Fish Oil and Toenails

By Sanjay Gupta

If you're confused by all the news about the health effects of eating fish, you're not alone. On one hand, the omega-3 fatty acids in fish are known to reduce the risk of heart disease, as the American Heart Association reminded us two weeks ago when it restated its recommendation that everybody eat at least two fish servings a week. On the other hand, fish that feed in contaminated waterways contain high levels of mercury, which can lead to cognitive problems in developing brains. That's why pregnant women and nursing mothers are advised to limit their consumption.

As if that weren't confusing enough, two new studies published last week in the New England Journal of Medicine investigated the possible effects of mercury on the heart, and they seem to have reached contradictory conclusions. One found no clear link between mercury levels and heart disease; the other found that men with high levels of mercury in their toenails were more likely to suffer a heart attack than those with low levels.

What are we to make of this? The first thing to remember is that this is how science proceeds, by fits and starts and seemingly contradictory results that get resolved only by further study. The second is that not all fish are created equal.

Compared with all the other things you might eat, fish are an excellent source of protein. They tend to eat algae as part of their natural life cycle, converting it into omega-3 fatty acids that can improve your cholesterol profile. But it's also true that our waterways have become increasingly contaminated with all sorts of pollutants, including mercury, and that these pollutants tend to accumulate at different levels in different species. The fish most at risk are predators high in the pelagic food chain, such as swordfish and sharks (see chart).

It was to test the effects of mercury on the heart that the two new studies compared the mercury levels in clippings from toenails, where heavy metals tend to be deposited. In one study, researchers led by Dr. Eliseo Guallar at Johns Hopkins found that European and Israeli men with the highest mercury levels were nearly 2.2 times as likely to have a heart attack as those with the lowest levels. The other study, led by Dr. Walter Willett at the Harvard School of Public Health, looked at a selection of American men and found no connection between mercury exposure and risk of heart disease, although Willett told me a "weak association" cannot be ruled out.

For most of us, eating two servings of fish a week should not pose any problems. Guallar, who hails from coastal Spain, continues to flavor his paella with salmon, which has negligible mercury levels. Willett eats swordfish only about twice a month--because of its expense, not any fear of mercury. Fish-oil supplements are high in omega3 fatty acids and probably don't contain as much mercury as whole fish. But they don't taste nearly as good.

Dr. Gupta is a neurosurgeon and cnn medical correspondent