Properties of acrylamide

Acrylamide (acrylic amide; International Union of Pure and Applied Chemistry (IUPAC) name 2-propenamide), an α,β-unsaturated (conjugated) reactive molecule, has the chemical formula C₃H₅NO^(10, Reference Fohgelberg, Rosén and Hellenäs¹¹⁾. It has a high mobility in soil and groundwater and is biodegradable⁽Reference Friedman¹²⁾. It is also used in industry to manufacture polyacrylamide or its co-polymers⁽Reference Friedman^12, Reference Wenzl, De La Calle and Anklam¹³⁾.

Acrylamide is inadvertently found in starchy foods such as crisps, chips and bread⁽Reference Miller, Carter and Sipes¹⁴⁾. Its discovery in heated foods has received a high level of attention from the public and the medical community. Acrylamide in fried and baked foods is produced by the reaction between asparagines and reducing sugars (fructose, glucose) or reactive carbonyls⁽Reference Viator and Muth^15, Reference Choe and Min¹⁶⁾. Coffee drinking and smoking are other major sources apart from the human diet⁽Reference Schettgen, Weiss and Drexler^17, Reference Lantz, Ternité and Wilkens¹⁸⁾. Boiling does not produce acrylamide, but baking, frying, deep-frying, over-cooking and microwaving produce large amounts of acrylamide in foodstuffs⁽Reference Miller, Carter and Sipes¹⁴⁾. On the other hand, water is one of the major sources of exposure, as polyacrylamide is used mainly for the purification of drinking water as a flocculating agent⁽Reference Friedman^12, Reference Wenzl, De La Calle and Anklam¹³⁾.

Acrylamide is classified as ‘probably carcinogenic to humans (group 2A)’ by the International Agency for Research on Cancer. There is inadequate evidence in humans for the carcinogenicity of acrylamide⁽¹⁰⁾.

Biotransformation of acrylamide

Excretion

The major pathway of metabolism for acrylamide is its conjugation with reduced GSH by glutathione S-transferase. Conditions such as malnutrition, oxidative stress and liver disease (alcoholic hepatitis, cirrhosis, and other malignant liver disorders) can decrease the GSH content of liver. Since a child's liver cannot carry as high a burden as an adult liver, especially under such conditions, a higher toxicity of acrylamide might be anticipated in children. Elimination occurs mainly in the urine as mercapturic acid conjugates. Greater than 90 % of absorbed acrylamide is excreted in the urine as metabolites. Less than 2 % is excreted as unchanged acrylamide. Smaller amounts are excreted in the bile and faeces. Approximately 60 % of an administered dose appears in the urine within 24 h⁽Reference Miller, Carter and Sipes¹⁴⁾. Biotransformation of acrylamide is summarised in Fig. 1.

Fig. 1 Biotransformation mechanism of acrylamide. CYP2E1, cytochrome 2E1; GST, glutathione S-transferase.

Toxicity of acrylamide

There are several health risks caused by the intake of acrylamide from several routes. Smoking by the mother is a major way by which the neonate is exposed to acrylamide⁽Reference Hilbig, Freidank and Kersting²⁶⁾. In addition, contaminated foodstuffs and drinking water are two other main ways to expose the baby or child to acrylamide. As children, especially those under the age of 2 years, are the most sensitive population, acrylamide intake levels must be monitored particularly in infant formulas and baby foods. On the other hand, mothers should be attentive while choosing the water with which they prepare food for their child. The health risks of acrylamide can be classified as follows.

Mutagenic, genotoxic and carcinogenic properties of acrylamide

The genotoxic, mutagenic and carcinogenic potentials of acrylamide have been studied extensively. Acrylamide itself reacts rapidly with thiol (–SH) and amino groups; this explains why its primary targets are proteins⁽Reference Carere²⁵⁾. Acrylamide has been shown to bind DNA by a Michael-type process in vitro with low activity⁽Reference Bergmark, Calleman and He^21, Reference Carere²⁵⁾. Nevertheless, there is sufficient evidence in the literature that both acrylamide and its metabolite glycidamide are mutagenic and clastogenic in mammalian cells⁽Reference Dearfield, Douglas and Ehling^19, Reference Jägerstad and Skog²²⁾. Data suggest that mice are more vulnerable to acrylamide tumorigenicity. The metabolic activation of acrylamide is more efficient and the detoxification process is poorer in mice than rats, since mice have higher levels of glycidamide and lower levels of GSH–glycidamide conjugates⁽Reference Sumner, Selvaraj and Nauhaus²⁷⁾.

Acrylamide causes induction of the following genotoxic effects⁽Reference Rice^28, Reference Hashimoto and Tanii²⁹⁾:

1. (a) gene mutations and chromosomal aberrations in germ cells of mice in vivo;

2. (b) chromosomal aberrations in germ cells of rats in vivo;

3. (c) chromosomal aberrations in somatic cells of rodents in vivo;

4. (d) gene mutations and chromosomal aberrations in cultured cells in vitro;

5. (e) cell transformation in mouse cell lines;

6. (f) somatic mutation in the spot test in vivo;

7. (g) heritable translocation and specific locus mutations in mice and dominant lethal mutations in both mice and rats;

8. (h) unscheduled DNA synthesis in rat spermatocytes in vivo; but not in rat hepatocytes. However, glycidamide induces unscheduled DNA synthesis in rat hepatocytes.

The studies on mutagenic, genotoxic and carcinogenic properties of acrylamide and glycidamide are summarised in Table 1⁽Reference Hashimoto and Tanii^29–Reference Mei, McDaniel and Dobrovolsky⁴³⁾.

Table 1 Summary of literature on mutagenicity, genotoxicity, carcinogenicity, neurotoxicity, reproductive and developmental toxicity of acrylamide

CHO, Chinese hamster ovary; CYP450, cytochrome 450; ppm, parts per million; GD, gestation day.

Dose–response and dose–effect relationships of acrylamide

Numerous investigators have looked at dose–response and dose–effect relationships in a variety of animal models. There do not appear to be significant differences between the mammalian species studied. The lethal dose, 50 % (LD₅₀) levels of acrylamide through various routes in different rodents are given in Table 2⁽Reference Manson, Brabec and Buelke-Sam⁷⁴⁾.

Table 2 Lethal dose, 50 % (LD₅₀) levels of acrylamide through different routes in different rodents

The WHO states that acrylamide belongs to the group of chemicals thought to have no reliably identifiable ‘threshold’ of effects, meaning that very low concentrations will also result in very low risks, but not in zero risk: some risk is always present when the chemical is ingested. However, for carcinogens such as acrylamide, risk is thought to increase with increasing exposure⁽⁷⁵⁾.

In June 2002, the FAO of the UN and the WHO issued a report about the health implications of acrylamide in food. The Consultation concluded that the no observed adverse effect level (NOAEL) for acrylamide neuropathy is 0·5 mg/kg BW per d and the NOAEL for fertility changes is four times higher than for peripheral neuropathy and 2000-fold greater than estimated dietary exposures⁽Reference Dybing and Sanner^24, Reference Konings, Baars and van Klaveren^{76, 77)}. The NOAEL for reproductive toxicity was estimated to be 2–5 mg/kg BW per d depending on the endpoint of fertility or embryonic death⁽⁷⁷⁾. No reproductive toxicities have been reported in humans. Therefore, it is highly unlikely that any reproductive toxicity in humans would result from dietary exposure to acrylamide, although concerns about the cumulative effects of low-level chronic exposure are increasing.

Evidence of neurological effects has been observed following single oral doses of 126 mg/kg in rats and rabbits and 100 mg/kg in dogs⁽Reference McCollister, Oyen and Rowe^78, Reference Kuperman⁷⁹⁾. Using chronic dosing schedules, it has been observed that cumulative oral doses of 500–600 mg/kg using daily doses of 25–50 mg/kg per d are required to produce ataxia in rats, dogs and baboons⁽Reference McCollister, Oyen and Rowe^78, Reference Thomann, Koella and Krinke^80, Reference Hopkins⁸¹⁾. Smaller daily doses do not produce a clinical effect until a larger cumulative dose is attained. It has been found that the administration of acrylamide at daily doses of 6 to 9 mg/kg does not produce evidence of neurotoxicity in rats until a cumulative dose of 1200 to 1800 mg/kg is attained, and that doses of up to 3 mg/kg per d for 90 d administered to rats do not result in adverse effects⁽Reference McCollister, Oyen and Rowe^78, Reference Fullerton and Barnes⁸²⁾. Spencer et al. ⁽Reference Spencer, Sabri and Schaumburg⁸³⁾ reported that Rhesus monkeys fed up to 2 mg/kg per d did not show any adverse clinical effects at 325 d.

Acrylamide appears to be a multi-organ carcinogen in rodents. This is consistent with its distribution throughout the whole body. However, it is noteworthy that there are no corresponding target organs in mice and rats. Rat thyroid follicular cell tumours and mammary tumours from two studies are considered of possible relevance for human health. Modelling of these data has allowed the determination of benchmark doses and benchmark dose lower confidence limits⁽Reference Johnson, Gorzinski and Bodner^84, Reference Friedman, Dulak and Stedham⁸⁵⁾. For a 10 % CI, the results were in the range of 300–1100 μg/kg per d for the mammary tumours and between 630 and 930 for the thyroid tumours using the Environmental Protection Agency's dose–response modelling for determination of a point of departure for a risk of 10 % lower exposure dose (LED₁₀)⁽Reference Parzefall^86, Reference Shipp, Lawrence and Gentry⁸⁷⁾. Relating the combined tumour data to Hb adduct data of acrylamide and glycidamide the doses 440 or 950 μg/kg per d were determined, respectively, which are close to the benchmark dose lower confidence limits⁽Reference Parzefall^86–Reference Exon⁸⁸⁾.

The estimated average chronic human dietary intake was calculated crudely as 0·3–0·7 μg/kg BW per d in 2002. However, the Food and Drug Administration calculated the exact intake as 0·4 μg/kg BW per d in 2003 and this value remained the same when the Food and Drug Administration updated this subject in 2004⁽⁸⁹⁾. This increase of acrylamide intake by age seems to reflect the increased consumption of fast food together with potato crisps or French fries.

As the NOAEL for neuropathy is given as 0·5 mg/kg BW per d, a woman weighing 132 lb (60 kg) could safely consume 30 mg acrylamide daily; a man weighing 180 lb (82 kg) could consume about 41 mg⁽Reference Rosén and Hellenäs⁹⁰⁾. On the other hand, if the same calculation is applied to the daily intake of a child, a child weighing 20 lb (9 kg) can consume food including 4·5 mg acrylamide. Whether it is appropriate to judge the daily intake of a child with the same calculation is a matter of debate considering that the susceptibility of a child towards several chemicals is much higher than that of an adult. It is anticipated that children will generally have intakes that are two to three times those of adults when expressed on a BW basis.

Studies on acrylamide contamination in baby foods and infant formulas and estimated daily exposures of acrylamide are given in Table 3⁽Reference Hilbig, Freidank and Kersting^26, Reference Alexy, Sichert-Hellert and Kersting^91–94).

Table 3 The daily intake of acrylamide of children in several European countries

BW, body weight; SCF, Scientific Committee on Food.

Determination of acrylamide and limits in foods

Potentially toxic acrylamide is largely derived from heat-induced reactions between the amino group of the free amino acid asparagine and carbonyl groups of glucose and fructose in cereals, potatoes, and other plant-derived foods⁽Reference Friedman and Levin⁹⁵⁾. Several methods have been described that determine acrylamide in several foodstuffs and water. The first reported data on acrylamide in foods available on the market were developed by the Swedish National Food Administration using liquid chromatography/tandem mass spectroscopy (LC/MS/MS)⁽⁹⁶⁾. Since then, several analytical procedures have been developed, including LC/MS/MS and GC/MS with different sensitivity, cost, speed and applicability⁽Reference Rosén and Hellenäs⁹⁰⁾.

There are a limited number of studies concerning the daily intake of acrylamide from foodstuffs. In the year 2002, the Swedish National Food Agency showed that certain fried, baked and deep-fried foods and coffee had high concentrations of acrylamide⁽⁹⁶⁾. In a Norwegian study, acrylamide exposure from foods and coffee was estimated to be 0·49 and 0·46 μg/kg BW in males and females, respectively⁽⁹⁷⁾. The New Zealand National Nutrition Survey showed that with a typical Western diet, including hot chips (French fries) and potato crisps, the calculated daily intake of acrylamide is 0·3 μg/kg BW, which is below the NOAEL⁽⁹⁸⁾.

On the other hand, as babies and children are a more sensitive population than adults, their consumption of foods including acrylamide must be limited. The studies calculating the daily intake of acrylamide of children in several European countries are summarised in Table 4⁽Reference Fohgelberg, Rosén and Hellenäs^11, Reference Rosén and Hellenäs^90, Reference Jiao, Zhang and Ren^99–103).

Table 4 Studies on acrylamide contamination in baby foods and infant formulas and estimated daily exposures of acrylamide

LC/MS/MS, liquid chromatography/tandem mass spectroscopy.

The European Union gives high importance to acrylamide contamination in food. After the Commission Recommendation 2007/331/EC, twenty-one member states of the European Union presented the results of acrylamide in several foodstuffs (presented under the titles of ‘French fries’, ‘potato crisps’, ‘potato products for home cooking’, ‘bread’, ‘breakfast cereals’, ‘biscuits’, ‘roasted coffee’, ‘jarred baby foods’, ‘processed cereal-based baby foods’ and ‘other products’) to the European Food Safety Authority in 2007. There were 2715 results reported for foods sampled in 2007, with a minimum of seventy-six reported for ‘processed cereal-based baby foods’ and a maximum of 854 reported for ‘other products’. The arithmetic mean acrylamide content ranged from 44 μg/kg for ‘jarred baby foods’ to 628 μg/kg for ‘potato crisps’, with the equivalent geometric means of 31 and 366 μg/kg. The 2007 results were compared with results collected by the European Commission Joint Research Centre's Institute for Reference Materials and Measurements in the years 2003 to 2006. There were 9311 results reported for foods sampled in the 4 years; however, there were only eight results reported for the food category ‘jarred baby foods’ and they were not included in the comparison. The arithmetic mean of acrylamide content ranged from 55 μg/kg for ‘cereal-based baby foods’ to 678 μg/kg for ‘potato crisps’, with the equivalent geometric means of 35 and 514 μg/kg. The highest 95th percentile value was reported for ‘potato crisps’ at 1718 μg/kg and the highest maximum for ‘other products’ at 7834 μg/kg⁽¹⁰²⁾.

Conclusion

Infants and children are susceptible towards the effects of several xenobiotics. The knowledge of all factors that affect the formation of glycidamide is important with respect to estimating risk from this known carcinogen. It is known that the metabolism and clearance of drugs are higher in children because of the larger liver:BW ratio and the higher blood flow through the liver compared with those of older adults⁽Reference Greim and Snyder¹⁰⁴⁾. As the major metabolite of acrylamide, glycidamide, is formed through biotransformation, it is likely that glycidamide can be formed at an higher rate than adults in children. In addition, the detoxification pathway through conjugation with GSH may be less functional due to low levels of GSH in a child's liver. Therefore, it is possible that the toxicity of acrylamide might be higher in children.

Children and adolescents should eat a balanced and assorted diet, which includes plenty of fruit and vegetables but also cereals. Also, parents should moderate their children's consumption of fried and fatty foods. Such a diet would also reduce the risk of high exposures to acrylamide, since acrylamide has not been determined in unheated or boiled foods⁽Reference Tareke, Rydberg and Karlsson¹⁰⁵⁾. Acrylamide presumably will also be present in low levels in a nutritionally balanced diet; however, it is in the power of parents to feed their children with more healthy foods. On the other hand, for small children who are merely fed with infant formula and baby food, it is obligatory for governments to take serious measures to monitor contaminants⁽⁸⁹⁾.

Taking into account all the knowledge given above, baby food and infant formulas should be routinely tested before use for the detection of acrylamide using improved techniques. Strict law enforcement should be undertaken regarding the detection of all the contaminants in infant formulas and baby foods. Furthermore, producers should be warned by regulatory authorities regarding good manufacturing practices to decrease the levels of contaminants, particularly acrylamide.