We observed an inverse association between acrylamide to hemoglobin adducts and birth weight and birth head circumference. The association was similar in newborns of mothers who did not smoke during pregnancy.
There was a stronger association between acrylamide’s metabolite glycidamide and birth weight and head circumference and glycidamide was additionally inversely associated with birth length. Glycidamide was stronger inversely associated with birth outcomes among newborns of non-smoking mothers, particularly for birth weight and head circumference.
Although with no statistically significant interaction between acrylamide and genotype, the inverse association between acrylamide exposure and birthweight and birth head circumference was stronger in children with a homozygous wildtype genotype for the studied SNPs in CYP2E1 and children with at least 1 variant allele in rs1138272 in GSTP1.
The order of magnitude of the effect size of the association between gestational acrylamide exposure and fetal growth in our study is similar to that of maternal smoking. For instance, the effect estimate in the highest quartile of glycidamide adducts (–225 grams for birth weight, –0.81 for length and –0.63 for head circumference in neonates from non-smoking mothers) is similar to the effect size of maternal smoking in our study (–219 grams for birth weight, –0.97 for length and –0.45 for head circumference).
Up to now, 3 other studies have investigated the association between acrylamide hemoglobin adduct levels and birth outcomes, among which was 1 study that also used acrylamide and glycidamide to hemoglobin adducts as a biomarker of exposure. All 3 studies observed an inverse association between gestational acrylamide exposure and indicators of fetal growth.4–6 The consistency between all 4 studies that have investigated the association thus far and between the studies using food frequency questionnaire data and biomarkers is rather remarkable. All 4 studies adjusted largely for the same covariables that we adjusted for in this study.
Out of the 3 studies, including the current study, that investigated the link with birth head circumference,5,6 ours is the second study to observe an inverse association between prenatal acrylamide exposure and head circumference.
We did not observe statistically significant interaction between acrylamide and genetic variants in genes involved in acrylamide metabolism. However, there were clear differences in the strength of the association between acrylamide and birth weight and head circumference between genotypes of CYP2E1 SNPs and a GSTP1 SNP (rs1138272) in newborns of non-smoking mothers. For the CYP2E1 SNPs, these differences suggest that glycidamide may be more important with regard to fetal growth than acrylamide because the wildtype allele of the studied SNPs is thought to have a higher enzymatic activity than the variant allele.10–12 A stronger effect of glycidamide than of acrylamide is reflected by the observation that the associations between glycidamide and birth outcomes in the current study were stronger than the associations with acrylamide. Strikingly, analyses on acrylamide and endometrial and ovarian cancer risks in a prospective cohort study showed that non-smoking women who were homozygous for the wild type CYP2E1 alleles (of rs2480258, rs915906 and rs11101888) had higher risks of both endometrial and ovarian cancer than women with variant alleles for those 3 SNPs.13 This pattern is similar to what we saw in the current study, where newborns of non-smoking mothers who were homozygous wildtypes for the same 3 SNPs showed stronger associations with birth weight and head circumference than newborns with variant alleles for these SNPs.
GSTP1 is an important phase II enzyme that protects against oxidative stress and it is involved of the metabolism of acrylamide, generating mercapturic acid derivatives of acrylamide and glycidamide that are excreted with the urine. The variant allele of the rs1138272 SNP probably leads to a reduced enzymatic activity of GSTP1.14 Therefore, it was expected that newborns with variant alleles of rs1138272 would show a stronger association between acrylamide and fetal growth.
We did not observe clear differences between the studied genotypes with regard to the glycidamide to acrylamide hemoglobin adduct ratio in cord blood. CYP2E1 gene transcription is induced or inhibited by numerous factors, such as liver function, fasting, obesity, alcohol intake, medication and various dietary factors.15 In addition, post-transcriptional processes have an important influence on CYP2E1 enzyme activity. A more subtle effect of the genotype may be masked by those other factors and processes that influence enzyme activity.15 Nevertheless, Pelle et al. found that carriers of the variant allele (T allele) of rs2480258 in CYP2E1 had a decreased enzyme activity, with a reduced CYP2E1 expression phenotype at the mRNA, protein, and CYP2E1 enzyme activity level. 10 In a separate study, they observed a decreased glycidamide to acrylamide ratio in homozygous carriers of the T allele.16 Duale et al. did not observe differences in the ratio of glycidamide to acrylamide for 2 SNPs in CYP2E1 (rs6413419 and rs2515641); rs6413419 is a missense mutation. In addition, they did not observe a difference in the ratio of glycidamide to acrylamide for rs1051740 in EPHX1, similar to our study.17 Also similar to our study, Doroshyenko et al. did not observe a difference in the ratio of glycidamide to acrylamide among genotypes of rs1695 and rs1138272 in GSTP1 in human volunteers but the sample size of their study was very small (n = 16).18 In a study on 51 persons occupationally exposed to acrylamide, Huang et al. observed no influence on the glycidamide to acrylamide ratio of variant alleles of the CYP2E1*5 SNP (rs2031920/rs3813867) nor of variant alleles of the rs1051740 SNP in EPHX1.19
Although the interactions between acrylamide and glycidamide and genotypes in the current study were not statistically significant because of the limited sample sizes and therefore have to be interpreted cautiously, they do point towards a role of CYP2E1 and GSTP1 in the association between gestational acrylamide exposure and fetal growth and thus give some suggestion that the association could be causal.
Currently, it is only possible to speculate about the possible mechanism or mechanisms behind acrylamide’s possible adverse effect on fetal growth. Both acrylamide and glycidamide can bind to thiol groups in proteins and when those thiol groups are in important positions, the function of the protein may be impaired.20 However, it is unknown what proteins could be involved in acrylamide’s putative adverse influence on fetal growth. Acrylamide has been shown to be associated with reduced serum insulin levels in both rats21 and human adults.22 Insulin is a growth factor that plays an important role in fetal growth. Acrylamide was also shown to decrease levels of plasma thyroid hormones in rats,23,24 and among Taiwanese adolescents, there was an inverse association between levels of a urinary acrylamide metabolite and serum free thyroxin (T4).25 Thyroid hormones are also involved in fetal growth and development. Acrylamide may impair the function of enzymes involved in insulin and thyroid hormone regulation.
Our study has some limitations. We cannot exclude the possibility of residual confounding by factors that are associated with both acrylamide and glycidamide hemoglobin adduct levels and birth outcomes, such as another dietary exposure or a generally less heathy diet. However, when we additionally adjusted the analyses for variables that can be thought of as proxies for a healthy or unhealthy diet (consumption of vegetables, fruits, fish and soda drinks), the associations were virtually unchanged. In addition, associations were found in different study populations in which the dietary sources of acrylamide differ and were still present after adjustment for confounders and after exclusion of children from smoking mothers. Therefore, it is not likely that other factors could fully account for the observed associations. In addition, the possible effect modification by SNPs in CYP2E1 eliminates a long list of potential confounding exposures that are not influenced by metabolism by CYP2E1.
In addition, we may have had too limited statistical power to observe statistically significant differences in the association between acrylamide hemoglobin adduct levels and birth outcomes in subgroups of genotypes.
Our study also has specific strengths. Our findings are generalizable as our study population is representative for the gestational segment of the population at large.7 Another advantage of our study was the use of acrylamide biomarkers. Biomarkers should not be assumed to always be superior to other methods of acrylamide exposure assessment, such as food frequency questionnaires, because a biomarker may not represent the exposure during the relevant period for disease etiology well, e.g., due to intra-individual variations of exposure in time. However, the hemoglobin adduct biomarker may well be superior in the case of a study on fetal growth because the hemoglobin adducts of acrylamide and glycidamide represent the exposure during the last 4 months of pregnancy. This part of the pregnancy is the period in which most growth takes place. The effects sizes in the study of Pedersen et al.6 and in the current study, which are the 2 studies that have used acrylamide biomarkers, are quite comparable but they are larger than the effect sizes in the studies that used food frequency questionnaires to estimate acrylamide intake. This may be due to the greater precision of the acrylamide exposure estimate that is reached by using the biomarker. Interestingly, the 2 biomarker studies were also the studies that observed an inverse association with birth head circumference, whereas the study that used a food frequency questionnaire did not.5
Our study strengthens the body of evidence that acrylamide intake at current dietary levels may have developmental effects. According to the Developmental Origins of Health and Disease hypothesis, suboptimal prenatal development likely predisposes to inferior health throughout life.26 Reduced fetal growth has been associated with among other increased incidences of diabetes mellitus (type 2), obesity and cardiovascular disease.27 Birth head circumference is an indication of fetal brain growth, and several studies have shown associations between decreased birth head circumference and decreased cognitive skills in childhood, even among the children within the normal range of birth size.28 It has also been shown to be associated with an increased risk of attention deficit hyperactivity disorder (ADHD).29 The association between acrylamide exposure and reduced birth head circumference suggests that prenatal acrylamide exposure may have cognitive effects in later life. This study suggests that reducing acrylamide exposure during pregnancy can be beneficial for child development.
Further studies on the association between acrylamide hemoglobin adduct levels and birth outcomes in newborns, and the interaction with CYP2E1 and GSTP1 SNPs are needed to better understand the potential effects of acrylamide on fetal growth. Furthermore, more epidemiological research is needed on possible mechanisms of action underlying the association between prenatal acrylamide exposure and fetal development. We strongly encourage specifically epidemiological studies on this topic because the animal data suggested that no developmental effects on humans would occur at dietary doses.1