We found improved performance or no association between prenatal MeHg exposure and 26 out of 27 primary endpoints measured in this study. We found only one adverse association with prenatal MeHg and this appeared in the lowest category of a multi-category endpoint (referrals to a school counselor), but there was no association for higher categories that represented more frequent referrals. These results do not support the hypothesis that prenatal MeHg exposure at levels achieved by fish consumption adversely affect adolescent development and are consistent with our earlier reports. On repeated and increasingly more detailed and extensive evaluations, we have found no consistent pattern of adverse associations between prenatal MeHg exposure and developmental outcomes. We have now studied over 70 primary and secondary endpoints including tests of global and specific neurocognitive domains, achievement and problematic behaviors. This finding is reassuring because the MeHg exposure in the SCDS Main Cohort is one of the highest in the world, over ten times that of samples in the United States (Schober et al., 2003
; McDowell, et al., 2004
) and Sweden (Björnberg, et al., 2003
). Our cohort’s mean prenatal hair MeHg level of 6.9 ppm was also higher than the median of 4.5 ppm reported in the Faeroe Islands Study (Grandjean, et al., 1997
As in our earlier studies (Davidson et al., 1998
, Myers et al., 2003
), we found improved performance with increasing prenatal MeHg exposure for some endpoints. There is no reason to believe that MeHg exposure at any concentration should improve performance, so it is most likely that prenatal MeHg measurements are a surrogate marker for fish consumption. For humans, fish is the primary source for some LCPUFA that are essential for neural development. The human body has a very limited ability to synthesize the n-3 LCPUFA docosahexaenoic acid (DHA) and eicosapentanoic acid (EPA), and fish is the primary human dietary source. These n-3 LCPUFA have been shown to improve cognitive and behavioral function in young children (Decsi and Koletzko 2005
, Dunstan et al., 2008
, Helland et al., 2003
, Judge et al., 2007
). Although maternal nutrient levels were not measured in this cohort, we did measure them in another SCDS cohort. In that study we found beneficial associations between total maternal n-3 LCPUFA and the BSID-II PDI score at 9 months and the association was further strengthened when the prenatal MeHg exposure level was included in the regression model (Davidson et al., 1998
). That study also found an adverse association with prenatal MeHg when nutrients were included in the model. Other studies have reported associations between total fish consumption and developmental outcomes, but they did not specifically measure LCPUFA (Davidson et al., 2006
, Decsi and Koletzko 2005
, Dunstan et al., 2008
, Schober et al., 2003
If improved performance with increasing prenatal MeHg exposure indeed reflects the influence of n-3 LCPUFA or other benefits associated with fish consumption on development, then these findings would suggest a modest but prolonged duration of such benefits. Improved scores on the W-J-II Calculation subtest and CANTAB IED total trials indicate enhanced problem solving ability. Our findings indicate benefits that are behavioral domain specific and extend to behaviors likely to positively influence a successful life trajectory.
Recent postnatal MeHg has been included in our analyses since fish consumption in Seychelles starts early and by age 5 years is fairly consistent. We have reported both adverse and beneficial associations with postnatal MeHg exposure when the Main Cohort was evaluated at earlier ages (Myers et al., 2009
). There was an adverse association with one of 27 endpoints in this study and likely represents a chance finding. We are cautious in interpreting associations with postnatal exposure since the SCDS was designed to determine associations with prenatal exposure. In addition, our postnatal measure is a convenience sample representing only one month exposure and there is no accepted metric of postnatal exposure.
Strengths of the current study include its high prenatal MeHg exposure, large cohort, high retention rate, longitudinal design, repeated testing at multiple ages and extensive characterization of the Seychellois study population. In addition, the tests we used have both clinical and environmental validity and include ones that are behavioral domain specific (Davidson et al., 2006
). One limitation of this cohort study is the absence of maternal nutritional information which would permit a direct assessment of prenatal nutrition influence on subjects’ development. We did not assay any biomarkers for maternal nutritional status during pregnancy, such as LCPUFA, nor did we collect any information on dietary behaviors. Fish are also high in selenium and when prepared in palm oil, as is the practice in the Seychelles, that may increase levels of vitamin E. Both selenium and vitamin E may act as antioxidants and reduce oxidative stress resulting from MeHg. These issues are being addressed in other Seychellois cohorts (Davidson et al., 2008
, Strain et al., 2008
The current findings are reassuring regarding the rewards and risks of fish consumption and indicate that exposure to MeHg during pregnancy from consumption of ocean fish is not adversely associated with neurocognitive and behavioral outcomes measured in late adolescence. Our findings should be of interest to policymakers and regulators as they formulate public health guidance concerning ocean fish consumption during pregnancy. Current guidelines appear to be limiting fish consumption by pregnant women (Bloomingdale et al., 2010
, Oken et al., 2003
) and thereby depriving the fetus of nutrients important to the developing brain.