Fish: Friend or Foe?

In addition to heart-healthy omega-3 fats, seafood can carry mercury and other toxins. For most people, the benefits of eating fish far outweigh the risks.

If you are an average American, you eat 16 pounds of seafood a year. Although that's only a fraction of U.S. chicken consumption, it still represents a lot of fish and shellfish — nearly 5 billion pounds a year. Many people eat seafood because they love it. But more and more are choking it down as a kind of health food.

Is it? Seafood is a great source of protein that's low in saturated fat, and many types have good-for-the-heart omega-3 fats. But fish can also contain mercury and other toxins. Two reports, coincidentally released on the same day, weigh the benefits and risks of eating fish and shellfish.

One report, from the Harvard School of Public Health, offered glowing conclusions about the benefits of eating fish. The other, from the national Institute of Medicine, was more cautious in its estimate of the benefits and worried more about the possible hazards of eating seafood. Although their tones differ, both basically say that eating fish once or twice a week is a good idea.

Both reports looked at fish over the entire human lifespan, from the impact of a pregnant woman's seafood consumption on her baby to the effects of seafood among older people with heart disease. We'll focus on the latter (see below for the Institute of Medicine's complete recommendations).

What we know

Fish has been touted as a heart-healthy food for years. But like many other foods, fish and shellfish can contain traces of toxins like mercury and polychlorinated biphenyls (PCBs). Let's look at what we know about the benefits and risks:

Known or probable benefits. The strongest evidence for fish as health food has to do with the prevention of heart-related death. Writing in the Journal of the American Medical Association, Drs. Dariush Mozaffarian and Eric Rimm of the Harvard School of Public Health make the case, based on a new synthesis of available data, that eating fish once or twice a week reduces the chances of dying from heart disease by one-third. That's on a par with what cholesterol-lowering statins can do. Experts believe that omega-3 fats in fish stabilize heart rhythms and prevent the sudden appearance of ventricular tachycardia, ventricular fibrillation, or other potentially deadly arrhythmias. The main omega-3 fats in fish are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

The Institute of Medicine report, Seafood Choices: Balancing Benefits and Risks, says that results "are not clearly supportive of a cardioprotective effect of EPA [or] DHA." The institute's main caution is that most of the evidence has come from observational studies, not the more rigorous randomized controlled trials (see "Is it the fish...").

Possible benefits. Eating seafood once or twice a week has also been linked with protection against stroke, atrial fibrillation, congestive heart failure, high blood pressure, age-related memory loss, and Alzheimer's disease. These connections, though, are preliminary since they come from small studies or from large ones with conflicting results.

Possible risks. Given our staggering production of pollutants — from factories, farms, cars, even homes — it is inevitable that some find their way into our food supply. The technology to measure toxins has become so sophisticated that vanishingly small amounts can be detected. The pollutants in fish that are highest on health experts' radar screens are mercury and man-made substances such as PCBs and dioxins.

These certainly aren't good for you. The question is, at what amount do they begin to harm health? Very high intake of mercury, such as the levels seen in industrial accidents, can damage nerves in adults, though the damage is usually reversed when mercury intake stops. Low levels of mercury may lead to subtle nerve damage or cardiovascular problems. But protective factors in fish seem to counteract these possible harmful effects.

What about PCBs, which people tend to think of as powerful promoters of cancer? The Institute of Medicine calls the cancer risk linked to PCBs "overrated," since it was based on experiments in which animals were given huge doses for long periods. No one really knows if, or how much, cancer is caused by the low levels found in fish. What's more, most (91%) of the PCBs in the American diet come from beef, chicken, pork, dairy products, vegetables, and eggs.

Known risks. Aside from getting a bone lodged in your throat, the main hazards of eating fish or shellfish are food poisoning (from spoiled seafood or a naturally occurring toxin), parasitic or viral infections, and allergic reactions. Of these, food poisoning is the most common. Seafood accounts for about 3% to 4% of food poisoning cases in the United States, about the same as for beef or chicken.

Overfishing is another known, but underappreciated, hazard. Demand for seafood in the United States has exceeded our supply, and we now import it from around the world. The United Nations estimates that world consumption will outstrip the yearly catch by the end of the 2000s. Popular species already facing commercial extinction include the Patagonian toothfish (known in restaurants as Chilean sea bass), Atlantic cod, grouper, snapper, and bluefin tuna.

Balancing act

Danger is almost always more memorable than safety. It is human nature to magnify risks, especially those outside our control. When Science magazine published a report in 2004 that salmon contained PCBs, and farmed salmon harbored more than wild salmon, some people stopped eating fish altogether. Others followed suit later that year when the FDA and the Environmental Protection Agency issued an advisory on mercury in fish and shellfish. Yet swearing off seafood may be throwing out the baby — or, more likely, the whole day care center — with the bathwater.

The salmon story illustrates this nicely. Using data from the EPA and elsewhere, Mozaffarian and Rimm estimated that PCB intake from eating farmed salmon twice a week for 70 years would cause an extra six cases of cancer per 100,000 people, while eating wild salmon would cause two extra cases. Yet eating either would prevent at least 7,000 deaths from heart disease. Even if the hazard was 10 times greater and the benefit one-tenth the size, the scales would still favor eating fish.

Putting it into practice

There are several ways to maximize the best of what seafood has to offer while minimizing your exposure to possible risks:

Choose wisely. Pick species that are rich in omega-3 fats and low in mercury and PCBs (see chart).

Two servings a week   Two servings a week   Here are the amounts of omega-3 fats, mercury, and PCBs you get from eating 6 ounces of these foods. Comparing omega-3 fats with mercury and PCBs is like comparing watermelons and walnuts. Omega-3s are measured in milligrams, while mercury and PCBs are measured in micrograms (a thousand times smaller than milligrams). So you can see all three, we adjusted the scales. Lines indicating recommended targets or limits put the levels into perspective.

Cut the fat. Removing belly and back fat from fish, and not eating the skin, can cut the PCB content of fish up to 40%.

Deep-six deep fried. When you eat out, avoid fried fish or fast-food fish burgers because the oils used in most restaurants still contain artery-damaging trans fats. At home, bake, broil, or sauté in olive or canola oil.

Break out of your rut. Sample the vast seafood smorgasbord. In addition to discovering wonderful new tastes, you will also minimize your intake of mercury and other contaminants.

Moderation is fine. You don't need to eat fish every day. For preventing heart trouble, aim for a total of 1,500–2,000 milligrams (mg) of omega-3 fats a week. That's one 3-ounce serving of farmed salmon, two 3-ounce servings of wild salmon, or larger portions of other seafood. For people with heart disease, the American Heart Association recommends 500–1,000 mg a day or 3,500–7,000 mg a week.

What about fish oil? Capsules of fish oil are definitely rich in omega-3 fats. Reputable brands contain little, if any, mercury or PCBs. If we could absolutely, positively say that the benefits of eating seafood came entirely from omega-3 fats, then downing these large pills daily would be an alternative to eating fish. It's possible, though, that you need the entire orchestra of fish fats, minerals, and supporting molecules, rather than the lone notes of EPA and DHA.

Yes, fish is good for you. And yes, some kinds of fish contain contaminants that you'd rather not be eating. What can you do? Eating seafood a couple times a week, and picking a wide variety of types, can maximize your benefit and minimize any risks. We'll give Dr. Mozaffarian, a Harvard-affiliated cardiologist and epidemiologist, the last word: "Among all the things in our food supply, on a calorie-for-calorie basis, fish is one of the most important foods you can choose for cardiovascular health."