Study selection

In order to achieve the aim of the present review, we conducted iterative searches in two databases, PubMed and Scopus. Selection criteria included original research articles, as well as review papers, meta-analyses, short communications, letters to the editors, and editorials, written in any language with an English abstract. The search period was set between January 1980 and March 2010. Medical subject heading (MeSH) keywords used to assist allocating the studies were: cereals, ready to eat cereals, breakfast, fortified, body mass index, overweight, obesity, nutritional status, and nutrient intake. After allocating the papers, we expanded the searches reviewing the reference lists and authors of ‘key’ papers. Selection of articles was restricted to those evaluating body size or weight managements in humans (i.e. BMI, weight in kg, or weight in pounds or body fat). Restriction based on the age of the study participants was not used because there is large variability in eligible age groups among studies. For prospective cohort studies, duration of ≥ 6 months was required for inclusion in the present review, to ensure that sufficient follow-up time was provided to effectively evaluate the relationship between RTEC consumption and weight change. For intervention studies, a sample size greater than twenty-five per intervention group was set, although the latter cannot ensure adequate statistical power. Thus, thirty-seven articles were initially allocated and expanded after reviewing the references lists. After reviewing the aim, design and results of the aforementioned papers, twenty-one research studies (i.e. fifteen observational and six clinical trials) that presented original information were selected and discussed here.

Association between consumption of ready-to-eat cereals with obesity/adiposity

Children and adolescents

Albertson et al. ⁽Reference Albertson, Anderson and Crockett¹³⁾ suggested that the consumption of RTEC at breakfast should be encouraged as a component of an eating pattern that promotes the maintenance of healthy body weight and nutrient intakes in children. This cross-sectional study was conducted on a sample of 603 children aged 4–12 years. Results showed that there was a statistically significant inverse relationship between BMI and frequency of RTEC consumption (P < 0·01) within each age group as well as for the total sample. The main limitation of the study was the fact that the food records, height and weight were self-reported. Findings from another recent cross-sectional study⁽Reference Panagiotakos, Antonogeorgos and Papadimitriou²¹⁾ conducted in 700 children aged 10–12 years provide evidence that daily consumption of breakfast and consumption of breakfast cereal as a most frequent choice are inversely associated with the prevalence of overweight or obesity.

Other researchers concluded that the consumption of breakfast cereals was associated with lower BMI levels and lower likelihood of overweight/obesity, in both sexes, in a sample of 2008 schoolchildren adolescents in a cross sectional study⁽Reference Kosti, Panagiotakos and Zampelas²²⁾. More specifically, the results showed that compared with no or rare consumption, the daily intake or the intake of more than two daily servings of cereals for breakfast was associated with lower BMI levels in boys and girls (P < 0·01). Consumption of cereals as a first choice for breakfast was associated with 33 (95 % CI 14, 48) % lower likelihood of overweight/obesity, irrespective of age, sex and physical activity status. Furthermore, another cross-sectional study⁽Reference Kafatos, Linardakis and Bertsias¹⁰⁾ conducted in a sample of 392 adolescents (183 boys, 209 girls) aged 15 (sd 0·4) years, showed among others that RTEC consumption was inversely related to the obesity indices. Others have reported similar associations⁽Reference Gibson and O'Sullivan^23–Reference Vågstrand, Barkeling and Forslund²⁵⁾. In particular, results from a survey conducted in a sample of 2075 British schoolchildren showed that BMI values were lower among frequent breakfast cereal eaters⁽Reference Gibson and O'Sullivan²³⁾. In addition, Ortega et al. ⁽Reference Ortega, Requejo and López-Sobaler²⁴⁾ found that intake of cereals at breakfast was significantly higher among normal-weight children aged 9–13 years⁽Reference Ortega, Requejo and López-Sobaler²⁴⁾. Similarly, Vågstrand et al. ⁽Reference Vågstrand, Barkeling and Forslund²⁵⁾ found that a low intake of breakfast cereals in boys was correlated significantly with a high body fat percentage (r _s ± 0·2)⁽Reference Vågstrand, Barkeling and Forslund²⁵⁾.

In addition, the findings from a cross-sectional study conducted by Williams et al. ⁽Reference Williams, O'Neil and Keast¹⁶⁾ in a sample of 1389 children aged 1–12 years and including African–Americans showed that the consumption of RTEC at breakfast was associated with improved body weight.

It is worth mentioning that the findings from the 10-year longitudinal biracial observational cohort study (National Heart, Lung, and Blood Institute Growth Health Study)⁽Reference Barton, Eldridge and Thompson^15, Reference Albertson, Thompson and Franko²⁶⁾ are in agreement with the aforementioned associations found in cross-sectional studies. In particular, the National Heart, Lung, and Blood Institute Growth Health Study was conducted in a cohort of females between the ages of 9 and 19 years. The ethnically diverse sample consisted of 2379 girls aged 9 and 10 years at baseline. Specifically, the results showed that the consumption of cereal breakfasts as part of an overall healthful lifestyle may play a role in maintaining a healthful BMI⁽Reference Barton, Eldridge and Thompson¹⁵⁾. Moreover, Albertson et al. ⁽Reference Albertson, Thompson and Franko²⁶⁾ concluded that girls who ate cereal on a greater percentage of days during childhood had lower percentage body fat and total cholesterol, and were more likely to exhibit high levels of physical activity and less television viewing during study year 10 (P values < 0·05)⁽Reference Albertson, Thompson and Franko²⁶⁾.

In addition, research findings from a recently conducted randomised clinical trial in a sample of 660 children aged 8–10 years at study entry showed, among others, that in boys higher RTEC consumption was associated with lower BMI⁽Reference Albertson, Affenito and Bauserman¹⁷⁾.

However, another intervention study⁽Reference Rosado, del R Arellano and Montemayor²⁷⁾ conducted in children aimed among others to determine if an increase in RTEC intake is an effective strategy to reduce excess body weight in overweight or at risk of overweight children. Results showed that a strategy to increase RTEC consumption, as a source of carbohydrate, to reduce obesity, is effective only when accompanied by nutrition education (Table 1).

Table 1 Summary of the reviewed studies relevant to the associations of ready-to-eat cereal (RTEC) with obesity/adiposity in children and adolescents

BF, body fatness.

Adults

Research by Cho et al. ⁽Reference Cho, Dietrich and Brown²⁸⁾, conducted on a sample of 16 452 individuals and consisting of data from the Third National Health and Nutrition Examination Survey (NHANES III), showed that not only is breakfast consumption important, but also, that the type of breakfast may be an important factor in influencing BMI. Eating cereal, whether cooked or ready to eat for breakfast, is associated with a significantly lower BMI in adults, when compared with either skipping breakfast or consuming other types of breakfast.

Other findings showed that RTEC breakfast consumption was associated with a desirable macronutrient profile for preventing obesity, and predicted weight status in women but not in men⁽Reference Song, Chun and Obayashi²⁹⁾. The study sample consisted of 4218 men and women aged ≥ 19 years (National Health and Nutrition Examination Survey). Results showed an inverse association between RTEC breakfast consumption and BMI in women.

Moreover, research findings from a prospective study⁽Reference Bazzano, Song and Bubes³⁰⁾ showed that BMI and weight gain were inversely associated with intake of breakfast cereals independently of other risk factors. The study sample consisted of 17 881 US male physicians aged 40–84 years. Results showed that over 8 and 13 years of follow-up, respectively, men who consumed breakfast cereal, regardless of type (whole or refined breakfast cereals), consistently weighed less than those who consumed breakfast cereals less often (P < 0·01).

However, some researchers⁽Reference Koh-Banerjee, Franz and Sampson^31, Reference Liu, Willett and Manson³²⁾ reported that this association was only attributed to the consumption of whole-grain cereals and not the refined-grain ones. In particular, Koh-Banerjee et al. ⁽Reference Koh-Banerjee, Franz and Sampson³¹⁾ conducted a study in a prospective cohort of 27 082 men aged 40–75 years at baseline. Results showed that an increase in whole-grain intake was inversely associated with long-term weight gain (P < 0·0001). A dose–response relationship was observed, and for every 40 g/d increment in whole-grain intake, weight gain was reduced by 0·49 kg. No associations were observed between changes in refined-grain intake and body weight. Similarly, in another prospective study⁽Reference Liu, Willett and Manson³²⁾ in a sample of 74 091 US female nurses aged 38–63 years, the authors concluded that weight gain was inversely associated with the intake of high-fibre, whole-grain foods, but positively related to the intake of refined-grain foods. This indicates the importance of distinguishing whole-grain products from refined-grain products to aid in weight control.

The protective effect of the consumption of RTEC against obesity has also been found in some intervention studies⁽Reference Waller, Vander Wal and Klurfeld^33–Reference Maki, Beiseigel and Jonnalagadda³⁶⁾. Particularly, findings from an intervention study⁽Reference Waller, Vander Wal and Klurfeld³³⁾ showed that eating RTEC after the evening meal may attenuate energy intake in night snackers and promote weight loss in compliant individuals. The study sample consisted of overweight/obese adults (aged 18–65 years) with self-reported night-snacking behaviours. Results showed that there was a correlation between the number of days of compliance with post-dinner cereal consumption and weight loss (r − 0·36; P < 0·057). In summary, the results of this short-term study show that offering a structured post-dinner snack to overweight or obese night snackers reduces post-dinner energy intake and the total daily energy intake and results in weight loss, which is proportional to compliance. Moreover, another intervention study⁽Reference Mattes³⁴⁾ in adults aimed to determine whether RTEC used as a portion-controlled meal replacement promotes weight loss. The authors concluded that RTEC may be used to promote weight loss when consumed as a portion-controlled meal replacement. Provision of a variety of brands does not compromise efficacy. In another randomised controlled clinical trial⁽Reference Wal, McBurney and Cho³⁵⁾, conducted in 164 eligible overweight and obese adults, the authors tested the effectiveness of three commercially available partial meal replacement products on weight-loss outcomes. The results showed that a variety of RTEC products may be safely and effectively used as meal replacements in weight-loss programmes. Moreover, in the most recently conducted randomised control study⁽Reference Maki, Beiseigel and Jonnalagadda³⁶⁾, conducted in 204 adults, the authors evaluated whether or not a whole-grain, ready-to-eat oat cereal containing viscous fibre, as part of a dietary programme for weight loss, lowers LDL-cholesterol levels and improves other CVD risk markers more than a dietary programme alone. The results showed that consumption of a whole-grain ready-to-eat oat cereal as part of a dietary programme for weight loss had favourable effects on fasting lipid levels and waist circumference (Table 2).

Table 2 Summary of the reviewed studies relevant to the associations of ready-to-eat cereals (RTEC) with obesity/adiposity in adults

Effects of consumption of ready-to-eat cereals on nutritional status

The strong association between RTEC consumption and vitamin and mineral intakes demonstrated in many studies suggests that RTEC make a major contribution to micronutrient intakes by increasing the intake of fibre, Ca, Fe, folic acid, vitamin C and Zn, and decreasing the intake of fat and cholesterol⁽Reference Kafatos, Linardakis and Bertsias^10–Reference Montenegro-Bethancourt, Vossenaar and Kuijper²⁰⁾. In particular, children who eat cereals consume significantly less fat and cholesterol⁽Reference Albertson, Anderson and Crockett¹³⁾. Moreover, in a randomised, controlled, multicentre, clinical trial, the results showed that except for energy, RTEC consumption was positively associated with all measures of nutrients for both sexes⁽Reference Albertson, Affenito and Bauserman¹⁷⁾. In particular, in boys, higher RTEC consumption was associated with lower total and LDL-cholesterol levels⁽Reference Albertson, Affenito and Bauserman¹⁷⁾. Since fortified breakfast cereals are consumed with milk it is not possible to separate the dietary effects of the two foods. Those who have a high intake of breakfast cereal almost certainly consume high milk levels⁽Reference van den Boom, Serra-Majem and Ribas^14, Reference Kosti, Panagiotakos and Zampelas^22, Reference Albertson, Thompson and Franko³⁷⁾ too.

Specifically, in the frame of the National Heart, Lung, and Blood Institute Growth Health Study, in a sample of 2379 adolescent girls, Albertson et al. ⁽Reference Albertson, Thompson and Franko³⁷⁾ tried to examine three possible explanations for the documented association between cereal consumption and positive health outcomes. In particular, the major findings were (1) cereal consumed at breakfast provided more fibre, Fe, folic acid and Zn and less fat, Na, sugar, and cholesterol, compared with the nutrients in foods eaten during non-cereal breakfasts. The cereal consumed provided less protein, carbohydrates and Ca, compared with foods consumed during non-cereal breakfasts; (2) eating cereal for breakfast appeared to facilitate milk consumption (leading to increased Ca intake) and to displace fats/sweets, quick breads, soda, and meat/eggs as breakfast foods. Eating cereal at breakfast was associated with increased consumption of fibre and carbohydrates and decreased consumption of fats throughout the day; and (3) greater physical activity was also associated with eating a cereal breakfast. Thus, the increased intakes of nutrients such as Ca and Zn of breakfast cereal eaters which are found in the literature are most likely due to the milk consumed with them, since breakfast cereals are not typically fortified with, or naturally rich in, these nutrients.

In addition, whole-grain breakfast cereals, fruits and vegetables are all important dietary sources of antioxidants⁽Reference Miller, Rigelhof and Marquart³⁸⁾. Indeed, literature findings showed that whole-wheat- and wheat bran-based ready-to-eat breakfast cereals could be important sources of dietary antioxidants. Phenolic acids from breakfast cereals possess strong antioxidant activity in vitro at concentrations that would be obtained from a normal serving of whole-wheat cereal. Current research on the antioxidant activity of wheat phenolics suggests that further research is warranted to determine the potential benefits of these dietary antioxidants⁽Reference Baublis, Lu and Clydesdale³⁹⁾. Moreover, several intervention studies showed that breakfast cereals may be of special help with respect to folate status, serum homocysteine levels and thiamin status in overweight/obese young women following energy-restricting diets⁽Reference Ortega, Lopez-Sobaler and Andres^40, Reference Ortega, Lopez-Sobaler and Andres⁴¹⁾ and that breakfast cereals are an important source of non-haeme Fe and could help to improve Fe status, especially in women with poor initial Fe levels⁽Reference Rodriguez-Rodriguez, Lopez-Sobaler and Andres⁴²⁾. Furthermore, Melanson et al. ⁽Reference Melanson, Angelopoulos and Nguyen⁴³⁾, who conducted an intervention study in a sample of 180 obese adults, found that a hypoenergetic diet with fibre-rich whole-grain cereal is effective for improving or maintaining other aspects of dietary quality during weight loss.

On the other hand, Whittaker et al. ⁽Reference Whittaker, Tufaro and Rader⁴⁴⁾ expressed concerns regarding the analysed values of Fe and folic acid in breakfast cereals, which were found to be considerably higher than the labelled values. However, other researchers concluded that concern for excessive nutrient intakes in those individuals consuming fortified cereals, even in children who have smaller safety margins, is unfounded⁽Reference Gibson^12, Reference van den Boom, Serra-Majem and Ribas¹⁴⁾. Moreover, there is a trend towards decreasing Na levels in RTEC contents⁽Reference Truswell⁴⁵⁾. Indeed, evidence has shown that there was no significant difference in Na intake between RTEC consumers and non-consumers⁽Reference van den Boom, Serra-Majem and Ribas¹⁴⁾.

A recently published paper has raised serious concerns relevant to the important differences observed in nutritional quality between children's cereals and non-children's cereals⁽Reference Schwartz, Vartanian and Wharton⁴⁶⁾. It is worth mentioning that, according to the authors' terminology, cereals that are primarily marketed to children (e.g. packaging contained a licensed character or contained an activity directed at children) were classified as children's cereals while cereals that are not marketed to children were classified as non-children's cereals. Specifically, the authors concluded that compared with non-children's cereals, children's cereals were denser in energy, sugar and Na, but were less dense in fibre and protein. In addition, the authors suggested that the majority of children's cereals (66 %) failed to meet US nutrition standards, particularly with respect to sugar content. However, research findings from recently conducted studies⁽Reference Kosti, Panagiotakos and Zampelas^22, Reference Albertson, Thompson and Franko²⁶⁾ in children and adolescents showed that cereal intake was associated with a variety of health outcomes (including lower body fat), after statistically adjusting for the sugar content of cereals consumed⁽Reference Albertson, Thompson and Franko²⁶⁾, and that consumption of pre-sweetened breakfast cereals was associated with lower BMI compared with non-pre-sweetened cereals, in both sexes (P < 0·001)⁽Reference Kosti, Panagiotakos and Zampelas²²⁾.

Possible explanations for the observed associations

The relationship between the consumption of RTEC with BMI is difficult to explain. Several explanations could be proposed for the association between cereal consumption and body weight. First, RTEC consumption may be a marker for other healthful lifestyle factors⁽Reference Albertson, Anderson and Crockett¹³⁾. For example, it has been reported⁽Reference Albertson, Anderson and Crockett¹³⁾ that breakfast consumers are less likely to snack during the morning, a habit, which if repeated daily, could lead to overconsumption of energy.

It is possible that consistent cereal eating is a marker for consistent intake of nutrient-rich foods and/or a tendency to consistently follow a generally healthful lifestyle⁽Reference Barton, Eldridge and Thompson¹⁵⁾. Indeed, research findings⁽Reference Albertson, Thompson and Franko³⁷⁾ showed that eating cereal at breakfast tends to displace consumption of other breakfast foods (for example, fats/sweets, quick breads, meats/eggs, soda), which have been shown⁽Reference Cho, Dietrich and Brown^28, Reference Striegel-Moore, Thompson and Affenito⁴⁷⁾ to be associated with high BMI.

Second, RTEC is most frequently consumed with milk⁽Reference van den Boom, Serra-Majem and Ribas^14, Reference Kosti, Panagiotakos and Zampelas^22, Reference Albertson, Thompson and Franko³⁷⁾, which is a good source of dietary Ca. Zemel⁽Reference Zemel⁴⁸⁾ showed that dietary Ca modulates circulating calcitriol (1,25-dihydroxyvitamin D) levels that in turn regulate intracellular Ca which affects fat metabolism in human adipocytes.

A portion of this additional anti-obesity bioactivity is attributable to the angiotensin-converting enzyme-inhibitory activity of dairy and to the high concentration of branched-chain amino acids which act synergistically with Ca to attenuate adiposity⁽Reference Zemel⁴⁸⁾. Thus, it is possible that the proposed mechanism⁽Reference Zemel⁴⁸⁾ could be a factor in contributing to the better intake regulation of frequent cereal eaters⁽Reference Albertson, Anderson and Crockett¹³⁾.

Furthermore, some studies⁽Reference Waller, Vander Wal and Klurfeld^33–Reference Wal, McBurney and Cho³⁵⁾ showed that consuming RTEC may contribute to reduced snacking and portion-controlled meal replacement, both of which might be mechanisms connecting RTEC consumption with reduced BMI.

In addition, it is worth mentioning that the findings from recently conducted prospective studies⁽Reference Albertson, Thompson and Franko^26, Reference Albertson, Thompson and Franko³⁷⁾ in children and adolescents showed that increased cereal intake is related to lifestyle habits such as high levels of physical activity and reduced television viewing, revealing that lifestyle issues might also have to be taken into serious consideration in our attempt to explain why RTEC intake is associated with lower BMI.

Furthermore, Harnack et al. ⁽Reference Harnack, Walters and Jacobs⁴⁹⁾ has shown that RTEC is an important food source of whole grains which have been found⁽Reference Good, Holschuh and Albertson^50, Reference van de Vijver, van den Bosch and van den Brandt⁵¹⁾ to be inversely associated with obesity, and that cereals and cereal products are the main sources of dietary fibre⁽Reference Suter⁴⁾ with an undoubtedly positive effect on BMI status⁽Reference Maki, Beiseigel and Jonnalagadda^36, Reference Lairon, Arnault and Bertrais⁵²⁾. Thus, these findings suggest that additional components in whole grains may contribute to favourable metabolic alterations that may reduce long-term weight gain⁽Reference Koh-Banerjee, Franz and Sampson³¹⁾, providing another possible explanation of the observed associations.

Indeed, the disruption and removal of the plant cell-wall material (dietary fibre), as occurs with many types of bakery products and breakfast cereals, results in the loss of their original complement of micronutrients, and the starch and sugar components are often made easily digestible⁽Reference Englyst and Englyst⁵³⁾. There is evidence that a greater consumption of slow-release carbohydrates is likely to be associated with health benefits⁽Reference Jenkins, Kendall and Augustin⁵⁴⁾. Thus, food processing techniques that retain or introduce characteristics that slow carbohydrate digestion should therefore generally be encouraged⁽Reference Englyst and Englyst⁵³⁾. In addition, concentrations of vitamins, minerals, essential fatty acids and phytochemicals that are important in carbohydrate metabolism are lower in refined grains⁽Reference Liu⁵⁵⁾. RTEC are high in unrefined carbohydrates (albeit, they may be sugar rich and thus higher in unrefined sugar) and are usually fortified with a range of micronutrients, including Fe, thiamin, riboflavin, niacin, vitamin B₆, folic acid, vitamin B₁₂ and vitamin D⁽Reference van den Boom, Serra-Majem and Ribas¹⁴⁾. However, several varieties of RTEC are high in dietary fibre, as they contain whole grains, bran or dried fruit⁽Reference van den Boom, Serra-Majem and Ribas¹⁴⁾.

Thus, the inherent high fibre content of most whole-grain foods may help prevent weight gain by increasing appetite control through producing a delay in carbohydrate absorption⁽Reference Porikos, Hesser and Van Itallie⁵⁶⁾. Moreover, multiple enzyme inhibitors that exist in a whole-grains-fibre complex might directly affect metabolic efficiency⁽Reference Liu⁵⁵⁾. This mechanism might serve as another way whereby whole grains could have beneficial effects on body weight⁽Reference Liu, Willett and Manson³²⁾.

In addition, the investigation of the effect of low-GI/glycaemic load diets in the prevention of obesity could be another possible, although not yet quite clear, mechanism of the observed associations.

There is now a large body of evidence comprising observational prospective cohort studies, randomised controlled trials and mechanistic experiments in animal models that provides robust support for low-GI carbohydrate diets in the prevention of obesity, diabetes and CVD⁽Reference Brand-Miller, McMillan-Price and Steinbeck⁵⁷⁾.

Carbohydrate foods with a lower GI may assist in weight management via several mechanisms⁽Reference Brand-Miller, Holt and Pawlak⁵⁸⁾. By reducing insulinaemia, low-GI foods may provide greater access to fatty acids as a source of fuel, promoting greater fat oxidation. Small differences in substrate oxidation have been found to predict long-term weight gain⁽Reference Brand-Miller, Holt and Pawlak⁵⁸⁾.

Another potentially very important line of research with the GI has been its influence on satiety, hunger, energy intake and ultimately obesity. The evidence from short-term studies suggests that low-GI diets increase satiety and decrease hunger compared with high-GI diets⁽Reference Brand-Miller, Holt and Pawlak^58, Reference Ludwig⁵⁹⁾. The longer-term benefits for energy balance with low-GI diets have been demonstrated in children as part of an obesity programme⁽Reference Spieth, Harnish and Lenders⁶⁰⁾.

However, results from other studies⁽Reference Cheng, Karaolis-Danckert and Libuda^61–Reference van Dam and Seidell⁶³⁾ provided no evidence to support an effect of a reduced-GI diet on body weight. Thus, findings from studies on the effect of the dietary GI on body weight have not been consistent⁽Reference van Dam and Seidell⁶³⁾.

Taking into account that modern carbohydrate staples, including potatoes, breads, breakfast cereals and other processed cereal foods have a high GI, even when high in fibre⁽Reference Foster-Powell, Holt and Brand-Miller⁶⁴⁾, it seems that the controversies observed between low- and high-GI/glycaemic load diets and health effects could not be explained by claims that the GI of the diet per se may have specific effects on body weight, and therefore such claims may be misleading⁽Reference Aston, Stokes and Jebb⁶²⁾.

Thus, although a large body of evidence supports the current popularity of low-GI diets, further research into the role of GI in the prevention and management of obesity and chronic disease is needed, since it seems that the associations between RTEC and obesity may be best explained via multiple mechanisms.