Study: The Heart Benefits of Fish May Outweigh the Mercury Risks

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Caren Brinkema

Eating fish is good for your heart, but there’s a catch: the fish richest in good-for-you omega-3 fats also tend to harbor the most mercury, which can be harmful for brain development, particularly among young children. That’s why pregnant women and women planning on starting a family are advised to limit their consumption of fish. Further, animal studies have linked high doses of mercury to some types of cancer and heart problems.

So does it make sense to eat fish, or would it be better to avoid the seafood counter entirely? A study of more 173,000 men and women may help you decide; the trial shows that mercury exposure from eating fish does not appear to increase heart-related problems.

The study’s scientists focused their attention on mercury in toenail clippings — yes, toenail clippings! — obtained from the participants. Mercury levels in toenails may seem an odd gauge, but it makes sense because they are a reliable source for measuring long-term exposure to the organic compound.

The researchers found that while people who reported eating more fish did indeed show higher levels of mercury in their nails, they did not suffer from any increase in heart disease or stroke. In fact, those with the highest concentrations of mercury enjoyed a 15% lower risk of heart disease over an average of 11 years of follow-up compared with those with the lowest concentrations.

The findings should reassure people who enjoy seafood — particularly deep-sea species such as tuna, swordfish and shark — for its heart-healthy benefits. The U.S. Dietary Guidelines recently released by nutrition experts also recommended for the first time that all Americans include at least two servings of fish each week in their diet, because the omega-3 fatty acids can lower risk of heart disease. But because of the animal studies on mercury’s potential health hazards, as well as conflicting results from smaller human studies, it hasn’t been easy to get comfortable with this advice.

“We were left with this big question mark on whether men and the majority of women in the population who are not pregnant or planning on becoming pregnant, should be concerned about mercury levels in fish,” says the study’s lead author Dr. Dariush Mozaffarian, a heart expert at the Harvard School of Public Health and Brigham and Women’s Hospital. “But in fact, in as robust a way possible, at levels of U.S. fish consumption, we saw no evidence of cardiovascular harm from mercury.”

What Mozaffarian’s group did see was that the omega-3 benefits of fish may be outweighing the small risk associated with mercury that might be contaminating the fish. “I think this really shuts the door on the worry about heart risk or toxicity to the heart at these exposure levels,” he says. “And that’s an important message, because there may be a lot of adults who are at risk of heart disease who may be confused or concerned about eating fish, or are shying away from fish because of their concerns about mercury. Our study suggests they don’t need to worry about heart risks.”

People with the highest amount of mercury, he estimates, may be consuming deep-water fish at least once a week, while those with the lowest levels may be eating fish only a few times a month.

But Mozaffarian isn’t suggesting that government health officials should stop monitoring fish for mercury contamination — indeed, even small amounts of mercury ingested from fish can cause temporary symptoms such as numbness and tingling in the limbs — so efforts by environmental authorities to lower mercury levels in deep-water fish should continue, he says.

When it comes to heart effects, however, at current levels of consumption, Americans don’t seem to be at any increased risk. Rather, people should be aware that eating fish can actually protect their heart and lower their risk of heart attack and stroke. “These are big killers in our society, and people should not avoid eating fish because of the concern over mercury,” he says.

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