The Dish on Fish: It’s Good for You—Except When It’s Not

To some scientists and doctors, fish is a miracle food, a treasure trove of heart-healthy fatty acids and disease-fighting vitamins. To others, it’s a culinary curse, the carrier of health-endangering toxins.

Which is it? The confusion is enough to make you go off the deep end.

Some things are clear. There’s little doubt, for example, that certain types—particularly cold-water fish such as salmon, herring, anchovies and Atlantic mackerel—rank as the top food source of omega-3 fatty acids, the fats believed to protect the heart, brain and other organs. Most seafood is also rich in other key nutrients and low in “bad” fats and calories.

On the flip side, headlines continue to warn that fish contains mercury, polychlorinated biphenyls (PCBs) and dioxins that have polluted some lakes, seas and rivers and are linked to a greater risk of liver disease, neurological problems and possibly cancer.

So, will eating seafood bolster your health or hurt it?

If you’re 50 or older, dive in, say experts: The benefits—and there are many, among them a lower risk of dementia, vision loss and dying from heart disease—far outweigh the risks.

“There is plenty of nutrition in fish, but most of the evidence points to omega-3s as the big-ticket benefit,” says Harvard Medical School cardiologist Dariush Mozaffarian, M.D., who as the author of several major medical studies may be the nation’s most prolific researcher on the health impact of seafood.

There are several types of omega-3s, some found in nuts and vegetable oils. But fish provides the highest amounts of two important types known as fish oil. DHA (docosahexaenoic acid), which is abundant in breast milk, accumulates in the brain, heart, eyes and other membranes to protect those organs. EPA (eicosapentaenoic acid) primarily helps keep immunity strong and is believed to fight inflammation. Together, as they are in most seafood, they provide a one-two punch against many age-related and other conditions—particularly heart disease. Most health benefits come from the equivalent of 250 to 500 milligrams a day of DHA and EPA—what’s found (and stored in the body) in about two servings of fatty fish a week.

“DHA and EPA are known as ‘essential fatty acids,’ meaning you have to eat them to get them. But as you age, the body doesn’t absorb and convert them as well,” says Ernst Schaefer, M.D., senior scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging and a professor of medicine and nutrition at Tufts University School of Medicine. “This makes for a stronger argument to eat more fish as you get older.”

Although omega-3 fish oils get most of the health-boosting kudos, they’re not the only reason why seafood rates high on the good-for-your-health list.

“Fish is a terrific source of protein and helps control blood sugar levels,” says nutritionist Joy Bauer, who regularly appears on the Today show and is author of the New York Times bestseller Food Cures. Fatty fish like salmon and sardines are also loaded with vitamin D, she says, with as much as 400 international units per serving, compared with just 100 IUs in a cup of milk. Bauer adds that certain types of fish—including tuna, halibut and salmon—contain selenium, an antioxidant that may reduce the risk of some cancers and is believed to neutralize the damage caused by mercury.

So why the concern that seafood may not be safe to eat?

Good question—if you’re a man or postmenopausal woman.

“You shouldn’t not eat fish because you are worried about contaminants,” says Mozaffarian. “The levels of PCBs and dioxins in fish are very low, and similar to those in everything else we eat. In fact, less than 10 percent of the PCBs and dioxins in the American food supply come from fish; 90 percent come from vegetables, meat and dairy.”

And mercury? When the Environmental Protection Agency and the Food and Drug Administration issued a warning about this toxin in certain fish and shellfish in March 2004, it was specific only to women who were or might become pregnant, nursing mothers and young children. That’s because mercury poisoning can cause birth defects and learning and memory problems in young children. However, a study published May 18 in the American Journal of Epidemiology shows that toddlers whose mothers ate more fish while pregnant with them scored higher on IQ tests than those whose mothers avoided seafood during their pregnancy.

Still, can you get too much of a good thing? “Older people should absolutely be concerned about mercury in fish,” says Florida neurologist David Perlmutter, M.D., author of The Better Brain Book. “Mercury causes memory loss and other cognitive problems. And there are other ways to get omega-3s besides eating fish.” His recommendation: Take algae-based DHA supplements, available in any health food store.

Tufts’ Schaefer adds that one high-quality fish oil supplement a day supplies the recommended amount of omega-3s that two servings of fish a week provide.

But Mozaffarian says those older than 50 should be worried about mercury only if they’re eating five or more seafood servings a week. “Then, they should have a variety of fish—the majority have low mercury levels—and minimze consumption of high-mercury shark, swordfish, tilefish and King mackerel. Overall, a fish meal is very beneficial.”

Sid Kirchheimer is the author of AARP/Sterling's new book Scam-Proof Your Life.