indices and physical activity among both sexes⁽Reference Raitakari, Mansikkaniemi and Marniemi²⁴⁷⁾; in multivariate analyses, the determinants of CRP concentration included obesity and smoking in men, and obesity, oral contraceptive use and physical activity in women. The study showed that about one in three of healthy women who used oral contraceptives had a CRP concentration exceeding 3 mg/l, which should be taken into account when studying younger females. Cross-sectional studies in men from the Aerobics Center Longitudinal Study have demonstrated that cardiorespiratory fitness levels are inversely associated with CRP concentration and also the prevalence of elevated CRP concentrations⁽Reference Church, Barlow and Earnest²⁴⁸⁾. Analyses with fibrinogen and white blood cell count showed similar results⁽Reference Church, Finley and Earnest²⁴⁹⁾. The competing effect of weight and fitness (assessed by submaximal graded exercise treadmill testing) on cardiorespiratory fitness levels was studied in the NHANES, which included 2112 US adults without previously diagnosed CVD⁽Reference Diaz, Player and Mainous²⁵⁰⁾. Both fitness and BMI were independently associated with increased fasting insulin and CRP concentrations. However, when patients with low, moderate and high fitness were further stratified as normal, overweight or obese, weight remained significantly associated with CRP, but fitness did not. This study concludes that ‘fat but fit’ subjects require weight-loss interventions to improve their CRP levels. Future interventions should emphasise weight control, even for those with high cardiorespiratory fitness.

In disease-free young populations, studies have assessed the interaction between inflammatory markers (CRP, IL-6 and TNF-α), physical activity or cardiorespiratory fitness and fatness⁽Reference Cook, Mendall and Whincup^47, Reference Isasi, Deckelbaum and Tracy^251–Reference Wärnberg²⁵⁸⁾. Organised leisure-time exercise (assessed by a questionnaire) in children has shown negative correlations with serum IL-6 concentrations, independently of adiposity and fat localisation⁽Reference Platat, Wagner and Klumpp²⁵⁶⁾, and in 10-year-old children, a borderline significant negative association was observed between CRP and self-reported physical activity, independently of ponderal index⁽Reference Cook, Mendall and Whincup⁴⁷⁾. US children and young adults (aged 6–24 years) from the Columbia University BioMarkers Study showed an inverse correlation between cardiovascular fitness and CRP concentration but only in boys, which remained after adjustment of confounders including BMI⁽Reference Isasi, Deckelbaum and Tracy²⁵¹⁾. Only one study has used accelerometry (an objective measure of total physical activity compared with leisure-time physical activity or exercise) instead of questionnaires as well as cardiovascular fitness⁽Reference Ruiz, Ortega and Warnberg²⁵⁷⁾. In this study of 9-year-old Swedish children, total physical activity was unrelated to CRP, fibrinogen, C3 or C4 concentrations, but exercise was. Nevertheless, once body fat was entered into the regression models, no associations with cardiovascular fitness or physical activity and the inflammatory markers measured were observed⁽Reference Ruiz, Ortega and Warnberg²⁵⁷⁾. Similarly, no associations were found between cardiorespiratory fitness or self-reported physical activity and CRP concentration in 12-year-old healthy Welsh children⁽Reference Thomas, Baker and Graham²⁵⁹⁾.

CRP, C3 and ceruloplasmin (but not C4) concentrations were negatively associated with muscle strength after controlling for sex, age, pubertal status, weight, height, socio-economic status and cardiorespiratory fitness, but did not remain when adjusting for body fat. Nevertheless, when stratifying according to overweight status, CRP (but not C3, C4 or ceruloplasmin) concentration was associated with muscle strength in overweight, but not in normal-weight, adolescents after controlling for potential confounders, including body fat and fat-free mass⁽Reference Ruiz, Ortega and Warnberg²⁶⁰⁾.

Hypoenergetic diet

One diet-dependent but apparently quite non-specific way of decreasing low-grade inflammation is energy restriction⁽Reference Holloszy and Fontana²⁶⁷⁾. Weight loss is accompanied by decreased concentrations of circulating mediators of inflammation, such as CRP, TNF-α, IL-6 and sICAM-1⁽Reference Plat, Jellema and Ramakers^155, Reference Kolb and Mandrup-Poulsen^268–Reference Madsen, Rissanen and Bruun²⁷⁰⁾, although it is difficult to dissect whether this effect is due to the weight loss per se or to the nature of the diet used to induce weight loss. On the other hand, it seems likely that reduced secretion of pro-inflammatory mediators from adipocytes or activated macrophages of adipose tissue contributes to the effect of weight loss⁽Reference Cancello, Henegar and Viguerie^31, Reference Curat, Wegner and Sengenes^36, Reference Cancello, Tordjman and Poitou^65, Reference Xu, Barnes and Yang^67, Reference Clement and Langin¹⁴²⁾. However, energy restriction itself may also play an anti-inflammatory role, with key mediators of the effect being proteins of the sirtuin and Forkhead box, sub-group O (FoxO) families which are induced/activated during states of limited energy supply. The sirtuins are NAD+-dependent deacetylases of substrates ranging from histones to transcriptional regulators. As a consequence, metabolic efficiency is improved, cell defences against stress are strengthened and inflammatory activities are dampened, notably by decreasing the activation of NF-κB⁽Reference Guarente and Picard^271–Reference Salminen, Ojala and Huuskonen²⁷⁴⁾. FoxO proteins are transcription factors which regulate the expression of genes involved in energy homeostasis, cell survival and inflammatory responses including NF-κB⁽Reference Salminen, Ojala and Huuskonen^274–Reference Nakae, Oki and Cao²⁷⁸⁾. Early studies have shown that reduced energy intake is paramount compared with the nature of low-energy food, i.e. decreased concentrations of inflammatory markers are observed with a low-energy, fat-rich as well as with a low-energy, carbohydrate-rich diet⁽Reference Sharman and Volek²⁷⁹⁾. However, it is conceivable that some dietary components are better regulators of sirtuin or FoxO activity than others. Resveratrol, present in red wine, was found to directly or indirectly interact with sirtuins and promote their deacetylase activity. This led to increased lifespan in model organisms and in animal models⁽Reference Baur, Pearson and Price^280, Reference Baur and Sinclair²⁸¹⁾. It is likely that dietary components will be identified that mimic or counteract the anti-inflammatory effects of energy restriction.

Mediterranean diet

The term Mediterranean diet refers to a traditional dietary pattern characteristic of many parts of Greece, Southern Italy, Southern Spain and elsewhere in the Mediterranean region. The traditional Mediterranean diet is rich in fruit, vegetables, whole-grains, legumes (beans), nuts, fish and low-fat dairy products, with moderate consumption of wine, and whose principal source of fat is olive oil⁽Reference Dai, Miller and Bremner^282–Reference Trichopoulou, Costacou and Bamia²⁸⁶⁾. Most studies have assessed the adherence to the Mediterranean diet by assigning a score in relation to the consumption of these foods. Others⁽Reference Esposito, Marfella and Ciotola^284, Reference Fung, McCullough and Newby²⁸⁵⁾ have used modified versions of this score by not considering certain food groups, i.e. dairy products.

Observational studies have examined the association of the Mediterranean diet with inflammatory markers in healthy persons⁽Reference Dai, Miller and Bremner^282, Reference Chrysohoou, Panagiotakos and Pitsavos^283, Reference Fung, McCullough and Newby²⁸⁵⁾, and they generally report inverse correlations. In a recent study⁽Reference Dai, Miller and Bremner²⁸²⁾ investigating psychological, behavioural and biological risk factors for subclinical CVD in 345 middle-aged male twins, an inverse association between adherence to the Mediterranean diet and inflammation, as measured by plasma IL-6 concentration, was noted. This association was independent of several known cardiovascular risk factors, and persisted when twins within pairs were compared, suggesting that the results were not confounded by shared environmental and genetic factors. Although a marginal relationship between CRP concentration and the Mediterranean diet was observed, this association was no longer significant when other cardiovascular risk factors were considered in the models. It is likely that IL-6 is a more sensitive marker of chronic low-grade inflammation and that CRP reflects a more downstream effect associated with IL-6. In a subsample of the Nurses’ Health Study⁽Reference Fung, McCullough and Newby²⁸⁵⁾, a Mediterranean diet index score was inversely associated with markers of inflammation (circulating IL-6 and CRP) as well as markers of endothelial dysfunction (the adhesion molecules sICAM-1, sVCAM-1 and soluble E-selectin (sE-selectin)); these associations persisted upon adjustment for traditional CVD risk factors. Similar findings were reported in the ATTICA study, involving 1514 men and 1528 women; specifically, subjects with greater adherence to the Mediterranean diet (those in the highest tertile) had 17 % lower IL-6 and 20 % lower CRP concentrations, compared with those in the lowest tertile in analyses that adjusted for other cardiovascular risk factors⁽Reference Chrysohoou, Panagiotakos and Pitsavos²⁸³⁾. Although a marginal association was noted between TNF-α and the Mediterranean diet, it was not significant in adjusted models⁽Reference Chrysohoou, Panagiotakos and Pitsavos²⁸³⁾. In another observational study with obese subjects (625 men and 712 women with abdominal adiposity), those with high CRP levels (>3 mg/l) were less likely to adopt the Mediterranean diet⁽Reference Pitsavos, Panagiotakos and Tzima²⁸⁷⁾. The authors reported that adoption of the Mediterranean diet in conjunction with moderate physical activity was associated with a reduced likelihood of having high CRP levels by 72 %, highlighting the potential importance of the Mediterranean diet in diminishing inflammation. In subjects at high risk for CVD, those with diabetes or with multiple CHD risk factors, however, no association between the Mediterranean diet and CRP or adhesion molecule (sICAM-1 and sVCAM-1) concentrations was seen⁽Reference Salas-Salvado, Garcia-Arellano and Estruch²⁸⁸⁾. However, a significant relationship between higher consumption of some typical Mediterranean diet components (cereals, fruit, nuts and virgin olive oil) and circulating inflammatory markers (IL-6) and markers of endothelial function was noted in these at-risk subjects.

Few intervention studies have been conducted to examine the effect of consuming the Mediterranean diet on markers of low-grade inflammation. In two cross-over studies involving short-term interventions (1–3 months) with healthy subjects, differential effects of the Mediterranean diet on markers of inflammation (and endothelial dysfunction) were observed. In the study by Ambring et al. ⁽Reference Ambring, Johansson and Axelsen²⁸⁹⁾, healthy subjects received their typical Swedish diet for about 4 weeks, and a Mediterranean-inspired diet for about 4 weeks in a randomised cross-over design, with 4 weeks of washout in-between. A marked reduction in the number of leucocytes, including monocytes, neutrophils and lymphocytes, and in the number of platelets after 4 weeks of the Mediterranean diet was noted, suggesting a lower inflammatory activity than during the Swedish diet period. On the other hand, IL-6 and CRP concentrations did not change with the Mediterranean diet; the authors speculated that the study may not have been powered to detect those effects. In another study⁽Reference Bellido, Lopez-Miranda and Perez-Martinez²⁹⁰⁾, twenty healthy young males were provided three dietary interventions, each lasting 4 weeks. First, all subjects consumed a diet high in saturated fat, next they were randomly assigned to either of the two intervention diets: a Mediterranean-style diet (MUFA-enriched diet, 22 % energy from MUFA) or a low-fat, high-carbohydrate diet ( < 30 % energy from fat and 55 % from carbohydrates). LDL was isolated and oxidised. Oxidised LDL from subjects on either the Mediterranean diet or the high-carbohydrate diet decreased TNF-α-induced VCAM-1 and E-selectin expression in human umbilical endothelial cells in vitro. A consistent decline in inflammatory markers has been observed in intervention studies involving obese subjects or those with the metabolic syndrome. In a randomised controlled study with 120 pre-menopausal obese women, the effects of a multidisciplinary approach (aiming at 10 % weight reduction with a combination of a low-energy, Mediterranean-style diet and increased physical activity) were evaluated compared with a control group (given general information about healthy choices and exercise)⁽Reference Esposito, Pontillo and Di Palo¹¹¹⁾. The intervention group received regular sessions (18 over a 2-year period) with a nutritionist to ensure compliance. Significant reduction in several markers of inflammation (CRP, IL-6 and IL-18) and an increase in adiponectin concentration were noted in the Mediterranean diet group compared with the control group. In a randomised controlled trial lasting 2 years, 180 subjects with the metabolic syndrome were assigned either to a Mediterranean-style diet (instructions were provided on increasing daily consumption of whole grains, fruit, vegetables, nuts and olive oil) or to a control group (prudent diet, with same macronutrient composition as the Mediterranean diet)⁽Reference Esposito, Marfella and Ciotola²⁸⁴⁾. After 2 years, body weight decreased more in the intervention group and inflammatory markers (CRP, IL-6, IL-7 and IL-18) decreased and endothelial function improved, compared with the control group. Interestingly, even after controlling for weight loss, the inflammatory markers declined more in subjects following the Mediterranean-style diet. In other related studies involving individuals with the metabolic syndrome, a consistent reduction in CRP concentration in the intervention group receiving the Mediterranean diet has been shown⁽Reference Esposito, Ciotola and Giugliano^291, Reference Esposito, Ciotola and Giugliano²⁹²⁾. In the Prevención con Dieta Mediterránea study, 772 asymptomatic subjects at high cardiovascular risk (diabetes or more than three CHD risk factors) were randomly assigned to a low-fat diet or one of two Mediterranean diets⁽Reference Estruch, Martinez-Gonzalez and Corella²⁹³⁾. Those allocated to the Mediterranean diets received nutritional education and either free virgin olive oil, or free nuts for 3 months. Both Mediterranean diets were beneficial in terms of significant reductions in serum IL-6, sICAM-1 and sVCAM-1 concentrations, while CRP concentration was reduced only in the Mediterranean diet supplemented with olive oil. Taken together, the results from these often large intervention studies strongly suggest that Mediterranean diets can lead to reductions in chronic low-grade inflammation and improvement in endothelial function, thereby offering cardioprotective effects.

Vegetarian diets

Observational studies have compared vegetarian with non-vegetarian diets in healthy subjects in relation to inflammatory markers. Dietary patterns consistent with vegetarianism were associated with lower concentrations of markers of chronic low-grade inflammation and endothelial function when compared with non-vegetarian diets⁽Reference Krajcovicova-Kudlackova and Blazicek^294–Reference Purschwitz, Rassoul and Reuter²⁹⁶⁾. In one study comparing a group of thirty Taoist adults who had been vegetarian for 5–55 years (average 22 years) with a group of thirty age- and sex-matched non-Taoist adults consuming a non-vegetarian diet, lower CRP concentrations were noted in the former group⁽Reference Szeto, Kwok and Benzie²⁹⁵⁾. Specifically, plasma CRP concentration averaged 0·77 and 1·30 mg/l in vegetarians and non-vegetarians, respectively. Another study with a similar design comparing vegetarians with non-vegetarians also found that the average CRP concentration was significantly lower in the vegetarian group (0·72 v. 1·62 mg/l)⁽Reference Krajcovicova-Kudlackova and Blazicek²⁹⁴⁾. Similarly, lower levels of the adhesion molecules sICAM-1 and sE-selectin have been reported in those following a vegetarian v. a non-vegetarian diet⁽Reference Purschwitz, Rassoul and Reuter²⁹⁶⁾. Thus, these cross-sectional studies suggest that a vegetarian-style diet can lead to lower chronic low-grade inflammation than an omnivorous diet. However, it is important to recognise that vegetarians may differ from non-vegetarians in aspects of lifestyle other than diet such as physical activity, smoking behaviour and socio-economic class. Studies considering vegetables and fruits are described in section ‘Vegetables and fruits’.

Eating patterns

The healthy eating index (HEI) was developed by the US Department of Agriculture, based on the Dietary Guidelines for Americans and the Food Guide Pyramid⁽Reference Kennedy, Ohls and Carlon²⁹⁷⁾. It has ten subcomponents: grains, fruits, vegetables, dairy, meats, fats, saturated fat, cholesterol, Na and dietary variety. Individuals are assigned scores for each of the ten components (from 0 to 10) based on their typical intakes; the maximum value for the HEI being 100. Recently, the HEI was revised as the Alternate HEI (AHEI) to focus on healthier items in the Food Guide Pyramid food groups⁽Reference McCullough, Feskanich and Stampfer²⁹⁸⁾ such as protein source, ratio of polyunsaturated:saturated fats and cereal fibre, as well as cis- v. trans-fat. Additionally, moderate alcohol consumption and long-term multivitamin use are also considered in the AHEI. The Diet Quality Index (DQI) is a composite score of an overall healthy diet that reflects an individual's adherence to the eight diet and health recommendations of the National Academy of Science. The revised DQI (DQI-R) is based on similar guidelines but also includes Fe and Ca intakes⁽Reference Haines, Siega-Riz and Popkin²⁹⁹⁾.

Using the data from NHANES III on a representative sample of the US population, Ford et al. ⁽Reference Ford, Mokdad and Liu³⁰⁰⁾ found a negative correlation between the HEI and CRP concentration after adjustment for several CVD risk factors including BMI and waist:hip ratio; when stratified by sex this finding was significant in women only. The authors noted that this association was primarily driven by grain consumption. The authors speculated that because HEI score was determined based on dietary intake data collected by a 24 h recall, the possibility of misclassification of individuals with respect to HEI status could have attenuated the associations. In a study by Fung et al. ⁽Reference Fung, McCullough and Newby²⁸⁵⁾, the association of several diet-quality scores, such as the HEI, AHEI, DQI-R and an alternate Mediterranean diet index with various markers of inflammation, was evaluated in a subsample of the Nurses’ Health Study (n 690 healthy women). The key findings were that the AHEI and alternate Mediterranean diet index scores were negatively associated with CRP, IL-6, sE-selectin and sICAM-1 concentrations, and that these associations persisted upon adjustment for potential confounding variables including BMI. In contrast, the HEI and DQI-R did not correlate with any of the inflammatory markers when potential confounders were taken into consideration in the regression models⁽Reference Fung, McCullough and Newby²⁸⁵⁾. A more recent study evaluated the impact of the AHEI among 1922 women from the Nurses’ Health Study (62 % of whom were overweight) who had no history of diabetes or CVD on plasma inflammatory marker concentrations⁽Reference Fargnoli, Fung and Olenczuk³⁰¹⁾. After adjustment for age and energy intake, women with the highest adherence to the AHEI had 24 % higher median total adiponectin and 32 % higher median high-molecular-weight adiponectin concentrations, as well as 16 % lower resistin, 41 % lower CRP and 19 % lower sE-selectin concentrations than did women with the lowest adherence to the AHEI. These associations remained significant after adjustment for potential confounders. Inverse associations between the AHEI and CRP, sTNFR2, IL-6, sICAM-1 and sVCAM-1 concentrations were evident, but they did not remain significant after adjustment for BMI. In a small cross-sectional study involving 114 ‘apparently healthy’ overweight or obese postmenopausal women, Boynton et al. ⁽Reference Boynton, Neuhouser and Wener³⁰²⁾ found little evidence of an association between dietary quality, as measured by the HEI and DQI, and markers of inflammation (CRP and SAA). Marginal associations were noted between the DQI and these inflammatory markers. These associations were, however, attenuated and no longer significant, after adjusting for adiposity (percentage of body fat or BMI), suggesting that the decrease in CRP or SAA concentration seen with a higher-quality diet is most probably mediated by obesity. The authors speculated that consuming a healthier diet may lead to decreased adiposity, which in turn could result in less low-grade inflammation.

Several studies have examined the relationship between consuming a ‘prudent diet’ and markers of low-grade inflammation. In one study, in 732 healthy women from the Nurses’ Health Study aged 43–69 years, a prudent pattern was characterised by higher intakes of fruit, vegetables, legumes, fish, poultry and whole grains, and a Western pattern was characterised by higher intakes of red and processed meats, sweets, desserts, French fries and refined grains⁽Reference Lopez-Garcia, Schulze and Fung³⁰³⁾. The prudent pattern was inversely associated with plasma concentrations of CRP and sE-selectin after adjustment for age, BMI, physical activity, smoking status and alcohol consumption. The Western pattern showed a positive relationship with CRP, IL-6, sE-selectin, sICAM-1 and sVCAM-1 after adjustment for all confounders except BMI; with further adjustment for BMI, the coefficients remained significant for CRP, sE-selectin, sICAM-1 and sVCAM-1.Using data from the Nurses’ Health Study, Schulze et al. ⁽Reference Schulze, Hoffmann and Manson³⁰⁴⁾ identified a dietary pattern that was significantly associated with increased concentrations of CRP, IL-6, sTNFR2, sE-selectin, sICAM-1 and sVCAM-1. This pattern, which was high in sugar-sweetened soft drinks, refined grains, diet soft drinks and processed meat but low in wine, coffee, cruciferous vegetables and yellow vegetables, was also associated with an increased risk of diabetes. Most recently, Hoebeeck et al. ⁽Reference Hoebeeck, Rietzschel and Langlois³⁰⁵⁾ evaluated the relationship between adherence to food-based dietary guidelines and inflammatory markers among 2524 healthy Belgian men and women aged 35–55 years. The dietary index consisted of three subscores (dietary quality, diversity and equilibrium) according to adherence to the Flemish food-based dietary guidelines, using data from a semi-quantitative FFQ. Higher dietary scores were inversely associated with IL-6 concentration and leucocyte numbers in the bloodstream. Nettleton et al. ⁽Reference Nettleton, Steffen and Mayer-Davis³⁰⁶⁾ examined relationships between dietary patterns and markers of inflammation and endothelial activation among 5089 non-diabetic participants in the Multi-Ethnic Study of Atherosclerosis. In this study, four dietary patterns were derived by using factor analysis. The fats and processed meats pattern (fats, oils, processed meats, fried potatoes, salty snacks and desserts) was positively associated with CRP and IL-6 concentrations. The beans, tomatoes and refined grains pattern (beans, tomatoes, refined grains and high-fat dairy products) was positively related to sICAM-1 concentration. In contrast, the whole grains and fruit pattern (whole grains, fruit, nuts and green leafy vegetables) was inversely associated with CRP, IL-6 and sICAM-1 concentrations, while the vegetables and fish pattern (fish and dark-yellow, cruciferous and other vegetables) was inversely related to IL-6 concentration. There are few such studies using data from outside of the USA or Europe. A cross-sectional study of 486 healthy Iranian women aged 40–60 years identified dietary patterns by factor analysis and related these to circulating markers of inflammation⁽Reference Esmaillzadeh, Kimiagar and Mehrabi³⁰⁷⁾. The healthy dietary pattern (high in fruits, vegetables, tomatoes, poultry, legumes, tea, fruit juices and whole grains) was inversely related to plasma CRP, sE-selectin and sVCAM-1 concentrations after controlling for potential confounders; with further adjustment for BMI and waist circumference, the associations remained significant for CRP and sVCAM-1. In contrast, the Western dietary pattern score (high in refined grains, red meat, butter, processed meat, high-fat dairy, sweets and desserts, pizza, potatoes, eggs, hydrogenated fats and soft drinks) was positively related to CRP, SAA, IL-6, sICAM-1 and sVCAM-1 concentrations; however, after additional control for BMI and waist circumference, the associations remained significant only for SAA and IL-6.The traditional dietary pattern (high in refined grains, potatoes, tea, whole grains, hydrogenated fats, legumes and casserole) was positively associated with the plasma IL-6 concentration after controlling for BMI and waist circumference. Nanri et al. ⁽Reference Nanri, Yoshida and Yamaji³⁰⁸⁾ investigated the relationship between dietary patterns and circulating CRP concentration in 7802 Japanese men and women with CRP <3 mg/l. The dietary patterns were derived from principal component analysis of the frequency of consumption of forty-nine food items ascertained by the FFQ. The following four dietary patterns were identified: healthy, high-fat, seafood and Westernised breakfast patterns. The healthy dietary pattern, characterised by high intakes of vegetables, fruits, soya products and fish, was inversely related to CRP concentrations, even after adjustment for age, BMI, smoking, alcohol consumption and physical activity in both men and women. Neither the high-fat dietary pattern nor the Westernised breakfast pattern was related to CRP concentrations. Most recently, the relationship between two dietary patterns (Healthy and Western), which were derived by principal component analysis using data collected by a FFQ from subjects in the Atherosclerosis Risk in Communities Study, and markers of inflammation has been described⁽Reference Nettleton, Matijevic and Follis³⁰⁹⁾. The measures of inflammation were quantified by flow cytometry in fresh whole blood from 1101 white adults. After multivariable adjustment, monocyte LPS receptor (CD14), monocyte TLR-2 and platelet glycoprotein IIb (CD41) showed inverse associations with the healthy dietary pattern. In contrast, the Western dietary pattern was positively associated with CD41 and platelet–granulocyte aggregates. Taken together, these studies suggest that healthy eating patterns are associated with lower concentrations of markers of chronic low-grade inflammation.

Whole grains/refined grains

Published studies have so far investigated a narrow and low range of whole grain intakes, which limits the interpretation of associations between whole grain intake and markers of low-grade chronic inflammation. Observational studies (Table 3)⁽Reference Ford, Mokdad and Liu^300, Reference Jensen, Koh-Banerjee and Franz^310–Reference Qi, van Dam and Liu³¹⁴⁾ including data from NHANES III have suggested that a high intake of whole grain is inversely associated with plasma CRP concentration (quintile (Q) 1 < 3·5 servings/d, Q5>9·7 servings/d)⁽Reference Ford, Mokdad and Liu³⁰⁰⁾. In contrast, Jensen et al. ⁽Reference Jensen, Koh-Banerjee and Franz³¹⁰⁾ reported no association between a moderate whole grain intake (Q1 8·2 g/d, Q5 43·8 g/d) and markers of inflammation (CRP, IL-6 and fibrinogen) in the Health Professionals’ Follow-Up Study. However, bran intake correlated inversely with CRP concentration and germ intake with IL-6 concentration⁽Reference Jensen, Koh-Banerjee and Franz³¹⁰⁾. Data from the Nurses’ Health Study showed lower CRP and sTNFR2 concentrations in diabetic women with a moderate intake of whole grain compared with those with a low intake⁽Reference Qi, van Dam and Liu³¹⁴⁾. In another study with diabetic males (n 780; from the Health Professionals’ Follow-Up Study), Qi et al. ⁽Reference Qi, Rimm and Liu³¹⁵⁾ showed that cereal fibre was positively associated with adiponectin concentration after controlling for a number of confounding factors. A higher intake of whole grain (low 0·8 servings/d, high 2·1 servings/d) within a Mediterranean diet in diabetic women was associated with increased adiponectin concentration⁽Reference Mantzoros, Williams and Manson³¹²⁾. Data from the Multi-Ethnic Study of Atherosclerosis (MESA) showed a higher whole grain intake (Q1 0·02 servings/d, Q5 1·39 servings/d) to be associated with lower CRP concentration in elderly subjects⁽Reference Lutsey, Jacobs and Kori³¹¹⁾.

Table 3 Observational studies on the association between whole grain intake and markers of low-grade inflammation

M, male; F, female; ↓ , decreased; CRP, C-reactive protein; = , no effect on; sTNFR, soluble TNF receptors; sICAM, soluble intercellular adhesion molecule; sE-selectin, soluble E-selectin; ↑ , increased; PAI-1, plasminogen activator inhibitor 1.

Evidence from intervention studies (Table 4)⁽Reference Esposito, Pontillo and Di Palo^111, Reference Andersson, Tengblad and Karlstrom^316–Reference Tighe, Duthie and Vaughan³²⁰⁾ includes a study whereby overweight and obese subjects consumed a hypoenergetic diet with or without whole-grain foods. CRP concentration decreased by 38 % (CRP at baseline 5·9–6·0 mg/l) in the whole-grain group independent of the observed weight loss⁽Reference Katcher, Legro and Kunselman³¹⁹⁾. However, plasma concentrations of IL-6 and TNF-α and PAI-1 activity did not change within the 12-week-study period. Based on the outcome of this study, the authors concluded that the changes seen in CRP concentration with a whole-grain diet were not part of a systemic anti-inflammatory effect⁽Reference Katcher, Legro and Kunselman³¹⁹⁾. In a study from Sweden with a similar design but only a 6-week intervention (whole-grain v. refined-grain products), the whole grain intake was not associated with changes in the concentrations of CRP and IL-6 or in PAI-1 activity of moderately overweight subjects⁽Reference Andersson, Tengblad and Karlstrom³¹⁶⁾. Baseline CRP concentration (2·03–2·86 mg/l) and BMI were much lower than in the previous study, which may explain the different outcomes between these studies. Recent intervention trials with whole grain (60–120 g/d) or wholemeal intake (30–40 g/d) and comparable CRP baseline concentrations also did not observe any changes in plasma markers of inflammation⁽Reference Brownlee, Moore and Chatfield^317, Reference Tighe, Duthie and Vaughan³²⁰⁾. In the study by Brownlee et al. ⁽Reference Brownlee, Moore and Chatfield³¹⁷⁾, whole-grain products did not substitute refined-grain products but were consumed in addition to the refined products. Bioprocessing of whole wheat affected the anti-inflammatory potential in a human intervention study when compared with unprocessed whole wheat⁽Reference Mateo, Aura and Selinheimo³²¹⁾. Bioprocessing modulated the ratio of pro-inflammatory:anti-inflammatory cytokines during 24 h after the intake of 300 g whole wheat bread. This study suggests that subtle changes in the processing of whole grains may be relevant for their anti-inflammatory activity, which could in part explain the contradictory findings of the human intervention studies.

Table 4 Intervention studies investigating the effect of whole grain intake on markers of low-grade inflammation

M, male; F, female; ↓ , decreased; = , no effect on; CRP, C-reactive protein; PAI-1, plasminogen activator inhibitor-1; sVCAM-1, soluble vascular cell adhesion molecule-1; sICAM-1, soluble intercellular adhesion molecule-1; sE-selectin, soluble E-selectin.

Replacing a refined-wheat flour pizza by a similar pizza prepared from whole-wheat flour resulted in a decreased postprandial concentration of the pro-inflammatory cytokine IL-18 in both non-diabetic and diabetic subjects⁽Reference Esposito, Nappo and Giugliano³¹⁸⁾.

In summary, whole grain intake appears to inversely associate with markers of low-grade inflammation. Processing status of whole-grain products should be more precisely defined in future studies. Potential mechanisms still have to be elucidated, as well as the active constituents which may include dietary fibre, minerals, vitamins and phytochemicals such as lignans and phenolic acids.

Vegetables and fruits

A number of cross-sectional studies have investigated the association between vegetable and fruit intake and biomarkers of inflammation (Table 5)⁽Reference Salas-Salvado, Garcia-Arellano and Estruch^288, Reference Bhupathiraju and Tucker^322–Reference Yannakoulia, Yiannakouris and Melistas³³⁰⁾. Study participants included healthy, normal-weight adults as well as overweight/obese adults with associated diseases. Each study applied different criteria to stratify the intake of vegetables and fruits which makes it difficult to compare the outcomes. Of the ten cross-sectional studies, seven reported an inverse association between a high intake of vegetables and fruits, either in combination or alone, and blood CRP concentrations⁽Reference Bhupathiraju and Tucker^322–Reference Gao, Bermudez and Tucker^325, Reference Holt, Steffen and Moran^327–Reference Wannamethee, Lowe and Rumley³²⁹⁾. In one study, no association with CRP was observed⁽Reference Salas-Salvado, Garcia-Arellano and Estruch²⁸⁸⁾, while in a second study, an inverse association was seen in men but not in women⁽Reference Holt, Steffen and Moran³²⁷⁾. For the other biomarkers reported, the outcome is less consistent. A recent study reported that a high intake of vegetables and fruits was associated with a lower peripheral blood mononuclear cell gene expression for several pro-inflammatory cytokines and adhesion molecules⁽Reference Hermsdorff, Zulet and Puchau³²⁶⁾. An important observation is that besides the quantity of total intake of vegetables and fruits, the variety consumed in a given time has a significant impact on these biomarkers. A high number of varieties of vegetables and fruits were inversely correlated with blood CRP levels⁽Reference Bhupathiraju and Tucker³²²⁾. This suggests that plant-specific constituents of vegetables and fruits such as the phytochemicals may contribute to the anti-inflammatory activities.

Table 5 Observational studies on the association between vegetable and fruit intake and markers of low-grade inflammation

M, male; F, female; ↓ , decreased; CRP, C-reactive protein; = , no effect on; sICAM-1, soluble intercellular adhesion molecule-1; sVCAM-1, soluble vascular cell adhesion molecule-1; ↑ , increased; IL-1ra, IL-1 receptor antagonist.

A number of intervention studies have investigated the impact of vegetables and fruits in total or of specific varieties on biomarkers of inflammation (Table 6)⁽Reference Basu, Du and Leyva^331–Reference Zern, Wood and Greene³⁴⁶⁾. Of the four studies focusing on vegetables and fruits as a food group, three reported a reduction in blood concentrations of different biomarkers of inflammation⁽Reference Esposito, Nappo and Giugliano^318, Reference Sanchez-Moreno, Cano and de^342, Reference Watzl, Kulling and Moseneder³⁴⁵⁾, while one study did not find any significant effect⁽Reference Freese, Vaarala and Turpeinen³³⁶⁾. In twelve studies with a specific focus on a single variety of vegetable or fruit, inconsistent results were reported. Most studies have used fruits or fruit extracts high in polyphenols. Results from such studies suggest an anti-inflammatory effect; however, mostly, only a single biomarker was affected, and never the complete set of inflammation biomarkers investigated⁽Reference Basu, Du and Leyva^331–Reference Dalgard, Nielsen and Morrow^334, Reference Karlsen, Retterstol and Laake^337–Reference Larmo, Alin and Salminen^340, Reference Zern, Wood and Greene³⁴⁶⁾. In conclusion, current evidence for specific effects of single vegetable and fruit varieties is not convincing, while a high overall intake of vegetables and fruits seems to be associated with a lower state of inflammation.

Table 6 Intervention studies investigating the effect of vegetable and fruit intake on markers of low-grade inflammation

M, male; F, female; ↓ , decreased; PAI-1, plasminogen activator inhibitor-1; = , no effect on; CRP, C-reactive protein; MCP-1, monocyte chemoattractant protein-1; sICAM-1, soluble intercellular adhesion molecule-1; sP-selectin, soluble P-selectin; sVCAM-1, soluble vascular cell adhesion molecule-1; sE-selectin, soluble E-selectin; RANTES, regulated on activation, normal T expressed and secreted; IFN, interferon; sTNFR, soluble TNF receptor; MIG, monokine-induced by IFN-γ; WBC, white blood cell; IL-1ra, IL-1 receptor antagonist; ↑ , increased.

Soya

Several randomised controlled intervention trials have shown that supplementation with different quantities of soya protein did not affect markers of inflammation (CRP, IL-6, IL-18, sICAM-1, sVCAM-1 and sE-selectin) (Table 7)⁽Reference Watzl, Kulling and Moseneder^345, Reference Azadbakht, Kimiagar and Mehrabi^347–Reference Zemel, Sun and Sobhani³⁶⁴⁾. Variations in soya protein isoflavone contents did not modulate the outcome in these studies. In one study, reduced plasma CRP, TNF-α and IL-18 concentrations were reported in postmenopausal Iranian women with the metabolic syndrome consuming soya nuts but not soya protein⁽Reference Azadbakht, Kimiagar and Mehrabi³⁴⁷⁾. The major difference between soya protein and soya nuts (comparable contents of isoflavones) was a much higher content of PUFA in the soya nuts, suggesting that rather than soya-specific constituents, the increased intake of PUFA may be responsible for the reduction in biomarkers of inflammation⁽Reference Azadbakht, Kimiagar and Mehrabi³⁴⁷⁾. The same group investigated in diabetic patients with nephropathy the anti-inflammatory effect of textured soya protein compared with animal protein after 4 years of intake. Consumption of soya protein significantly lowered plasma concentrations of CRP⁽Reference Azadbakht, Atabak and Esmaillzadeh³⁴⁸⁾. In another study investigating soya nuts in normotensive and hypertensive postmenopausal women, no effect on CRP, IL-6, sICAM-1 or matrix metalloproteinase-9 concentrations was observed compared with the control diet⁽Reference Nasca, Zhou and Welty³⁶¹⁾. However, in hypertensive women, levels of sVCAM-1 were significantly lower after soya nut consumption⁽Reference Nasca, Zhou and Welty³⁶¹⁾. Again, the increased PUFA intake may have caused this effect. Long-term soya exposure did not affect CRP, IL-6, leptin and adiponectin concentrations in postmenopausal women⁽Reference Maskarinec, Steude and Franke³⁵⁷⁾, or CRP, IL-6, leptin, adiponectin, MCP-1 or MIP-1β concentrations in men⁽Reference Maskarinec, Oum and Chaptman³⁶⁵⁾. Short-term soya intervention in postmenopausal women with or without exercise-induced inflammation had no effect on IL-1β, IL-6 or TNF-α concentrations⁽Reference Beavers, Serra and Beavers^349, Reference Beavers, Serra and Beavers³⁵⁰⁾. Among men with prostate cancer undergoing androgen deprivation therapy, soya intervention had no effect on the concentrations of several inflammatory markers⁽Reference Napora, Short and Muller³⁶⁶⁾. Overall, these data suggest that soya protein does not affect markers of inflammation, that soyabean processing may affect the anti-inflammatory potential of soyabean constituents and that the health status of the study subjects might determine the anti-inflammatory efficacy of specific foods.

Table 7 Intervention studies investigating the effect of soya intake on markers of low-grade inflammation

F, female; = , no effect on; sIL-2R, soluble IL-2 receptor; sE-selectin, soluble E-selectin; sP-selectin, soluble P-selectin; sICAM-1, soluble intercellular adhesion molecule-1; sVCAM-1, soluble vascular cell adhesion molecule-1; CRP, C-reactive protein; M, male; ↓ , decreased; ↑ , increased; MCP, monocyte chemoattractant protein; S-hs CRP, serum high-sensitivity C-reactive protein; DASH, dietary approaches to stop hypertension.

Nuts

To date, only few studies have investigated the effect of nuts on inflammatory markers. The MESA reported that a high intake of nuts and seeds ( ≥ 5 times/week) compared with a low intake (rarely or no consumption) was associated with lower plasma concentrations of CRP, IL-6 and fibrinogen⁽Reference Jiang, Jacobs and Mayer-Davis³⁶⁷⁾. In contrast, data from the Nurses’ Health Study suggest that nut intake is not associated with inflammatory markers including sTNFR2, CRP, fibrinogen, sICAM-1 and sE-selectin⁽Reference Li, Brennan and Wedick³⁶⁸⁾. An 8-week intervention with walnuts or cashews (63–108 g/d) in subjects with the metabolic syndrome observed no effect on serum CRP concentrations⁽Reference Mukuddem-Petersen, Stonehouse and Jerling³⁶⁰⁾. A pistachio intervention (60–100 g/d providing about 20 % of total energy) for 4 weeks in healthy young subjects had no effect on plasma CRP or TNF-α concentrations, while the concentration of IL-6 was significantly reduced⁽Reference Sari, Baltaci and Bagci³⁶⁹⁾. A recent intervention study with two doses of almonds (10 and 20 % of total energy) in healthy adults reported reduced serum concentrations of sE-selectin and CRP, but IL-6 and fibrinogen were not affected⁽Reference Rajaram, Connell and Sabate³⁷⁰⁾. No clear dose–response relationships were observed. In contrast, an earlier study in hyperlipidaemic subjects providing two doses of almonds did not observe an effect on CRP concentrations⁽Reference Jenkins, Kendall and Jackson³⁵⁶⁾. A systematic review on walnut consumption and inflammatory markers also reported inconsistent results for plasma CRP, while walnuts added to a Mediterranean diet resulted in significantly lower sVCAM-1 concentrations⁽Reference Banel and Hu³⁷¹⁾. This suggests that rather than a general anti-inflammatory effect, walnuts may exert anti-inflammatory effects primarily in the endothelium. Major contributors to any anti-inflammatory activity of nuts are likely to include PUFA, Mg and phytochemicals including ellagic acid⁽Reference Ros³⁷²⁾.

Fish

Increased frequency of fish consumption was associated with lower CRP and IL-6 concentrations in a cohort of 727 adults in the USA⁽Reference Lopez-Garcia, Schulze and Manson³⁷³⁾. sICAM-1 and sE-selectin concentrations also decreased but the effect of fish consumption was weaker than what was observed for CRP and IL-6. sTNFR2 concentration was not associated with fish consumption. A study in 379 adults in Denmark reported a lack of association between fish consumption and CRP concentration, even though the subjects displayed a wide range of intakes including very high intakes (e.g. over 30 % of subjects ate fish more than once per d)⁽Reference Madsen, Skou and Hansen³⁷⁴⁾. Data from 5037 adults in the NHANES showed no association of fish consumption with CRP concentration after adjusting for a range of confounders⁽Reference King, Egan and Geesey³⁷⁵⁾. In this study, 25 % of subjects consumed fish more than 3 times per month. Most recently, fish consumption among a cohort of 4077 Australian adults was reported not to be associated with CRP concentration before or after adjustment for confounders⁽Reference Hickling, Hung and Knuiman³⁷⁶⁾. In contrast to these findings, a study in 3102 Greek adults reported that fish consumption was ‘dose-dependently’ associated with lower CRP, IL-6, TNF-α and SAA concentrations and white blood cell counts; individuals consuming >300 g fish/week (n 259; 9 %) had significantly lower concentrations of CRP ( − 33 %), IL-6 ( − 33 %), TNF-α ( − 21 %), SAA ( − 28 %) and leucocytes ( − 4 %) than seen in individuals not consuming fish (n 319; 11 %)⁽Reference Zampelas, Panagiotakos and Pitsavos³⁷⁷⁾. It is possible that the observations of Zampelas et al. may be due to greater fish consumption than in the other populations studied and/or to the type of fish consumed. No studies have discriminated between consumption of lean v. fatty fish. A 4-week intervention study with herring (150 g/d, 5 d/week) in overweight and obese adults in Sweden (n 13) reported a trend towards lower CRP concentration compared with the concentration seen when the subjects consumed a reference diet, but the 30 % reduction in CRP was not significant⁽Reference Lindqvist, Langkilde and Undeland³⁷⁸⁾. The lack of statistical significance of the effect seen may be due to the small sample size.

Tea

A cross-sectional study from Japan reported no relationship between green tea consumption and the concentrations of several inflammatory markers⁽Reference Maki, Pham and Yoshida³⁷⁹⁾, while a cross-sectional study from Belgium detected significantly lower CRP and SAA concentrations in regular tea drinkers after multivariate analysis⁽Reference De Bacquer, Clays and Delanghe³⁸⁰⁾. The effect of tea consumption (green or black tea) has been investigated in several small intervention studies usually with a fairly short duration (Table 8)⁽Reference Basu, Du and Sanchez^381–Reference Widlansky, Duffy and Hamburg³⁸⁹⁾. Most studies indicate that tea consumption has no significant effect on the markers of inflammation reported⁽Reference de Maat, Pijl and Kluft^382–Reference Ryu, Lee and Lee^386, Reference Sung, Min and Lee^388, Reference Widlansky, Duffy and Hamburg³⁸⁹⁾. However, a longer supplementation period of 6 weeks with black tea in healthy non-smoking men reduced plasma CRP concentrations significantly⁽Reference De Bacquer, Clays and Delanghe^380, Reference Steptoe, Gibson and Vuononvirta³⁸⁷⁾. Overall, there are no consistent data that tea consumption (black or green) has a beneficial effect on inflammatory status, but studies of longer duration than those currently reported need to be performed.

Table 8 Intervention studies investigating the effect of black or green tea intake on markers of low-grade inflammation

M, male; F, female; = , no effect on; CRP, C-reactive protein; sICAM-1, soluble intercellular adhesion molecule-1; sVCAM-1, soluble vascular cell adhesion molecule-1; ↓ , decreased; SAA, serum amyloid A.

The modest or even absent effects of tea on inflammatory and oxidative stress markers in vivo are surprising in view of the potent inhibitory effects of tea components such as catechins on the expression of pro-inflammatory mediators in vitro ⁽Reference Khan, Afaq and Saleem^390–Reference Syed, Afaq and Kweon³⁹²⁾. This may be due to the fact that the majority of tea catechins undergo methylation, glucuronidation and sulfation during uptake which may limit bioavailability⁽Reference Feng³⁹³⁾. Furthermore, studies in vitro have often used catechin concentrations >10 μm, whereas plasma concentrations of catechins after ingestion of tea rarely exceed 1 μm⁽Reference Renouf, Guy and Marmet³⁹⁴⁾.

Coffee

Habitual coffee consumption was analysed for association with markers of low-grade inflammation in cross-sectional epidemiological studies which yield conflicting results (Table 9)⁽Reference Schulze, Hoffmann and Manson^304, Reference Hamer, Williams and Vuononvirta^395–Reference Zampelas, Panagiotakos and Pitsavos³⁹⁸⁾. Consumption of decaffeinated coffee was not associated with changes in systemic levels of soluble adhesion molecules, but CRP concentrations were lower in decaffeinated coffee drinkers in one of the two studies⁽Reference Lopez-Garcia, van Dam and Qi³⁹⁶⁾. The contradictory findings from observational studies may reflect the fact that coffee contains a mixture of bioactives with divergent effects on physiology⁽Reference Rodrigues and Klein^399–Reference Battram, Arthur and Weekes⁴⁰³⁾. A recent intervention study providing two doses of coffee (4 and 8 cups of filtered coffee/d) in the same individuals confirmed in part the observations from the cross-sectional studies. While IL-18 (decreased) and adiponectin (increased) were significantly modulated by coffee consumption, concentrations of CRP, leptin, SAA, IL-6, MIF and IL-1ra were not affected⁽Reference Kempf, Herder and Erlund⁴⁰⁴⁾. No dose–response relationship was seen in this study. Plasma caffeine concentrations were positively associated with plasma adiponectin concentrations⁽Reference Kempf, Herder and Erlund⁴⁰⁴⁾. Taken together, the available data do not allow a firm conclusion as to whether coffee consumption modulates low-grade inflammation.

Table 9 Observational studies on the association between coffee intake and markers of low-grade inflammation

M, male; F, female; ↑ , increased; CRP, C-reactive protein; SAA, serum amyloid A; = , no effect on; sE-selectin, soluble E-selectin; sICAM-1, soluble intercellular adhesion molecule-1; sTNFR, soluble TNF receptor; ↓ , decreased; sVCAM-1, soluble vascular cell adhesion molecule-1.

Cocoa

Cocoa has a high content of monomeric (epicatechin and catechin) and oligomeric (procyanidin) flavanols⁽Reference Adamson, Lazarus and Mitchell^405–Reference Natsume, Osakabe and Yamagishi⁴⁰⁷⁾. These latter polymeric fractions are present in higher concentrations/amounts in cocoa compared with other flavanol-rich foods such as red wine or green tea⁽Reference Lazarus, Adamson and Hammerstone^408, Reference Lazarus, Hammerstone and Schmitz⁴⁰⁹⁾. Thus, certain cocoa-based products are rich in flavanols⁽Reference Vinson, Jang and Dabbagh^410, Reference Hammerstone, Lazarus and Schmitz⁴¹¹⁾, some of which have been found in model systems to possess potential anti-inflammatory activities. However, the effect of cocoa flavanols and their related procyanidins appears to be related to the degree of polymerisation, and opposing effects on inflammatory cytokine production in vitro of low- and higher-degree of polymerisation flavanols have been reported⁽Reference Mao, Van de Water and Keen^412, Reference Mao, Powell and Van de Water⁴¹³⁾. Because of the focus on in vitro exploration of the effects of cocoa flavanols, there is a need for well-designed human studies, using cocoa properly characterised in terms of flavanol content. In an Italian study of over 10 000 people, of whom half were free of any chronic disease, 1317 people reported having eaten any chocolate during the past year; 824 ate chocolate regularly in the form of dark chocolate only⁽Reference di Giuseppe, Di Castelnuovo and Centritto⁴¹⁴⁾. Regular consumption of small doses of dark chocolate seemed to decrease inflammation: after adjustment for multiple confounders, dark chocolate consumption was inversely associated with CRP concentration. Serum CRP concentrations were 1·32 (1·26–1·39 mg/l) in chocolate non-consumers and 1·10 (1·03–1·17 mg/l) in consumers. A J-shaped relationship between dark chocolate consumption and serum CRP was observed; consumers of up to 1 serving (20 g) of dark chocolate every 3 d had significantly lower serum CRP concentrations than non-consumers or those consuming more than 20 g chocolate per d. In a small uncontrolled intervention study, healthy subjects (n 25) consumed dark chocolate (36·9 g/d) and a cocoa powder drink (30·95 g powder/d) for 6 weeks; there was no change in concentrations of CRP, TNF-α, IL-1β, IL-6 or soluble P-selectin (sP-selectin)⁽Reference Mathur, Devaraj and Grundy⁴¹⁵⁾. Another small (n 28), uncontrolled intervention study with dark chocolate for 1 week found a reduction in CRP concentration in women but not in men⁽Reference Hamed, Gambert and Bliden⁴¹⁶⁾. Most recently, Monagas et al. ⁽Reference Monagas, Khan and Andres-Lacueva⁴¹⁷⁾ reported the effect of a 4-week randomised cross-over trial of 40 g cocoa powder in skimmed milk daily v. skimmed milk in forty-two older subjects: serum concentrations of sICAM-1 and sP-selectin were lower after the cocoa powder intervention. Thus, there is some evidence that cocoa and cocoa-rich foods reduce low-grade inflammation.

Alcohol

The regular consumption of alcohol-containing beverages such as wine or beer has been reported to be inversely associated with several markers of low-grade inflammation, in a dose-dependent manner: i.e. moderate daily intake (1–2 drinks/d) was often found to be associated with decreased concentrations of inflammatory markers, although this was not the case in all studies (Table 10) ⁽Reference Albert, Glynn and Ridker^418–Reference Wannamethee, Lowe and Shaper⁴³²⁾.

Table 10 Observational studies on the association between alcohol intake and markers of low-grade inflammation

M, male; F, female; U, U-shaped relationship; CRP, C-reactive protein; ↑ , increased; = , no effect on; ↓ , decreased; sICAM-1, soluble intercellular adhesion molecule-1; SAA, serum amyloid A; iU, inverse U-shaped relationship; sTNFR, soluble TNF receptor; sVCAM-1, soluble vascular cell adhesion molecule-1; PAI-1, plasminogen activator inhibitor 1.

The short-term effect of alcohol intake on markers of low-grade inflammation has been studied in intervention trials which either determined acute effects during the next hours following ingestion or changes compared with baseline after 2–4 weeks of daily alcohol consumption (Table 11)⁽Reference Beulens, de Zoete and Kok^433–Reference Williams, Sutherland and Whelan⁴⁴⁸⁾. Although most studies were of small size, findings were quite consistent in that there was little change in low-grade inflammation in the hours following alcohol intake. In one study, a decrease in NF-κB activation was reported after consuming a fat-rich meal when red wine was also consumed compared with consuming the fat-rich meal⁽Reference Blanco-Colio, Valderrama and Alvarez-Sala⁴³⁸⁾. In general, findings are fairly similar for trials with wine, beer, vodka or ethanol (see Table 11). After 2–4 weeks of daily intake of alcoholic beverages, changes in low-grade inflammation were noted in five out of eight trials, with decreases in concentrations of CRP, cytokines and soluble adhesion molecules, and an increased concentration of adiponectin (see Table 11). These effects were seen in trials with white or red wine, or beer. The three ‘negative’ trials⁽Reference Beulens, de Zoete and Kok^433–Reference Watzl, Bub and Pretzer⁴³⁶⁾ used red wine or beer. Some variation of outcome between trials is to be expected, notably because the lifestyle and characteristics of participants will not be identical between trials, in particular with regard to the local diet and the amount of daily physical activity. Nevertheless, when taken together, these studies suggest some beneficial long-term effects of moderate daily consumption of wine or beer on low-grade inflammation. It is not clear whether such effects are to due to the alcohol content of the beverages tested, or to other components, such as phenolic compounds in these fermented beverages.

Table 11 Intervention studies investigating the effect of alcohol intake on markers of low-grade inflammation

M, male; F, female; ↓ , decreased; = , no effect on; sP-selectin, soluble P-selectin; ↑ , increased; CRP, C-reactive protein; sICAM-1, soluble intercellular adhesion molecule-1; sVCAM-1, soluble vascular cell adhesion molecule-1; TGF, transforming growth factor; MCP-1, monocyte chemoattractant protein-1; sCD40L, sCD40 ligand.

Animal studies

In spontaneously diabetic db/db mice⁽Reference Hofmann, Dong and Li⁵⁰⁷⁾ and in normal mice fed high-fat diets⁽Reference Sandu, Song and Cai⁵⁰⁸⁾, feeding of a low-AGE diet reduced serum AGE levels and improved insulin sensitivity, compared with a high-AGE diet. In addition, in non-obese diabetic mice, a fetal or neonatal low-AGE environment prevented autoimmune diabetes, possibly by reducing antigenic stimulus for T-cell-mediated injury or by attenuating β-cell damage⁽Reference Peppa, He and Hattori⁵⁰⁹⁾. In both type 1 and 2 diabetes models (i.e. non-obese diabetic and db/db mice, respectively) low dietary AGE content provided sustained protection towards the development of diabetic nephropathy⁽Reference Zheng, He and Cai⁵¹⁰⁾. Likewise, in genetically hypercholesterolaemic apoE^− / − mice, low AGE content in the diet attenuated the development of atherosclerosis after induction of diabetes with streptozotocin⁽Reference Lin, Choudhury and Cai⁵¹¹⁾ and neointimal formation after arterial injury⁽Reference Lin, Reis and Dore⁵¹²⁾. Finally, in db/db mice, wound healing was accelerated by a low-AGE diet, with predominant wound contraction, compared with animals on a high-AGE diet, which favoured the epithelialisation mode of wound closure⁽Reference Peppa, Brem and Ehrlich⁵¹³⁾.

Human studies

In healthy younger (aged < 45 years) and older (aged >60 years) human subjects, dietary AGE, but not energy intake, correlated with serum AGE levels and markers of oxidative stress (8-isoprostanes), which in turn correlated with serum CRP concentration and insulin resistance⁽Reference Negrean, Stirban and Stratmann^514–Reference Vlassara, Cai and Crandall⁵²¹⁾ (Table 12). It is noteworthy that in this study, some healthy individuals showed AGE levels of similar magnitude as found in diabetic patients.

Table 12 Studies on the association between advanced glycation end products (AGE) intake and markers of low-grade inflammation

M, male; F, female; ↑ , increased; CRP, C-reactive protein; sVCAM-1, soluble vascular cell adhesion molecule-1; = , no effect on; PAI-1, plasminogen activator inhibitor-1; sICAM-1, soluble intercellular adhesion molecule-1; ↓ , decreased.

AGE-rich diets used for intervention studies in human subjects are usually generated by more severe thermal treatment of food, particularly by frying, broiling and baking, whereas AGE-poor food is prepared by steaming or simply without heating at all. Therefore, unless specified, these are the methods that were used for the preparation of test and control diets in the studies reviewed below. Taken together, these studies show that dietary AGE content correlates with circulating levels of AGE and with markers of inflammation, oxidative stress, endothelial dysfunction and renal function. One caveat that needs attention for future human studies is the significant contribution of smoking to the endogenous AGE load⁽Reference Cerami, Founds and Nicholl^462, Reference Venkatesan, Hemalatha and Bobby⁵²²⁾.

Diabetic subjects with and without nephropathy were given a single meal of egg-white, cooked with or without fructose, with the AGE-rich food containing about three times more AGE than a single meal of a regular diet⁽Reference Koschinsky, He and Mitsuhashi⁴⁵⁶⁾. There was a significant correlation between dietary AGE content and serum AGE levels. Moreover, the serum AGE levels were directly related to the degree of renal dysfunction, as estimated by the increase in albuminuria and the decrease in creatinine clearance, and renal AGE excretion was inversely correlated with the degree of renal dysfunction. The relationship between dietary AGE intake, serum AGE levels and renal AGE clearance was confirmed in a cross-sectional study performed in long-term haemodialysis and peritoneal dialysis patients with or without diabetes. This study showed that circulating AGE correlated significantly with the AGE content of a regular diet, as estimated by means of 3 d dietary records and food questionnaires, as well as with blood urea N level⁽Reference Uribarri, Peppa and Cai⁴⁵⁹⁾. Likewise, in non-diabetic patients on maintenance peritoneal dialysis randomly assigned to two groups consuming either a high- or low-AGE diet for 4 weeks, serum AGE levels correlated with AGE consumption as well as with blood urea N, serum creatinine, total protein, albumin and P, with AGE restriction profoundly reducing circulating AGE⁽Reference Uribarri, Peppa and Cai⁵²³⁾. Finally, cross-sectional studies of healthy subjects and of patients undergoing haemodialysis showed a positive relationship between dietary AGE intake and plasma inflammatory markers⁽Reference Peppa, Uribarri and Cai^515, Reference Uribarri, Cai and Peppa⁵²⁰⁾.

A number of acute studies examining the effect of a single high-AGE meal on inflammatory markers have been conducted (see Table 12). Although these have been conducted in a variety of subject/patient groups, they show rather similar effects: typically, there is an elevation of various inflammatory markers (CRP, cytokines and soluble adhesion molecules) in the hours after consumption of a high-AGE meal, and this increase does not occur with a low-AGE meal (see Table 12). Furthermore, healthy and diabetic volunteers receiving a single dose of a high-AGE (caffeine-free) cola drink showed an increase in serum PAI-1⁽Reference Uribarri, Stirban and Sander⁵¹⁹⁾.

In patients on haemodialysis or peritoneal dialysis randomly assigned to a 4-week high- or low-AGE diet, circulating levels of AGE as well as markers of inflammation and endothelial dysfunction (which were markedly elevated at baseline, although not correlated with AGE) decreased significantly in response to AGE restriction, i.e. the low-AGE diet⁽Reference Peppa, Uribarri and Cai⁵¹⁵⁾. The findings of this latter study are in accordance with earlier data in diabetic subjects with normal renal function who showed a decrease in several inflammatory markers following 2 or 6 weeks of a low-AGE diet⁽Reference Vlassara, Cai and Crandall⁵²¹⁾.

SFA

In vitro studies have suggested that SFA may promote inflammatory processes. Exposure of myotubes or adipocytes to the SFA palmitic acid (16 : 0) increased IL-6 mRNA expression and subsequent protein production, possibly via activation of NF-κB⁽Reference Weigert, Brodbeck and Staiger^572, Reference Ajuwon and Spurlock⁵⁷³⁾. Monocytes are activated directly by SFA, especially lauric acid (12 : 0), via TLR-4 and through this mechanism induce NF-κB activity⁽Reference Lee, Plakidas and Lee^574, Reference Weatherill, Lee and Zhao⁵⁷⁵⁾. A limited number of observational studies have investigated the relationship between SFA exposure and circulating markers of inflammation (Table 13)⁽Reference Lopez-Garcia, Schulze and Manson^373, Reference Madsen, Skou and Hansen^374, Reference Eschen, Christensen and Toft^576–Reference Yli-Jama, Seljeflot and Meyer⁵⁸⁵⁾. Fernandez-Real et al. ⁽Reference Fernandez-Real, Broch and Vendrell⁵⁷⁷⁾ did not see any relationship between serum SFA and CRP or IL-6 in lean individuals, while in overweight individuals, serum SFA were positively associated with IL-6 concentration, and the ratio of SFA:n-6 or n-3 PUFA was positively associated with IL-6 and CRP concentrations, respectively. A study in overweight adolescents showed positive relationships between total SFA in plasma phospholipids or cholesteryl esters and IL-6, but not CRP, concentration⁽Reference Klein-Platat, Drai and Oujaa⁵⁷⁹⁾. Thus, there is general agreement between two studies in overweight subjects that SFA exposure is associated with higher IL-6 concentration. A study in lean individuals does not show this. An intervention study feeding diets rich in stearic acid (18 : 0) or in lauric, myristic (14 : 0) and palmitic acids to men aged 25–60 years for 5 weeks showed higher concentrations of CRP, fibrinogen, IL-6 and sE-selectin compared with a diet enriched in oleic acid (18 : 1n-9)⁽Reference Baer, Judd and Clevidence⁵⁸⁶⁾. There are few other intervention studies chronically increasing SFA intake in human subjects and reporting inflammatory markers.

Table 13 Observational studies on the association between fatty acid intake or status and markers of low-grade inflammation

M, male; F, female; αLNA, α-linolenic acid; = , no effect on; CRP, C-reactive protein; ↓ , decreased; sICAM-1, soluble intercellular adhesion molecule-1; sVCAM-1, soluble vascular cell adhesion molecule-1; ↑ , increased; sTNFR, soluble TNF receptor; sP-selectin, soluble P-selectin; IL-1ra, IL-1 receptor antagonist; TGF, transforming growth factor.

Trans-fatty acids

Using a subgroup of the Nurses’ Health Study, Lopez-Garcia et al. ⁽Reference Lopez-Garcia, Schulze and Meigs⁵⁸⁰⁾ identified significant positive associations between the intake of trans-fatty acids in the diet and the concentrations of all six inflammatory markers assessed, including CRP, IL-6 and three soluble adhesion molecules. In a 5-week intervention in healthy men, a trans-fatty acid-enriched diet resulted in higher CRP and IL-6 concentrations than diets rich in oleic acid, stearic acid or the combination of lauric, myristic and palmitic acids⁽Reference Baer, Judd and Clevidence⁵⁸⁶⁾. Furthermore, the concentration of sE-selectin was higher than in all other dietary groups including the stearic acid and lauric+myristic+palmitic groups. Thus, an association study and an intervention study both demonstrate that dietary trans-fatty acids elevate the concentrations of a range of inflammatory markers including CRP, IL-6 and adhesion molecules.

Conjugated linoleic acids

In vitro and animal feeding studies have suggested marked effects of conjugated linoleic acids (CLA) on inflammation⁽Reference Li, Barnes and Butz^587–Reference Moloney, Toomey and Noone⁵⁸⁹⁾. However, results from human intervention studies using CLA-rich capsules are equivocal (Table 14)⁽Reference Mullen, Moloney and Nugent^590–Reference Tricon, Burdge and Kew⁵⁹⁶⁾. For example, two studies demonstrate that CLA, especially the trans-10, cis-12 isomer, increases CRP concentration, but not the concentrations of cytokines or soluble adhesion molecules⁽Reference Riserus, Basu and Jovinge^593, Reference Smedman, Basu and Jovinge⁵⁹⁴⁾. However, at least four other studies have failed to show an effect of CLA on CRP concentration (see Table 14). Duration of intervention is not likely to be an explanatory factor for differences in the findings, but, since most studies have used mixtures of CLA isomers in different proportions, the precise dose of the more potent isomer (perhaps the trans-10, cis-12 isomer) may be an explanation. The two studies showing increased CRP with CLA provided 2·1⁽Reference Smedman, Basu and Jovinge⁵⁹⁴⁾ and 2·7⁽Reference Riserus, Basu and Jovinge⁵⁹³⁾ g/d of trans-10, cis-12 CLA. The four studies showing no effect of CLA on CRP used between 0·4 and 2·5 g/d of this isomer (see Table 14). The question of whether CLA per se, or whether specific CLA isomers, increase low-grade inflammation requires further study.

Table 14 Intervention studies investigating the effect of conjugated linoleic acid intake on markers of low-grade inflammation

M, male; CLA, conjugated linoleic acid; c, cis; t, trans; ↑ , increased; CRP, C-reactive protein; = , no effect on; F, female; sICAM-1, soluble intercellular adhesion molecule-1; LT, leukotriene; sVCAM-1, soluble vascular cell adhesion molecule-1; sTNFR, soluble TNF receptor.

Linoleic and α-linolenic acids

Linoleic (18 : 2n-6) and α-linolenic (18 : 3n-3) acids constitute the majority (>95 %) of PUFA in most Western diets, with the former usually being present in an excess of approximately 10-fold over the latter. These two fatty acids are the parent n-6 and n-3 PUFA, respectively. Because of the role of linoleic acid as the precursor of arachidonic acid (20 : 4n-6), which is, in turn, the substrate for the synthesis of pro-inflammatory eicosanoids such as PGE₂ and 4-series leukotrienes, it is widely considered that elevated n-6 and low n-3 fatty acids (i.e. a high n-6:n-3 PUFA ratio) in the diet will promote inflammation. However, available evidence does not support this contention.

Dietary intakes of linoleic acid were not associated with CRP, IL-6, sTNFR1 or sTNFR2 concentrations in subgroups of the Physicians’ Health Study and the Nurses’ Health Study⁽Reference Pischon, Hankinson and Hotamisligil⁵⁸³⁾. The concentration of linoleic acid in blood lipids⁽Reference Ferrucci, Cherubini and Bandinelli^578, Reference Klein-Platat, Drai and Oujaa⁵⁷⁹⁾ or granulocytes⁽Reference Madsen, Skou and Hansen³⁷⁴⁾ was not associated with CRP or IL-6 concentration, although a large Swedish study reported an inverse association between linoleic acid in cholesteryl esters and CRP concentration⁽Reference Petersson, Basu and Cederholm⁵⁸²⁾. Linoleic acid makes a major contribution to the fatty acids within blood lipids, especially cholesteryl esters, where it is the predominant n-6 PUFA. Total n-6 PUFA in serum fatty acids in overweight, but not in lean, subjects were inversely associated with IL-6 but not CRP concentration⁽Reference Fernandez-Real, Broch and Vendrell⁵⁷⁷⁾. The ratio of SFA:n-6 PUFA in serum lipids or in plasma phospholipids was positively associated with IL-6 but not CRP concentration in overweight subjects⁽Reference Fernandez-Real, Broch and Vendrell^577, Reference Klein-Platat, Drai and Oujaa⁵⁷⁹⁾. This suggests that decreasing SFA status while increasing n-6 PUFA status might reduce low-grade inflammation. An intervention study in thirty-eight healthy male and female volunteers (mean age 27 years) compared diets with 18 % energy from oleic acid and 4 % from linoleic acid or 4 % energy from oleic acid and 12 % from linoleic acid; plasma sICAM-1 concentrations were not different between these two groups⁽Reference Turpeinen, Basu and Mutanen⁵⁹⁷⁾, suggesting that exchange of oleic acid for linoleic acid, while keeping SFA intake constant, does not affect this marker of inflammation.

Dietary α-linolenic acid intakes were not associated with CRP, IL-6, sTNFR1 or sTNFR2 concentrations in one study on subgroups of the Physicians’ Health Study and the Nurses’ Health Study⁽Reference Pischon, Hankinson and Hotamisligil⁵⁸³⁾. In a second study on another subgroup of the Nurses’ Health Study, α-linolenic acid intakes were not associated with CRP, sICAM-1, sE-selectin or sTNFR2 concentrations but were associated with lower IL-6 and sVCAM-1 concentrations⁽Reference Lopez-Garcia, Schulze and Manson³⁷³⁾. The concentration of α-linolenic acid in blood lipids⁽Reference Ferrucci, Cherubini and Bandinelli^578, Reference Klein-Platat, Drai and Oujaa^579, Reference Petersson, Basu and Cederholm⁵⁸²⁾ or granulocytes⁽Reference Madsen, Skou and Hansen³⁷⁴⁾ was not associated with CRP or IL-6 concentration. Data from an Italian study showed no association between α-linolenic acid in plasma fatty acids and several cytokines including IL-6 and TNF-α, but there was an inverse association with CRP⁽Reference Ferrucci, Cherubini and Bandinelli⁵⁷⁸⁾. These association studies suggest a modest anti-inflammatory effect of α-linolenic acid.

Several intervention studies have involved high α-linolenic acid intakes usually by providing flaxseed oil in capsules or in liquid form or foodstuffs made using flaxseed oil⁽Reference Burdge and Calder⁵⁹⁸⁾. Frequently, these studies have used a control group with a high intake of linoleic acid, with the comparison essentially being replacement of linoleic acid with α-linolenic acid. Thus, the focus of these studies is essentially to explore the importance of the n-6:n-3 PUFA ratio of the diet. If linoleic and α-linolenic acids have similar effects on low-grade inflammation, then studies exchanging one of these fatty acids with the other would be likely to see little effect. Table 15 summarises intervention studies with α-linolenic acid that have measured markers of low-grade inflammation as an outcome. Findings are inconsistent with a number of studies identifying the effects of α-linolenic acid on some markers and not others, and some studies finding no effects (Table 15)⁽Reference Bemelmans, Lefrandt and Feskens^599–Reference Zhao, Etherton and Martin⁶⁰⁶⁾. Many studies have used very high intakes of α-linolenic acid, relative to typical habitual intakes. The study of Paschos et al. ⁽Reference Paschos, Rallidis and Liakos⁶⁰¹⁾ is enlightening because it showed that α-linolenic acid (8·1 g/d for 12 weeks) was less effective (i.e. induced fewer and smaller effects) against a more healthy, than against a less healthy, background diet. Thus, dose, duration, sample size and the nature of the background diet are possible contributors to the varied findings of studies with α-linolenic acid. However, what is apparent from these observations is that a substantial increase in the intake of α-linolenic acid can decrease low-grade inflammation as indicated by circulating CRP, IL-6 or soluble adhesion molecules (see Table 15). The study of Zhao et al. ⁽Reference Zhao, Etherton and Martin⁶⁰⁶⁾ provides further insight: the change in CRP and sVCAM-1 concentrations was correlated with the change in the concentrations of different serum n-3 PUFA (α-linolenic acid, EPA, docosapentaenoic acid (22 : 5n-6) and DHA) that had occurred. The only significant relationships were inverse and between the change in EPA status and the changes in CRP and sVCAM-1 concentrations. This suggests that the anti-inflammatory effect seen with the very high intake of α-linolenic acid in this study is due to the conversion of α-linolenic acid to EPA. A suitable conclusion may be that high intakes of α-linolenic are anti-inflammatory acting via the α-linolenic acid derivative, EPA.

Table 15 Intervention studies investigating the effect of α-linolenic acid (αLNA) intake on markers of low-grade inflammation

M, male; F, female; = , no effect on; CRP, C-reactive protein; sL-selectin, soluble L-selectin; sP-selectin, soluble P-selectin; PAI, plasminogen activator inhibitor; sICAM-1, soluble intercellular adhesion molecule-1; ↓ , decreased; sVCAM-1, soluble vascular cell adhesion molecule-1; SAA, serum amyloid A; MCSF, macrophage colony-stimulating factor.

Several of the intervention studies with α-linolenic acid have involved a group consuming a high intake of linoleic acid, frequently as the control for the high α-linolenic acid intake. These groups provide some information about the impact of linoleic acid on low-grade inflammation. The study of Rallidis et al. ⁽Reference Rallidis, Paschos and Liakos^603, Reference Rallidis, Paschos and Papaioannou⁶⁰⁴⁾ involved a control group consuming about 11 g/d of linoleic acid in addition to the dietary intake; this approximately doubled linoleic acid intake. The increase in linoleic acid intake did not alter the concentrations of CRP, IL-6, SAA, sICAM-1 or sE-selectin, but the concentration of sVCAM-1 was decreased⁽Reference Rallidis, Paschos and Liakos^603, Reference Rallidis, Paschos and Papaioannou⁶⁰⁴⁾. In the study of Paschos et al. ⁽Reference Paschos, Zampelas and Panagiotakos⁶⁰²⁾, the control group consumed 11·2 g/d of linoleic acid on top of their normal intake, which was approximately 11 g/d. This doubling of linoleic acid intake did not affect TNF-α or adiponectin concentrations. These studies did not alter any aspect of diet but required subjects to consume oil providing linoleic or α-linolenic acids on top of the normal diet. Thus, these studies show that markedly increasing linoleic acid intake in those consuming on average about 10 g/d does not increase low-grade inflammation.

The study of Zhao et al. ⁽Reference Zhao, Etherton and Martin⁶⁰⁶⁾ included a high linoleic acid intervention group (13 % energy) and the control group consumed what is described as an ‘average American diet’. This provided 12·7 % of energy from SFA, 7·7 % from linoleic acid and 0·8 % from α-linolenic acid. The corresponding values for the linoleic and α-linolenic acid rich diets were 8·5, 12·6 and 3·6 and 8·2, 10·5 and 6·5, respectively. Thus, the linoleic acid-rich diet contained more α-linolenic acid than the control diet, while the α-linolenic acid diet contained more linoleic acid than the control diet and almost as much linoleic acid as the linoleic acid diet. Essentially, what this means is that the linoleic acid diet is examining the effect of replacing some SFA with the combination of linoleic and α-linolenic acids and that the α-linolenic acid diet is examining the effect of replacing some linoleic acid with α-linolenic acid. In the linoleic acid group, the concentrations of CRP and sICAM-1 decreased by 45 and 25 %, respectively; sVCAM-1 and sE-selectin also fell (by 15 and 7 %) but these changes were not significant. Thus, replacing about one-third of SFA with linoleic acid plus α-linolenic acid decreases low-grade inflammation. This is consistent with the associations described above and confirms that relative to SFA, the combination of linoleic and α-linolenic acids is anti-inflammatory. The changes in inflammatory markers seen with the α-linolenic acid diet were greater than with the linoleic acid diet (75, 80, 37·5 and 12 % decreases in CRP, sVCAM-1, sICAM-1 and sE-selectin concentrations, respectively). Since this diet is effectively replacing some linoleic acid with α-linolenic acid, relative to the amounts in the linoleic acid diet, these results suggest that α-linolenic acid is more potent than linoleic acid with regard to reducing inflammation.

The studies just described have focused on increasing intake of linoleic and α-linolenic acids to study their effects. The study of Liou et al. ⁽Reference Liou, King and Zibrik⁶⁰⁷⁾ used a different approach. They kept the amount of α-linolenic acid in the diet of men aged 20–45 years constant over the study period at about 1 % of energy. Linoleic acid intake was either 10·5 or 3·8 % of energy. Saturated fat was also constant across the two diets at about 8 % of energy. The manipulation of linoleic acid was at the expense of oleic acid (17 or 10 % of energy). The intervention duration was 4 weeks and a random order, cross-over design was used. CRP and IL-6 concentrations were not different after 4 weeks on either diet. This finding is in accordance with that of Turpeinen et al. ⁽Reference Turpeinen, Basu and Mutanen⁵⁹⁷⁾.

Arachidonic acid

Arachidonic acid intake in the diet is low relative to that of its metabolic precursor linoleic acid (approximately 500 mg/d v. approximately 11 g/d, respectively). Nevertheless, arachidonic acid is the most prevalent n-6 PUFA and PUFA in the membranes of inflammatory cells and other cells that might be involved in low-grade inflammation such as endothelial cells and platelets. This reflects the important functional role of arachidonic acid as a precursor of the eicosanoid family of lipid mediators; this family includes the 2-series PG and the 4-series leukotrienes. Since these eicosanoids are classically associated with inflammatory processes and are targeted by common anti-inflammatory therapies, it is generally considered that arachidonic acid will enhance inflammation. However, observations that classical pro-inflammatory mediators such as PGE₂ can also exert anti-inflammatory effects and that arachidonic acid gives rise to anti-inflammatory mediators such as lipoxin A₄ have started to challenge the earlier view⁽Reference Calder⁶⁰⁸⁾. Several studies have examined the association between arachidonic acid status and markers of low-grade inflammation. There was no association between arachidonic acid in granulocytes and CRP concentration⁽Reference Madsen, Skou and Hansen³⁷⁴⁾. Serum free arachidonic acid was not associated with sICAM-1 or sE-selectin concentrations and was actually inversely associated with sVCAM-1 concentration⁽Reference Yli-Jama, Seljeflot and Meyer⁵⁸⁵⁾. Ferrucci et al. ⁽Reference Ferrucci, Cherubini and Bandinelli⁵⁷⁸⁾ reported no association between arachidonic acid in plasma and CRP, TNF-α, IL-1β, IL-10 and sIL-6R concentrations, while there was an inverse association with IL-6 and IL-1ra concentrations and a positive association with TGF-β concentration. These observations suggest either that plasma arachidonic acid has little impact on low-grade inflammation (does not affect CRP or TNF-α) or that it is anti-inflammatory (lowers IL-6; increases TGF-β).

There are very few intervention studies with arachidonic acid reporting on low-grade inflammation. In an uncontrolled study, Kelley et al. ⁽Reference Kelley, Taylor and Nelson⁶⁰⁹⁾ reported higher granulocyte numbers in the blood of a group of ten healthy men (aged 20–38 years) taking a supplement of 1·5 g/d of arachidonic acid for 100 d compared with numbers after a run-in diet providing 200 mg/d of arachidonic acid. In another small, but controlled, study, eight subjects aged 55–75 years consumed capsules providing 700 mg/d of arachidonic acid for 12 weeks⁽Reference Thies, Miles and Nebe-von-Caron⁶⁰⁵⁾; there was no effect on plasma sVCAM-1, sICAM-1 or sE-selectin concentrations.

Marine-derived long-chain n-3 PUFA

The long-chain n-3 PUFA EPA and DHA are found in seafood, especially oily fish. They are also present in fish oils and in certain algal oils; in some preparations, the fatty acids are in a more concentrated form than in natural fish oils. In fish oils, the fatty acids are in the TAG form, but other forms of long-chain n-3 PUFA are also available, for example, as phospholipids or ethyl esters. Increased intake of long-chain n-3 PUFA results in increased proportions of those fatty acids in inflammatory cell phospholipids⁽Reference Yaqoob, Pala and Cortina-Borja^610–Reference Calder⁶¹⁴⁾. The incorporation of EPA and DHA into human inflammatory cells is partly at the expense of arachidonic acid, resulting in less substrate available for the synthesis of the classic inflammatory eicosanoids such as PGE₂. Through altered eicosanoid production, n-3 PUFA could affect inflammation and inflammatory processes, although they also exert non-eicosanoid-mediated actions on cell signalling and gene expression. The effects of long-chain n-3 PUFA have been examined in many model systems and findings from cell-culture systems and from animal models are generally consistent in identifying anti-inflammatory actions⁽Reference Calder⁵⁶⁹⁾. Furthermore, clinical trials have demonstrated anti-inflammatory effects and clinical benefit from fish oil administration in diseases with a frank inflammatory basis including rheumatoid arthritis⁽Reference Calder⁶¹⁵⁾, inflammatory bowel diseases⁽Reference Calder⁶¹⁶⁾ and childhood asthma⁽Reference Kremmyda, Vlachava and Noakes⁶¹⁷⁾.

Data from subgroups of the Physicians’ Health Study and the Nurses’ Health Study showed inverse associations between the dietary intake of EPA+DHA and concentrations of CRP, sTNFR1 and sTNFR2⁽Reference Pischon, Hankinson and Hotamisligil⁵⁸³⁾, and CRP, sICAM-1, sVCAM-1 and sE-selectin⁽Reference Lopez-Garcia, Schulze and Manson³⁷³⁾. The concentration of either EPA or DHA in granulocyte membranes was inversely associated with CRP concentration in one study⁽Reference Madsen, Skou and Hansen³⁷⁴⁾; the effect of DHA was stronger than that of EPA. Serum non-esterified EPA and DHA were both inversely associated with concentrations of sVCAM-1 and sICAM-1 in patients at risk of CHD⁽Reference Yli-Jama, Seljeflot and Meyer⁵⁸⁵⁾; EPA was also inversely associated with sE-selectin concentration. Plasma cholesteryl ester EPA was inversely associated with CRP concentration in overweight subjects⁽Reference Klein-Platat, Drai and Oujaa⁵⁷⁹⁾. In an elderly Italian population, plasma EPA was inversely associated with IL-6 concentration and positively associated with the concentrations of the anti-inflammatory cytokines IL-10 and TGF-β⁽Reference Ferrucci, Cherubini and Bandinelli⁵⁷⁸⁾. Furthermore, plasma DHA was inversely associated with IL-6 and TNF-α concentrations and was also positively associated with the concentrations of IL-10 and TGF-β⁽Reference Ferrucci, Cherubini and Bandinelli⁵⁷⁸⁾. Thus, observational studies suggest that both EPA and DHA are anti-inflammatory.

The ready availability of fish oil capsules has facilitated numerous supplementation studies of long-chain n-3 PUFA in various subject groups; these are summarised in Table 16⁽Reference Jellema, Plat and Mensink^164, Reference Thies, Miles and Nebe-von-Caron^605, Reference Abe, El-Masri and Kimball^618–Reference Yusof, Miles and Calder⁶⁴⁶⁾. Studies have shown that long-chain n-3 PUFA lower the concentrations of CRP⁽Reference Browning, Krebs and Moore^621, Reference Ciubotaru, Lee and Wander^624, Reference Rasic-Milutinovic, Perunicic and Pljesa⁶³⁹⁾, IL-6⁽Reference Browning, Krebs and Moore^621, Reference Ciubotaru, Lee and Wander^624, Reference Rasic-Milutinovic, Perunicic and Pljesa⁶³⁹⁾, TNF-α⁽Reference Rasic-Milutinovic, Perunicic and Pljesa⁶³⁹⁾, IL-18⁽Reference Troseid, Arnesen and Hjerkinn⁶⁴³⁾, sICAM-1⁽Reference Eschen, Christensen and De^625, Reference Hjerkinn, Seljeflot and Ellingsen^628, Reference Yamada, Yoshida and Nakano^645, Reference Yusof, Miles and Calder⁶⁴⁶⁾, sVCAM-1⁽Reference Thies, Miles and Nebe-von-Caron^605, Reference Yamada, Yoshida and Nakano⁶⁴⁵⁾ and sE-selectin⁽Reference Abe, El-Masri and Kimball⁶¹⁸⁾ in various subject/patient groups (see Table 16). For example, one study showed an increase in adiponectin concentration when a weight-loss programme and 4·2 g/d EPA+DHA were combined in overweight, insulin-resistant women⁽Reference Krebs, Browning and McLean⁶³²⁾. Thus, there is quite a lot of evidence for anti-inflammatory effects of supplemental long-chain n-3 PUFA. In a group of overweight women with type 2 diabetes, 8 weeks of a moderate dose of long-chain n-3 PUFA (1·8 g/d of EPA+DHA) decreased adiposity and reduced expression of a number of inflammation-related genes in the subcutaneous adipose tissue⁽Reference Kabir, Skurnik and Naour⁶³⁰⁾. The parallelism between the down-regulation of these genes and the reduction in adiposity and adipocyte diameter by n-3 PUFA treatment suggests a positive relationship between adipose cell size and adipose tissue inflammation, agreeing with other observations⁽Reference Cancello, Henegar and Viguerie^31, Reference Weisberg, McCann and Desai⁶⁶⁾. Also these findings in type 2 diabetic women are paralleled by those from rodent studies: 6 weeks of n-3 PUFA supplementation prevented adipose tissue inflammation induced by a high-fat diet⁽Reference Todoric, Loffler and Huber⁶⁴⁷⁾, and the presence of n-3 PUFA in the diet of long-term insulin-resistant, sucrose-fed rats decreased adipocyte diameter⁽Reference Rossi, Lombardo and Lacorte⁶⁴⁸⁾ and significantly reduced several inflammation-related genes (unpublished results, Guerre-Millo M, Naour N, Lombardo Y, Clement K, Rizkalla S). These studies suggest that reducing adiposity with n-3 PUFA could decrease adipose tissue inflammation and macrophage infiltration. The beneficial effects of n-3 PUFA may be linked to local blunting of adipose tissue inflammation.

Table 16 Intervention studies investigating the effect of marine n-3 PUFA intake on markers of low-grade inflammation

M, male; FO, fish oil; = , no effect on; IL-1ra, IL-1 receptor antagonist; F, female; EE, ethyl ester; sICAM-1, soluble intercellular adhesion molecule-1; sVCAM-1, soluble vascular cell adhesion molecule-1; ↓ , decreased; sE-selectin, soluble E-selectin; ↑ , increased; sP-selectin, soluble P-selectin; PAI-1, plasminogen activator inhibitor-1; CRP, C-reactive protein; sTNFR, soluble TNF receptor; SAA, serum amyloid A.

Despite the large number of positive studies with long-chain n-3 PUFA, there are a number of studies that have failed to replicate these findings (see Table 16). Furthermore, two early studies showed an enhancement of selected inflammatory markers following long-chain n-3 PUFA administration⁽Reference Johansen, Seljeflot and Hostmark^629, Reference Seljeflot, Arnesen and Brude⁶⁴²⁾. Seljeflot et al. provided 4·8 g/d of EPA+DHA in ethyl ester form to hyperlipidaemic male smokers for 6 weeks, while Johansen et al. provided 5·1 g/d of EPA+DHA in ethyl ester form to patients with CHD for 24 weeks. Both studies identified an increase in sVCAM-1 ( < 10 %) and sE-selectin (20 %) concentrations, with no effect on sP-selectin and a decrease in von Willibrand factor and thrombomodulin concentrations. In both cases, the authors ascribed the effect of EPA+DHA on sVCAM-1 and sE-selectin to increased oxidant stress in these subjects.

Thus, although the overwhelming view is that EPA+DHA given at sufficient doses are anti-inflammatory, the evidence from measurements of markers of low-grade inflammation is not entirely consistent. The lack of consistency may be related to differences in: duration of treatment; sample size; characteristics of the populations studied (e.g. age, healthy v. diseased, type of disease, smokers v. non-smokers); background diet; dose of EPA+DHA used; relative contribution of EPA and DHA, since they may have different anti-inflammatory potencies; chemical formulation (e.g. TAG v. ethyl ester); degree of oxidative stress present. One other factor that has been recently identified is genetic differences among individuals, which may have an impact on the ability of n-3 PUFA to exert an anti-inflammatory effect. This was first identified by Grimble et al. ⁽Reference Grimble, Howell and O'Reilly⁶⁴⁹⁾ who showed that the ability of fish oil to lower the LPS-stimulated production of TNF-α by blood mononuclear cells was determined in part by polymorphisms within the TNF-α and TNF-β genes. Another example of such an interaction was identified by Shen et al. ⁽Reference Shen, Arnett and Peacock⁶⁵⁰⁾. They first identified that the IL-1β 6054 G>A SNP was significantly associated with CRP and adiponectin concentrations and with the prevalence of the metabolic syndrome among a group of 1120 men and women with a mean age of 49 years. There was also a significant interaction between this polymorphism and erythrocyte membrane n-3 PUFA content. Among subjects with low erythrocyte n-3 PUFA content (below the median), the 6054 G allele was associated with increased risk of the metabolic syndrome (OR 3·29, 95 % CI 1·49, 7·26 for GG and OR 1·95, 95 % CI 0·85, 4·46 for GA) compared with the AA genotype, but there were no significant genotype associations among subjects with high erythrocyte n-3 PUFA content (above the median). The results suggest that IL-1β genetic variants are associated with measures of chronic low-grade inflammation and the risk of the metabolic syndrome, and that genetic influences were more evident among subjects with low erythrocyte n-3 PUFA status and so, most probably low n-3 PUFA intake.

Acute postprandial effects of hyperglycaemia

Hyperglycaemia, elevated TAG/NEFA and hyperinsulinaemia are events that are mutually inherent to the initial development of a diabetic state. The effects of the individual events are difficult to study since they always present together. However, some observations have suggested that hyperglycaemia and hypertriacylglycerolaemia/elevated NEFA levels induce independent effects and, when present together, act synergistically to generate oxidative stress, inflammation, impaired endothelial function and vascular disease⁽Reference Ceriello, Quagliaro and Piconi^651–Reference Kempf, Rose and Herder⁶⁵³⁾. Oxidative stress is known to induce damage to cell membranes, internal cell structures and DNA as well as to induce inflammatory responses (Fig. 8) ⁽Reference Giugliano, Ceriello and Esposito⁶⁵⁴⁾. Ceriello et al. ⁽Reference Ceriello, Taboga and Tonutti⁶⁵²⁾ observed that after meals LDL oxidation increases and that this phenomenon is correlated with the degree of hyperglycaemia. This clearly points to hyperglycaemia-induced free radical production that has an impact on a broad range of metabolic events. NO, a potent vasodilator, is assumed to play a key role in this respect. It has been suggested that hyperglycaemia leads to an increased oxidation of NO, thereby reducing NO levels which leads to impairment in vasodilatation⁽Reference Davidson and Duchen⁶⁵⁵⁾. Indeed, alloxan diabetic rats are observed to have reduced NO levels in blood⁽Reference Mohan and Das⁶⁵⁶⁾, and in human subjects, acute hyperglycaemia attenuated endothelium-dependent vasodilation⁽Reference Williams, Goldfine and Timimi⁶⁵⁷⁾. Postprandial hyperglycaemia is also correlated with impaired myocardial perfusion in diabetic patients⁽Reference Scognamiglio, Negut and De Kreutzenberg⁶⁵⁸⁾. The fact that such a decrease can be prevented by supplying various antioxidants such as vitamin C, vitamin E and α-lipoic acid⁽Reference Jacob, Ruus and Hermann^659–Reference Evans, Goldfine and Maddux⁶⁶²⁾ or l-arginine⁽Reference Mohan and Das^656, Reference Giugliano, Marfella and Coppola⁶⁶³⁾, the precursor of NO, suggests that increased NO oxidation and the related NO drain have an impact on cells of the vasculature. This is supported by other evidence. Restoration of NO availability results in normalisation of endothelial function as well as insulin sensitivity⁽Reference Jacob, Ruus and Hermann^659, Reference Caballero⁶⁶⁰⁾. During an oral glucose tolerance test, plasma antioxidant status, measured as total plasma radical trapping capacity, significantly decreased in normal as well as in diabetic individuals. The consumption of a glycaemic meal increases oxidative stress and reduces antioxidant defences, with the increase being significantly greater with higher levels of hyperglycaemia⁽Reference Ceriello, Bortolotti and Motz^169, Reference Ceriello, Bortolotti and Motz^664, Reference Ursini, Zamburlini and Cazzolato⁶⁶⁵⁾. Acute post-meal hyperglycaemia was observed to induce the formation of nitrotyrosine, a marker of oxidative stress, in healthy, non-overweight individuals⁽Reference Marfella, Quagliaro and Nappo⁶⁶⁶⁾. It has been hypothesised that acute hyperglycaemia may induce a drain of vitamin C from cells because vitamin C and glucose share a common transport system⁽Reference Price, Price and Reynolds⁶⁶⁷⁾ and oxidative stress leads to a use of intracellular vitamin C. Accordingly, Chen et al. ⁽Reference Chen, Hutchinson and Pecoraro⁶⁶⁸⁾ provided evidence in vitro that acute hyperglycaemia leads to a significant decrease in leucocyte vitamin C content. Evans et al. ⁽Reference Evans, Goldfine and Maddux⁶⁶²⁾ developed a unique oxidative stress hypothesis suggesting that chronic elevation of hyperglycaemia (and NEFA) induces an activation of the NF-κB, p38 MAPK and NH₂-terminal Jun kinase/stress-activated protein kinase pathways. This happens along with the activation of the RAGE, protein kinase C and sorbitol stress pathways. The authors suggest that these events play a key role in causing late complications in type 1 and type 2 diabetes, going along with insulin resistance and impaired insulin secretion in type 2 diabetes. Evans et al. ⁽Reference Evans, Goldfine and Maddux⁶⁶²⁾ provided evidence that elevated glucose causes oxidative stress due to increased production of mitochondrial ROS, non-enzymatic glycation of proteins and glucose auto-oxidation. They also pointed out that elevated NEFA can cause oxidative stress due to increased mitochondrial uncoupling and β-oxidation, the latter leading to an increased production of ROS and to an activation of stress-sensitive signalling pathways, which in turn impair insulin secretion and action. Thus, oxidative stress induced by acute and chronic elevations in glucose and NEFA plays a key role in causing insulin resistance and β-cell dysfunction. A range of studies has shown that acute and lasting hyperglycaemia leads to an elevation in dicarbonyls and protein glycation (measured as fructosamine or HbA1c). Glycosylated proteins, as well as their metabolites, AGE, which bind to the AGE receptor RAGE, induce a cascade of signals that initiate an inflammatory response⁽Reference Beisswenger, Howell and O'Dell⁶⁶⁹⁾. Accordingly, pro-inflammatory transcription factors, inflammatory markers including CRP, IL-6, IL-8, TNF-α and matrix metalloproteinase and markers of endothelial dysfunction VCAM-1, ICAM-1 and E-selectin have been observed to be elevated in response to acute and persistent hyperglycaemia⁽Reference Bierhaus, Humpert and Morcos^474, Reference Bierhaus, Rudofsky and Humpert^475, Reference Negrean, Stirban and Stratmann^514, Reference Vlassara, Cai and Crandall^521, Reference Kempf, Rose and Herder^653, Reference Evans, Goldfine and Maddux^662, Reference Cai, He and Zhu^670, Reference Kempf, Rose and Herder⁶⁷¹⁾.

Fig. 8 Schematic representation of the general mechanisms by which hyperglycaemia can affect inflammation. PKC, protein kinase C. Reproduced with permission from Giugliano et al. ⁽Reference Giugliano, Ceriello and Esposito⁶⁵⁴⁾.

Glycaemic index, glycaemic load and inflammation

Dietary glycaemic index (GI) and glycaemic load (GL) were both positively associated with plasma CRP concentration in 18 137 healthy women aged >45 years without diagnosed diabetes⁽Reference Levitan, Cook and Stampfer⁶⁷²⁾. In another study in 974 subjects aged 42–87 years, dietary GI was positively associated with CRP concentration⁽Reference Du, van der and van Bakel⁶⁷³⁾. Qi et al. ⁽Reference Qi, Rimm and Liu³¹⁵⁾ examined the associations of dietary GI and GL with plasma adiponectin among 780 diabetic men from the Health Professionals’ Follow-Up Study. After adjustment for other factors, dietary GI and GL were both significantly inversely associated with plasma adiponectin concentration in a dose-dependent fashion. Adiponectin levels were 13 % lower in the highest quintile of dietary GI than in the lowest quintile. For dietary GL, adiponectin levels were 18 % lower in the highest quintile than in the lowest. Several intervention studies have examined the impact of dietary GI or GL on markers of chronic low-grade inflammation. In one study, thirty-four healthy overweight adults aged 24–42 years received energy-restricted diets with high or low GL for 6 months⁽Reference Pittas, Roberts and Das⁶⁷⁴⁾; more subjects in the low GL group showed a decline in serum CRP concentration than in the high GL group. In another study, Shikany et al. ⁽Reference Shikany, Phadke and Redden⁶⁷⁵⁾ found no differences in CRP, TNF-α, IL-6, fibrinogen, sTNFR2 or PAI-1 concentrations when overweight or obese men consumed diets with high or low GI for 4 weeks. Likewise, 11-week interventions of high- or low-GL diets in fifteen overweight subjects had no differential effects on CRP, TNF-α, IL-6 or MCP-1 concentrations⁽Reference Vrolix and Mensink⁶⁷⁶⁾. Thus, evidence from large observational studies is highly suggestive that there is a positive association between GI/GL and low-grade inflammation, but intervention trials do not support this convincingly, perhaps because of the small size of most of the latter trials.

Dietary fibre

In NHANES 1999–2000 in the USA, dietary fibre intake was inversely associated with serum CRP concentrations (OR 0·49 for 32 g/d v. 5·1 g/d) in diabetic women (n 3920)⁽Reference Ajani, Ford and Mokdad⁶⁷⁷⁾, as well as in subjects with diabetes, hypertension and obesity (n 7891)⁽Reference King, Mainous and Egan⁶⁷⁸⁾. Similar findings were reported in a cross-sectional study with healthy subjects (n 205) and in subjects with metabolic disorders (n 1653) in Italy⁽Reference Bo, Durazzo and Guidi⁶⁷⁹⁾. In the Seasonal Variation of Blood Cholesterol Levels Study involving 524 subjects, who had multiple measurements of CRP and dietary intake, dietary total fibre and soluble and insoluble fibre were all inversely related to CRP concentration in the regression models adjusted for several traditional cardiovascular risk factors, current infection status and season of year⁽Reference Ma, Griffith and Chasan-Taber⁶⁸⁰⁾. In contrast, the same authors observed no correlation between dietary fibre intake and blood CRP concentrations with a single measurement at baseline in the Women's Health Initiative Observational Study⁽Reference Ma, Hebert and Li⁶⁸¹⁾. However, there was an inverse association between fibre intake and IL-6 and sTNFR2 concentrations in this female population. In a study with diabetic males (n 780, Health Professionals’ Follow-Up Study), cereal fibre was positively associated with adiponectin levels after controlling for a number of confounding factors⁽Reference Qi, Rimm and Liu³¹⁵⁾. King et al. ⁽Reference King, Egan and Woolson⁶⁸²⁾ conducted an intervention study with thirty-five lean normotensive and seventeen obese hypertensive adults that involved a randomised cross-over design. The two intervention diets constituted either the Dietary Approaches to Stop Hypertension (DASH) diet (naturally high in fibre i.e. 30 g fibre/d) or a fibre-supplemented usual diet (30 g psyllium fibre/d) each for a 3-week period. Both diets caused a reduction in CRP concentration (14 and 18 %, respectively), although this was significant only in lean normotensive subjects in either intervention arm. The data generally support that dietary fibre intake is associated with reduced low-grade inflammation.

Deficiency v. excess

Fe has been called a double-edged sword, as both Fe deficiency and excess can have deleterious effects⁽Reference Weiss⁶⁹⁵⁾. Fe is important in immune/inflammatory responses including neutrophil activation, macrophage effector functions, and T-helper cell type-1 and -2 (Th1/Th2) patterns (reviewed in Weiss⁽Reference Weiss⁶⁹⁵⁾). Fe deficiency is associated with alterations in the immune response and increased risk of infections⁽Reference Weiss^695–Reference de Silva, Atukorala and Weerasinghe⁶⁹⁷⁾. On the other hand, because Fe is a redox-active transition metal, it may contribute to the production of ROS, oxidative stress and inflammation⁽Reference Wood^698, Reference Wagener, Volk and Willis⁶⁹⁹⁾, and thus Fe excess can augment the risk for diabetes and CVD. It is important to indicate that ROS are produced by free, and not bound, Fe⁽Reference McCord⁷⁰⁰⁾ and the body has evolved a metabolic system that minimises the availability of free Fe⁽Reference Wood⁶⁹⁸⁾. Most Fe in the body is not free, and is bound to proteins such as ferritin, transferrin and Hb; thus, serum ferritin, transferrin, transferrin saturation and Hb measurements do not reflect the availability of ‘free’ or ‘labile’ Fe that is implicated in the production of ROS. In recent years, the measurement of non-transferring-bound labile Fe⁽Reference Lee and Jacobs⁷⁰¹⁾ has been introduced; however, few studies have utilised this measurement to date due to technical, cost and standardisation issues.

Iron status and low-grade inflammation

There is some epidemiological evidence that higher Fe intake, particularly that of haem Fe, and higher Fe status are associated with increased risk of type 2 diabetes, atherosclerosis and CHD⁽Reference Rajpathak, Ma and Manson^702–Reference Ahluwalia, Genoux and Ferrieres⁷¹⁴⁾, although not all of the literature is consistent. It is not clear whether the association of Fe intake and status with diabetes or CVD is mediated through the effects of Fe on inflammatory pathways. In a small study with thirty-one carbohydrate-intolerant patients, quantitative phlebotomy was used to induce Fe depletion to near-deficiency levels⁽Reference Facchini and Saylor⁷¹⁵⁾. The induced Fe deficiency was associated with reductions in several CVD risk factors as well as in the inflammatory marker fibrinogen. While uncertainty exists as to whether Fe plays a causative role in the aetiology of low-grade inflammation and its associated pathologies, past and emerging evidence indicates that chronic low-grade inflammation is associated with poor Fe status in obese persons⁽Reference Selzer and Mayer^716–Reference Yanoff, Menzie and Denkinger⁷²¹⁾. A concomitant improvement in Fe status (as measured by transferrin saturation) and decrease in inflammation (CRP and orosomucoid concentrations) has been observed after bariatric surgery intervention in morbidly obese women⁽Reference Anty, Dahman and Iannelli⁷²²⁾. In total, emerging evidence indicates that the low-grade inflammation of obesity may be associated with low Fe status; however, further investigation of this relationship is warranted.

A small number of studies have investigated the effects of increasing total Fe or haem Fe intake on markers of inflammation⁽Reference Hodgson, Ward and Burke^723–Reference Schumann, Kroll and Weiss⁷²⁵⁾. In a small study involving three healthy volunteers who received 120 mg Fe/d for a week, there were no changes in circulating CRP concentration or leucocyte counts, or in urinary neopterin concentration⁽Reference Schumann, Kroll and Weiss⁷²⁵⁾. Furthermore, postpartum Fe supplementation (80 mg/d) for 12 weeks in non-anaemic Fe-deficient women did not significantly alter CRP concentration or leucocyte counts⁽Reference Krafft, Perewusnyk and Hanseler⁷²⁶⁾. In a small study in 8–11-year-old Guatemalan children who received twice the recommended daily amounts of Fe for 8 weeks (n 20) or placebo (n 20), no differences in CRP or orosomucoid concentrations were noted, although α-1 antichymotrypsin levels were increased with the Fe supplement⁽Reference Rosales, Kang and Pfeiffer⁷²⁴⁾. In another study examining the effect of increasing lean red meat intake, participants were either assigned to a control group (maintain their usual diet) or to partially replace energy from carbohydrate-rich foods with 200 g/d of lean red meat for 8 weeks⁽Reference Hodgson, Ward and Burke⁷²³⁾. In this short intervention, increased red meat intake was not associated with increased oxidative stress or inflammation. Taken together, the evidence from these limited studies examining the effect of Fe supplementation or increased red meat intake on inflammatory markers suggests no major effect on inflammation; however, there is a need for further larger well-designed studies to clarify this effect.