The Mercury Question: How to Understand Risk vs. Reward When Eating Fish

Tuna steak
Do the benefits of eating fish outweigh the risks of potential mercury poisoning?

Should you stop, or drastically reduce, eating fish to avoid mercury? Depending on what you read or watch on TV, the answers might be contradictory. According to James K. Hammitt, program director of Environmental Health Risk: Analysis and Applications, the risk analysis isn’t as simple as avoiding fish entirely—in fact, people are ultimately missing out on key nutrients when taking that approach.

The Current FDA Recommendations of Mercury in Fish

Eating fish has significant benefits. In fact, fish are often considered the healthiest protein one can eat, particularly because of nutrients like vitamin D and selenium. Omega-3 fatty acids can help protect against heart disease, reduce inflammation, and improve organ health, among other things.

The FDA has documented which species of fish, on average, have the highest levels of mercury. Generally speaking, depending on the amount of consumption, fish like swordfish, shark, and certain kinds of tuna—large fish that eat a lot of smaller fish—have higher levels of methylmercury (the only form of mercury to bioaccumulate in the human body). At very high exposures, this can lead to fatigue, muscle weakness, and dizziness and damage organs like the kidneys and liver. Chronic low-level exposure to methylmercury has also been shown to impair brain function.

“There are benefits to eating fish, and there’s nothing good about having methylmercury in them. It may be true that there’s value in eating mercury-rich fish, but it would be better to eat mercury-poor fish,” says Hammitt.

So, as far as formal advice, one should avoid heavy consumption of those specific species, and instead eat food lower on the food chain that weren’t caught in areas contaminated by high methylmercury levels. In other words: eating fish has healthy benefits, and the fear of mercury shouldn’t prevent you from choosing fish as a food source.

There are benefits to eating fish, and there’s nothing good about having methylmercury in them. It may be true that there’s value in eating mercury-rich fish, but it would be better to eat mercury-poor fish.

Specific Recommendations About Methylmercury for Pregnant Women

In 2001, the FDA came out with mercury-in-fish recommendations in relation to women who were pregnant, and cautioned that they should avoid exposure to methylmercury. It’s now well-known that there are negative effects on children if their mothers were exposed to high amounts during pregnancy.

Some research indicated that households that ate a significant amount of seafood decreased fish consumption by 21%, and that this led to a 17% reduction in exposure to mercury, but there was a cost—a 21% reduction in omega-3 fatty acids. However, other research showed that exposure declined significantly in women of childbearing age who were the most highly exposed to methylmercury, but that their seafood consumption remained similar as before. In other words, the latter research shows that the advisory was effective at helping pregnant women curb their methylmercury intake, but they continued to reap the benefits.

One of Hammitt’s studies suggests that if women of child-bearing age avoided all fish consumption for many years (and if men of child-bearing age do as well), the net health harms to adults can offset the benefits of reducing cognitive risk to children if women reduce their fish consumption.

Thus, eating fish, or taking n-3 fatty acid supplements, provides clear benefits; it’s the methylmercury in the environment, and subsequently the fish that live in that environment, that’s the problem.

If you’re sitting at a sushi restaurant wondering what to eat, one meal isn’t necessarily going to make a difference.

The Risks of Too Much Methylmercury to the Body

Mercury poisoning from fish does occur. One legal case—centered around actor Jeremy Piven—was connected to his alleged mercury poisoning from eating too much sushi. In that case, Piven apparently ate sushi twice a day for years according to his doctor. In a medical case involving Richard Gelfond, he too ate significant amounts of fish in the service of eating a healthier diet. In his case he continues to suffer aftereffects and symptoms, even after he changed his diet to remove the relevant fish from his diet.

So, the risk is present and the impact can be significant. However, “If you’re sitting at a sushi restaurant wondering what to eat, one meal isn’t necessarily going to make a difference,” says Hammitt.

Studying the negative impacts can be nuanced. There’s evidence that mercury increases the risk of heart attacks, more significant than the potential cognitive effects for infants. But fish provide nutrients that can help ward off heart disease. Then again, the nutritional benefit of fish plateaus, but the risk associated with methylmercury continues to increase at high exposures.

Cooking is known to reduce mercury content in fish by up to 30 percent. Additionally, the half-life of methylmercury is only two months for human beings and less for other species, so large intake of those species in the past isn’t necessarily an irreversible problem.

Monitoring methylmercury in the environment, especially as climate change continues, is essential to ensure that the positive benefits from fish continue.

The Unknowns of Methylmercury in Fish

Unfortunately, as the oceans warm due to climate change, there’s research led by Elsie Sunderland (who also teaches at Environmental Health Risk) to suggest that certain kinds of fish in certain environments like the Gulf of Maine now have increased mercury levels. Thus, the levels, and the recommendations, may change over time, and it’s critically important to try and remove mercury and carbon dioxide from the environment.

Monitoring methylmercury in the environment, especially as climate change continues, is essential to ensure that the positive benefits from fish continue. Global warming may act as a sort of wild card, throwing off existing numbers and changing the makeup of fish’s food sources—particularly for large, predator fish. Smaller fish, and vegetarian fish, have less risk of exposure.

It’s important for the individual person to watch the scientific community for updates as well as local advisories about the safety of fish, and also to pay attention to what goes in one’s body. With a bit of awareness and careful consideration, one can still keep fish in one’s diet. But, reductions in both mercury and carbon dioxide are needed to protect the integrity of seafood as an important food source.


Harvard T.H. Chan School of Public Health offers Environmental Health Risk: Analysis and Applications, which explores the basics of health risk assessment and applications of risk assessment to diverse problems.