With all the news on potential methylmercury exposure via fish consumption, should we really be scared to eat fish? This question has been raised with increased frequency since the 1956 discovery in Minamata, Japan, of severe, debilitating birth defects resulting from methylmercury exposure via contaminated fish. Motivated by the “Minimata Disease” findings, scientists have made great efforts to study and evaluate methylmercury levels worldwide.
Two study cohorts, focusing on fetuses exposed to methylmercury via their mothers’ consumption of seafood, as in Minimata, have received the most attention: the Seychelles Child Development Study and the Faroe Islands birth cohort. Regulatory agencies have used these longitudinal studies to measure prenatal exposure to methylmercury and assess the risk of adverse neurodevelopmental outcomes, such as impaired motor skills, language capabilities, and behavioral functions.
Launched in the late 1980s, both the Seychelles Child Development Study and the Faroe Island birth cohort study have provided a wealth of information on exposure and risk. Recent re-evaluations of each study cohort, with the children now age 9 and 14 years, respectively, have been performed to determine the continued neurodevelopmental effects of methylmercury. The findings, however, only served to fuel controversy: the Faroese cohort found an association between prenatal methylmercury exposure and neurodevelopmental effects, while data from the Seychellois cohort did not support this hypothesis.
Both studies have received a fair share of criticism as to their methodology, with differences in study findings blamed on study protocols. The cohorts differed in the marker for prenatal exposure (e.g., maternal hair samples versus umbilical cord blood), measurement of neurodevelopmental effects, and several other confounding factors. Maternal hair samples, as used in the Seychelles studies, have been frowned upon because of the difficulty in determining the dose that may be delivered to the fetus and the lack of evidence to confirm that such samples are adequate biomarkers. Cord blood samples, collected at birth, which served as the prenatal biomarker for estimating methylmercury exposure in the Faroe Islands cohort, are criticized for two reasons: of the 50-day half-life of cord blood and the inability of cord blood to reflect levels of methylmercury during the first trimester due to fluctuations caused by binge eating. Currently, there is no consensus on the most appropriate biomarker for assessing prenatal exposure to mercury.
Another contenious issue between the cohorts is the form of methylmercury exposure and the resulting potential confounders. Methylmercury exposure among the Seychellois is primarily via contaminated fish, while the Farose consume whale meat and blubber in addition to fish. Proponents of the Seychelles cohort claim that as exposure to methylmercury is primarily via fish consumption, the study findings are more reflective of potential health events in other countries. In contrast, the Faroese diet includes whale meat and blubber that have been found to have higher levels of methylmercury (up to 3 ppm) due to bioaccumulation. A further confounder with consumption of whale meat is the exposure to other environmental contaminants, such as PCBs and dioxins, both of which have potential for adverse human health effects, including developmental neurotoxicity.
Interestingly, amongst the null results of the latest Seychellois report, researchers found a positive association between methylmercury exposure and better performance on one of the neurodevelopmental tests. This finding could be a yet another confounder, i.e., that increased methylmercury exposure served as a marker for increased fish consumption and better nutrition, in the form of protein and omega-3 fatty acids. With all these potential confounders and differences in methodology, it is difficult to have a solid argument that fetal mercury exposure via normal fish consumption is associated with neurodevelopmental effects.
That said, should people avoid the consumption of fish and exclude this valuable source of protein and fatty acids from their diets? To address this potential question, the FDA has imposed the action level of 1 ppm for methylmercury in the edible portion of fish – a level that would limit consumers’ methylmercury exposure to one-tenth of the lowest level associated with adverse effects observed in the poisoning incidents. The FDA has established this level so that consumers may enjoy the benefits of eating fish, while the scientific community continues to debate the methylmercury issue.