We have all heard about the nutritional benefits of fish, especially those fish high in beneficial omega-3 fats. But a diet high in fish has one often-overlooked downfall—mercury exposure. Mercury is a heavy metal that accumulates in fish up the food chain. Small fish are eaten by medium-sized fish, which are then eaten by large fish, all the while mercury bioaccumulates in higher and higher concentrations. People who eat a diet high in fish—especially large fish—are at increased risk of high mercury exposure well above the Environmental Protection Agency’s reference intake level of 5.8 µg/mL.

In a recent study published in the journal Integrative Medicine, researchers studied 384 healthy, well-educated men aged 23 to 65 in an effort to understand whether fish intake was associated with a protective or negative effect on cognitive function, and how mercury levels related to this finding. Their findings were interesting. In men with modestly elevated mercury levels who had an increased intake of fish, the researchers actually found the highest cognitive function. These findings suggest that the nutrients obtained from moderate fish intake may outweigh the negative impact of increased mercury consumption.

The researchers also found, however, that as fish intake increased—of tuna, grouper, snapper, bass, swordfish, and shark, in particular—mercury levels rose above 15 µg/mL and so did cognitive dysfunction. “This finding suggests that high levels of mercury can overwhelm the protective effects of omega-3 fatty acids,” stated the researchers. They caution individuals with a high fish intake, “in particular people who eat more than three servings of fish weekly or more than three to four servings per month of large-mouthed fish [mentioned above].”

Studies on the harmful effects of mercury are conflicting, and more research is needed to determine what levels pose risk. Although this study found that a moderately increased mercury level did not negatively harm cognitive health, the participants of the study were already very healthy, so we do not know what effect moderately elevated mercury levels would have on people in less-than-perfect health. Also, we only know what effects the mercury had on particular cognitive tests. It may affect other areas of health that were not fully investigated in this study.

I have always been in favor of minimizing exposure to mercury. If you want to eat fish, stick with salmon, sardines, and herring, three fish known to be highest in omega-3 while also being low in mercury. And to be sure you are getting enough omega-3, take a high-potency, purified fish oil supplement.