Unimpaired Neurodevelopment in Adolescents of Mothers with High Prenatal Fish and Methylmercury Intakes

The benefits of fish consumption during pregnancy, which provides nutrients and long-chain omega-3 PUFAs (n-3 LC-PUFAs), consistently outweigh the potentially harmful effects related to the exposure of environmental contaminants, particularly methylmercury, in fish and shellfish. Not all women and health care providers see it that way, but scientists familiar with the literature do. Evidence from populations in the Arctic and Seychelle Islands who consume large amounts of ocean fish indicates that children exposed to high DHA intakes and environmental contaminants from seafoods have better visual acuity into late childhood and exhibit no clinically meaningful deficits in several neurodevelopmental assessments. Some developmental scores were positively associated with prenatal methylmercury exposure, presumably a reflection of higher nutrient intakes.

The Seychelles Child Development Study has continued to examine the cognitive and behavioral outcomes of the children recruited into the original cohort. The offspring are now 17 years of age and investigators continue to evaluate the effects of their prenatal exposure to methylmercury and nutrients. Of the original 779 infant-mother pairs, 705 adolescents were eligible for evaluation and 600 participated. Recent postnatal exposure to methylmercury was measured in a 1-cm length of hair taken close to the scalp. Prenatal methylmercury exposure was measured in maternal hair. The participants’ average age was 17.1 years (range 15.7 to 18.4 years). Behavior assessments were ascertained using a confidential questionnaire adapted from World Health Organization and US Centers for Disease Control and Prevention, the California Verbal Learning Test and 5 subtests of the Woodcock-Johnson Test of Scholastic Achievement-II. School records of problem behaviors were also included. Additional computer-based tests included subtests from the Cambridge Neuropsychological Test Automated Battery (CANTAB) and the Wisconsin Card Sorting Test.

Primary end points for each of the evaluations were identified and examined for their association with prenatal methylmercury exposure. All statistical models were adjusted for sex, socioeconomic status, maternal intelligence and recent postnatal methylmercury exposure. Other adjustments in the analysis of neurocognitive endpoints and behaviors were made, such as the child’s age at testing and IQ at 107 months of age. Only individuals with a full set of covariate data were included in the analyses.

Increasing exposure to prenatal methylmercury was associated with fewer trials to complete the Cambridge Neuropsychological Test Battery and with improved Woodcock-Johnson Scholastic Achievement scores, as shown by regression analysis. These observations suggest enhanced problem-solving ability. There is no reason to think that methylmercury itself improves performance, as the authors pointed out, so it is more likely that higher methylmercury exposure reflects fish or nutrient intake. The authors suggest that n-3 LC-PUFAs would be likely candidates for the associations with improved cognitive performance.

Regression analysis also showed improved scores with higher prenatal methylmercury exposure for several behavioral assessments. Significant associations were observed for fewer reports of substance abuse especially for females incurring the highest number of reports (5 and 6 reports per year) and for a reduction in the highest category of behavioral incidents (≥ 13 incidents per year) reported to a school counselor. The only adverse association was an increase in the lowest category of referrals (1 to 3 per year) to a counselor with higher prenatal methylmercury exposure. Overall, there was improved performance or no association with prenatal methylmercury exposure in 26 of the 27 primary outcomes assessed in this study.

The compelling aspect of these observations is the lack of evidence for harmful cognitive or behavioral outcomes associated with high levels of prenatal methylmercury exposure from maternal fish consumption. Years of follow-up studies with mother-child cohorts in the Seychelles Islands have documented “no consistent pattern of adverse associations between prenatal [methylmercury] exposure and developmental outcomes.” These observations are important because the Seychellois consume 10 times more seafood from a variety of ocean species as US women eat. In spite of having high hair methylmercury content (6.9 ppm), which is greater than in the Faeroe Islands (4.5 ppm) or the U.S. (0.36ppm or 0.2 ppm according to NHANES data), the offspring of Seychellois mothers exhibit no clinically meaningful adverse effects of prenatal methylmercury exposure primarily from eating fish.

These findings are important for supporting the safety of eating a variety of ocean fish and shellfish. All fish contain some methylmercury, although the amount varies with the species and age of the fish. Methylmercury is a potent neurotoxin found mainly in fish and shellfish. However, two large cohort studies have reported no adverse effects in infant and child neurodevelopment from maternal consumption of fish during pregnancy. Consumption of marine mammals (e.g., pilot whale) and very large predator fish (e.g., some sharks) with high levels of methylmercury may increase the risk of adverse effects of methylmercury exposure. However, these foods are not generally found in the seafood marketplace and can be avoided. Seafood contains an array of nutrients that contribute to the health benefits associated with seafood consumption, some of which, e.g., selenium, may mitigate the potential harm of methylmercury. Data such as these from the Seychelles Islands provide evidence that eating a variety of ocean fish and shellfish species supports rather than harms human health.

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