The dish on fish

Content provided by the Faculty of the Harvard Medical School

The dish on fish

Benefits outweigh risks, according to two recent reviews.

We know, we know: We're supposed to eat fish. Several guidelines say a healthful diet should include two servings (usually defined as 3 ounces) per week. Fish contains eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), two long-chain omega-3 fats that aren't found in appreciable amounts in any other sort of readily available food. Those omega-3 fats may have all kinds of benefits, but so far the best evidence is that they're protective against fatal cardiac arrhythmias and heart attacks. In addition, DHA may be important to early brain development and to healthy pregnancies.

But there are contaminants to consider. It's well known that mercury (more precisely, methylmercury) accumulates in the lean tissue of some species. Polychlorinated biphenyls (PCBs) and dioxins are found in fatty tissue. Mercury crosses the placenta, and high doses cause serious brain damage. In adults, the metal may harm the heart. PCBs and dioxin cause cancer in animal experiments, and there's evidence that they're human carcinogens, too. They may also throw the immune system out of kilter and cause neurologic defects.

So which way does the fish scale tip, toward benefit or risk?

Late in 2006, the Institute of Medicine (IOM) and two researchers at the Harvard School of Public Health, Dariush Mozaffarian and Eric Rimm, weighed in on that question with extensive reviews of the existing medical research. Their conclusions differed in some respects. The Harvard researchers found plenty of evidence that EPA and DHA have health benefits. The IOM committee of experts found the evidence unimpressive and suggested an alternative explanation for the positive cardiovascular findings: People who eat fish regularly may not eat as much meat, so their intake of harmful saturated fat is lower.

But the IOM and Harvard experts agreed on the critical question: Eating fish is worth whatever risks the contaminants might entail. In general, the levels of pollutants are below levels at which the FDA would take action. The IOM experts said that for most people, a healthful diet should include two 3-ounce servings per week. The Harvard researchers said that for adults, the benefits of "modest" fish consumption — which they defined as one or two servings per week — outweigh the risks.

Mercury vs. the omega-3s *Calculated from limit of 1 microgram per gram (1 part per million). Source: Based on Journal of the American Medical Association, Oct. 18, 2006, pp. 1890–91.

Here are some other important points made in the two reviews:

Fish when you're eating for two. The Harvard researchers stressed the importance of DHA to fetal development (the IOM committee was more cautious). Mercury "climbs" the food chain, so the metal accumulates more in the flesh of large, long-lived predatory fish than it does in smaller, short-lived creatures. The Harvard researchers said women who are or may become pregnant and nursing mothers should steer clear of some types of fish (king mackerel, tilefish — also known as golden bass — shark, and swordfish) to minimize exposure to mercury but should eat 12 ounces of other types of fish and shellfish per week to make sure they're getting enough DHA.

Not created equal. A 3-ounce serving of farmed salmon contains over 2,000 milligrams of EPA and DHA omega-3 fats. A serving of catfish that size delivers less than a tenth of that amount (about 150 mg). So if omega-3 fats are what we're after when we eat fish, it's not just any fish that will do. Besides salmon, the best omega-3 providers include Atlantic mackerel (which is different from king mackerel), herring, and sardines. Swordfish and golden bass throw us right back into the benefit-risk quagmire, because while they're ample providers of omega-3s, they also rank high in mercury content. Americans eat more shrimp than any other type of seafood, but a 3-ounce serving only has about 250 mg of omega-3 fats. Lobster, at 71 mg per 3 ounces, is also an omega-3 lightweight.

Salmon is the best deal. One knock against fish is that it's expensive, so the Harvard researchers set out to calculate out how much it would cost to get 250 mg of EPA and DHA daily from eating fish. They surveyed prices in six cities and collected data on EPA and DHA content. Salmon turns out to be the best deal: It would cost you just nine cents a day to get 250 mg of EPA and DHA daily from canned pink salmon. Other sorts of salmon were almost as inexpensive. Catfish may be cheap by the pound, but not if you're shopping for omega-3 content. According to the Harvard researchers' calculations, you'd have to pay over $2 per day for a daily dose of 250 mg of EPA and DHA from catfish.

Farmed versus wild salmon. The omega-3 content of farmed salmon depends on the amount of fish oil in the feed, so if feeding practices change, so might omega-3 content. But for now, farmed salmon has considerably more omega-3 content than its wild comrades, according to IOM and Harvard reviews (4,504 mg versus 1,774 mg in a 6-ounce serving, according to the Harvard paper). But some bad comes with the good — the Harvard researchers also cited studies showing that farmed salmon had more PCB contamination than wild salmon.

White versus light tuna. Canned tuna is sold either as white tuna — sometimes called albacore — or as light tuna. White tuna is almost always packed in water in solid form (it's a single, solid piece of loin, cut to fit in the can), whereas light tuna is often sold as chunk tuna (a mixture of cut pieces) in either oil or water. White tuna contains three times the omega-3 fats of light tuna, but alas, also has three times the mercury.

The law of diminishing returns applies. The Harvard researchers say that after about 250 mg per day — or roughly 1,500–2,000 mg per week — the cardiovascular benefits of additional omega-3 fat intake start to plateau. This "threshold effect" means there may not be much point to consuming large amounts of fish, at least for cardiovascular health. Eat 6 ounces of salmon, or another fairly fatty fish, per week and apparently you'll be experiencing most of the benefit.

Selenium may keep mercury from rising. There's some evidence that selenium blocks some of the harmful effect of mercury and also keeps it from accumulating in tissue. If this research holds up, it's good news for seafood lovers because most fish and shellfish (especially mussels) are rich in selenium.

Don't bank on plant sources. EPA and DHA aren't the only omega-3 fats. Alpha-linolenic acid (ALA), which is found in flaxseed, soybeans, and walnuts, is a member of the exclusive club. ALA may have benefits of its own, but the body converts relatively small amounts of it into EPA or DHA, so sources of ALA like flaxseed oil and walnuts aren't a substitute for eating fish.

The chicken and egg question. Poultry can be a source of omega-3 fats, depending on what it's fed. But the IOM report says that fish isn't used to make chicken feed these days, so there's a negligible amount of EPA and DHA in chicken. Eggs are a different story. You've probably seen omega-3 eggs for sale at the supermarket. Egg-laying hens convert some ALA to DHA, so putting flaxseed or some other source of ALA in their feed can increase the DHA (and ALA) content of eggs considerably.

Scum to the rescue. Someday we may be depending on big vats of algae for our omega-3s rather than schools of fish. Two types of algae, Schizochytrium species and Crypthecodinium cohnii, are already being used commercially to produce DHA, according to the IOM review. Several companies sell algae-derived DHA supplements.

Last updated:	February 08, 2007