Fish and shellfish are great sources of lean protein, and many types are rich in heart-healthy omega-3 fatty acids. But there's a catch: some species of fish contain worrisome amounts of methylmercury, a toxin that's especially dangerous to developing brains. That's why women who are or could become pregnant and young children shouldn't eat high-mercury fish such as swordfish, shark, king mackerel, and tilefish. A new study hints that eating too much—or the wrong kind—of salmon and tuna can also boost mercury levels.
The Dietary Guidelines for Americans advise us to eat eight ounces of seafood a week (12 ounces a week for women who are pregnant). That would deliver enough omega-3 fatty acids to help brain and nerve growth and protect the heart. But eight ounces is more than double the amount of fish the average American eats in a week.
The new study, published in the May issue of The American Journal of Clinical Nutrition, offers clues about how often and what types of seafood Americans eat, and how that affects mercury levels in the bloodstream. Researchers with the National Health and Nutrition Examination Surveys asked a nationwide sample of 10,673 adults what seafood they had eaten in the previous month. Shrimp was the most popular choice (46%), followed by tuna (34%), and salmon (27%). Only 2% said they had eaten high-mercury fish species.
Most of the participants (95%) had blood levels of mercury in the safe zone—under 5.8 micrograms per liter (μg/L). Not surprisingly, the more fish people ate, the higher the levels of mercury in their blood. Those who consumed swordfish, shark, and other high-mercury fish were the most likely to have blood levels of mercury above 5.8 μg/L. But some who ate only salmon or tuna also had high mercury levels.
Having a blood mercury level of 5.8 μg/L isn't necessarily harmful for an adult, explains Dr. Emily Oken, an associate professor of population medicine at Harvard Medical School who has studied women's fish consumption during pregnancy. "It's very complicated to tease out the harmful effects of mercury because the primary source is from fish, and fish has nutrients that are beneficial to the brain and the heart, the same organs that mercury may harm," she says.
Eat seafood, go for variety
Eating some fish is good. Eating lots of it might not be. Take, for example, the story of IMAX Corporation CEO Richard Gelfond. According to the Wall Street Journal, he ate sushi twice a day for two decades. After noticing numbness, balance, and coordination problems—all signs of mercury toxicity—he was found to have a blood mercury level of 72 μg/L. That's 12 times higher than the safe level.
The health effects of blood mercury levels somewhat above the healthy range simply isn't known. So what's a fish aficionado—or just the average person hoping to follow a heart-healthy diet—supposed to do?
"As a physician, I recommend that people eat fish," says Dr. Oken. Choose different types, and stay away from high-mercury species. If you eat canned tuna, look for chunk light, which is lower in mercury than other varieties. Don’t be overly concerned about salmon; the increased mercury level attributed to salmon consumption was quite small, and it’s possible that salmon eaters also ate more of the other high-mercury fish that contributed to the elevated level, says Dr. Oken. The table below, adapted from a study she led to promote healthy fish consumption in pregnant women, can help you make choices. It features low-mercury varieties and includes how much to eat to get suggested levels of omega-3 fatty acids.
If sustainability of fish populations is a concern for you, check out Seafood Watch from the Monterey Bay Aquarium. It aims to help consumers and businesses make choices for good health and healthy oceans.
Low-mercury seafood choices
There are many different ways to get the recommended weekly amount of omega-3 fatty acids. You can do it with a single meal of fish high in omega-3 fatty acids, or multiple meals of species with lesser amounts. (An expanded version of this table is available in the online Nutrition Journal.)
One 6-oz serving per week of
OR two 6-oz servings per week of
OR three 6-oz servings per week
|salmon (farm raised, wild caught, or canned)
trout (farm raised)
trout (wild caught)