Fish: What’s the Catch?
For: Wellness Made Easy, September 2003
Studies showing the benefits of eating fish are almost too numerous to count. Fish may lower the risk of stroke and heart disease. It has been linked (by preliminary evidence) to higher mental functioning in older people, and possibly to a lower risk for prostate cancer. It contains substances essential for brain development in the fetus and infant. The fat in fish may help relieve the inflammatory symptoms of rheumatoid arthritis, and it helps lower triglycerides in the blood.
What’s the source of all this fish power? Fatty fish are high in polyunsaturated fatty acids called omega-3s. Like aspirin, these omega-3s make platelets in the blood less likely to stick together, and they may reduce the process of inflammation. Thus, they may reduce clotting and the risk of heart attack and stroke. Besides the omega-3s, fish has other virtues: it is a good protein source, relatively low in calories and saturated fat, and rich in B vitamins, iodine, phosphorus, iron, selenium, and zinc.
Americans are eating more fish these days, and not just canned tuna. The American Heart Association, among others, urges everybody to eat at least two servings of fish a week. In restaurants and at home, health-conscious people often choose fish over beef, pork, and lamb. Many people have learned to like newly popular varieties with exotic names like branzino, mahi-mahi, and tilapia, and chefs as well as home cooks have learned to prepare fresh fish in appetizing new ways (forget about breaded, deep-fried fillets). Along the way, fish-eaters may have decided that if two servings a week are good, then five or six are better.
Reasons to worry
Yet there are reasons to worry about fish. For one thing, fish and shellfish can contain dangerous microorganisms and parasites; fortunately these can be wiped out by cooking. That’s why we advise against eating raw fish or shellfish.
But fish may also contain other contaminants, such as organo-chlorine compounds, including DDT, PCBs, and dioxin, as well as heavy metals such as mercury. Organochlorines are particularly likely to show up in freshwater fish from lakes and local rivers, but they’re in the ocean, too. Mercury, on the other hand, is more likely to show up in large saltwater fish, but it’s often found in lakes and rivers as well. Contaminants like organochlorines are just as likely to be present in farmed fish as in wild—indeed, some studies find that farmed salmon, for example, has higher levels of pesticides than salmon in the wild.
These chemicals come from agricultural runoff, animal feed lots, incineration, and many kinds of manufacturing processes. The use of organochlorine compounds is declining in North America, and there are now bans or limits on some organochlorine compounds, but they are still accumulating in the environment and in fish. Fortunately, there are agencies that keep track of this kind of contamination, and it’s thought that levels in fish are low, and that people would be at risk only if they ate large amounts of the same species from the same source—which may be contaminated. Fortunately, too, organochlorine compounds accumulate in fatty tissue and skin, so you can eliminate a large portion of them by not eating the skin and visible fat. Of course, discarding the fat would remove some of the beneficial omega-3s.
Mercury is a different problem. Ubiquitous in the environment, mercury tends to accumulate throughout the food chain, and accumulates in the flesh of the fish. The higher a fish is on the food chain, the greater the concentration of mercury—thus large predators like swordfish and sharks are most likely to contain high concentrations. So are fish such as mackerel and tuna, also high on the food chain. Mercury at high levels can cause neurological and vision problems, and even death. Lower levels may damage the developing nervous system in fetuses, infants, and children.
That’s why the government tells pregnant or breastfeeding women, women who might become pregnant, and children, to limit fish consumption and to avoid some species entirely. The EPA and National Academy of Sciences have revised their estimates of "safe" levels of mercury downward. But some people wonder whether the same levels of mercury that may harm children and fetuses also harm other people. Or whether a few people, including some of the most health-conscious, are simply eating too much fish.
Expensive mercury
One study that recently made news came from Dr. Jane Hightower of the California Pacific Medical Center in San Francisco. Concerned about a female patient who complained of hair loss, Dr. Hightower tested for mercury and found high levels in the woman’s blood. She was eating fish nine times a week. During the next year, Dr. Hightower tested 116 other heavy fish-eaters and discovered that 60 had levels high enough to be at risk for mercury poisoning. One child had suffered brain damage. Their only known exposure to mercury was through fish—and they usually ate expensive fresh-caught marine fish, not lake fish, canned fish, or breaded fish sticks (though the brain-damaged child had also eaten canned tuna). Elevated mercury levels—higher than the safe maximum recommended by the government—tended to correlate with eating lots of fresh swordfish, tuna steaks, and salmon.
Dr. Hightower’s conclusion: Fish is good food, but doctors should tell people not to eat too much, particularly such species as swordfish, and should test for mercury in patients at risk because of high fish consumption.
Another study appeared last fall in the New England Journal of Medicine. Subjects from eight European countries and Israel who tested positive for high levels of mercury (presumably from fish, though that was not determined) also had a high risk of heart attack. Were the supposed heart-protective qualities of fish being offset, ironically, by the mercury they contained? In the same issue another study found no connection between mercury exposure and heart disease. However, the researchers noted that a weak connection between the two could not be ruled out. The question will no doubt be studied further.
A recent government report found that the vast majority of Americans do not have high levels of mercury in their bodies. At the same time, a new study in the Journal of the American Medical Association concluded that while this was true for most people, 8% of women of childbearing age do have higher levels than the government now regards as safe. This is why it’s important for women planning to become pregnant, pregnant women, and young children to follow the guidelines in the box on page 5. Actually everybody should eat fish in reasonable amounts and choose a variety of fish.
What to do
• Fish is still an excellent food. But as with all good things, moderation is the key—limit yourself to 12 ounces of fish a week, on average (from stores and restaurants, not local lakes.) That’s two or three servings, depending on serving size.
• Vary your intake. Choose shellfish sometimes, then other kinds of fish, and some canned fish. Don’t stick with one species all the time. Smaller ocean fish are likely to have less mercury.
• Check with the health department in your area before you eat freshwater fish caught locally. If the health department has no information, limit your consumption of fish from local waters to no more than one serving per week. Information for all 50 states regarding local advisories is available at the EPA website.
• Choose fish lower in the food chain. Smaller non-predatory fish with shorter life spans tend to have lower levels of mercury. These include cod, whitefish, pollock, mahi-mahi, ocean perch, halibut, haddock, flounder, sole, and all shellfish (clams, shrimp, oysters, mussels, lobsters). But watch for information—the government often revises its advice. This is why it would helpful, as the California Medical Association has proposed, for fish (fresh, canned, or frozen) to be labeled for mercury content.
• Don’t eat fish or shellfish raw. Cook fish until it turns opaque and flakes when tested with a knife or fork. Shellfish such as shrimp and mussels will cook in five minutes. Steam mussels and clams past the point when the shell opens. This won’t reduce any mercury, but it will kill bacteria.
• Some plant foods (flaxseed and canola oil, walnuts, leafy greens) contain fats similar to the omega-3s in fish. These are good for you, but are not the same as the omega-3s in fish (we will discuss this in an upcoming issue).
Pregnant women and children
If you are pregnant, potentially pregnant, or nursing eat no more than 12 ounces of cooked fish per week, on average, and chose from a variety of different species, fresh or canned. This is also safe for children 16 and younger. Don’t eat shark, swordfish, tilefish, or king mackerel. These large game fish are likely to be high in mercury. Check with the local health department regarding recommendations (advisories) for locally caught fish. If there are none, limit your consumption of fish from those waters to no more than once a week. Until more is known about mercury levels in canned tuna, we advise you to eat it, too, no more than once a week.
UC Berkeley Wellness Letter, July 2003
