Methylmercury and omega-3 fatty acids: co-occurrence of dietary sources with emphasis on fish and shellfish.

Despite many claims of broad benefits, especially for in utero development, derived from the consumption of fish as a source of omega-3 fatty acids, individual species of fish and shellfish provide substantially varied levels of these fatty acids. Likewise, mean methylmercury (MeHg) concentrations for fish and shellfish species differ by greater than an order of magnitude. Consideration of within-species variability would increase this variation farther. Exposures to both MeHg and to the omega-3 fatty acids reflect dietary choices including species consumed, frequency of consumption, and portion size. In view of these sources of variability, data on dietary patterns and blood mercury (microg/L) among women of child-bearing age (e.g., 16-49 years) provided an indication of exposures in the United States. Utilizing data from the National Health and Nutrition Examination Survey (NHANES) for survey years 1999--2002, calculated consumption of MeHg and omega-3 fatty acids from fish and shellfish have been estimated based on results from 3614 women who provided 30-day dietary recall and 24-hours records. Statistics from NHANES when appropriately weighted are representative of the US population. The association between dietary MeHg from fish and shellfish and dietary fish intake yielded a Pearson correlation of 0.68. The Pearson correlation between estimated 30-day intake from fish/shellfish consumption for omega-3 fatty acids and MeHg was 0.66. Evaluation of the most commonly consumed fish and shellfish species as sources of MeHg and omega-3 fatty acids indicated that salmon followed by shrimp are principal sources of omega-3 fatty acids and are lesser sources of MeHg, in contrast with tuna which provides omega-3 fatty acids, but considerably higher levels of MeHg. These data can be used to guide selection of individual fish and shellfish species that are higher in omega-3 content and low in MeHg concentrations. This more refined dietary approach contrasts with generic recommendations that simply advise increasing fish consumption as a path toward improving cardiovascular health and providing benefits for in utero development or avoiding fish altogether.