Mercury in Seafood: Bad for Adults, Too

Eating lots of fish? You're probably eating lots of mercury, and that could be bad for your heart.

April 28, 2014

If you're eating fish more than once a week, you could have mercury levels high enough to put you at risk for some serious heart complications, finds a new study published in the American Journal of Clinical Nutrition.

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It's one of those ironies of nutrition: Eating fish high in omega-3 fatty acids is reportedly beneficial for your long-term heart health, but those same fish are often larger predatory fish, such as tuna and salmon, that are also high in mercury. And while moms and moms-to-be have long been warned that eating high-mercury fish could put their kids at risk of neurological problems, adults aren't fully aware of the metal's heart-damaging effects.

The new study was fairly basic and intended to find out whether adults who eat a lot of fish also have higher-than-average mercury levels. So they compared food-frequency questionnaires with blood measurements of the metal and found that, yes, frequently, that's the case. Adults who eat any kind of seafood at all more than five times a month, about 40 percent of their study sample, had mercury levels that were 41 percent higher than people who ate fish just once or twice a week.

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The difference was much smaller in people who said they ate shellfish only (think shrimp, crabs, lobster, mussels and oysters, among others.), but much greater in people who said they ate fish only; that category included tuna, salmon, mackerel, Pollock, swordfish, and other varieties of big fish, many of which are known to be high in mercury. In that group, mercury levels in people who said they ate fish only more than five times a month were nearly double those in people who ate fish just once or twice a month.

The study didn't ask how much fish these people were eating, and it also didn't make distinctions between species that may or may not have high levels of mercury depending on variety and where they were caught. For example, albacore and Bluefin tuna have very high levels of mercury, as do tuna caught in the Atlantic and Gulf of Mexico, whereas canned light tuna and tuna caught in the Pacific have lower levels. And all those factors can influence how much mercury you consume when you eat fish. 

Regardless, their results raise a few red flags. People with the highest fish consumption averaged more than 5.8 micrograms/liter (mcg/L) mercury in their blood, enough to do damage, according to a 2012 study from Syracuse University. In that study, the researcher found that levels around 3.27 mcg/L limited levels of cortisol, a stress hormone. Although that sounds like a good thing, cortisol prevents your body from generating an inflammatory response, so if it can't do its job, inflammatory diseases, such as cardiovascular disease and high blood pressure, can flourish.

The good news? You can have your omega-3s and your heart-health too. It's just a matter of eating fish that are both high in fatty acids and low in mercury.

Your best bets for those, according to a recent analysis by the nonprofit Environmental Working Group:
• Anchovies
• Herring
• Mussels
• Wild Alaskan salmon*
• Sardines
• Shad
• Trout

*There's some evidence that farmed Atlantic salmon have detectable levels of mercury, although not nearly as much as other high-mercury fish, such as albacore tuna or swordfish. Stick with wild Alaskan salmon to be on the safe side; it's also a more sustainable choice than farmed salmon.