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Seafood Cuts Risk for Heart Disease

By Luis Treacy | Heart Disease | Rating:

The nomadic hunters of the North American Arctic called themselves Inuit, which simply means "the people." Their more familiar name—Eskimos—comes from their ancient enemies, the Cree. It means "people who eat raw meat."

Raw or cooked, the traditional Eskimo diet—seal, whale, fish, caribou, and other Arctic wildlife—has become in the past few years the unlikely springboard for a new surge in dietary advice. Increasingly, medical and nutrition specialists are encouraging Americans to eat more fish in the hope of reducing the risk of heart attack. And, unlike earlier advice, which stressed "lean" fish for cholesterol watchers, the new consensus is that any fish—lean or fatty, finfish or shellfish—will do. In fact, it seems the fatter, the better.

Classic Eskimo fare would appear to be a cardiologist's nightmare. It's very high in animal fat and cholesterol and nearly devoid of vegetables, fruit, and grain. Yet heart disease is uncommon among Eskimos. Their blood cholesterol levels tend to be relatively low, with high concentrations of "good cholesterol". One reason for this paradox is the type of fat Eskimos consume. Beef and pork, both staples of the traditional American diet, are relatively high in saturated fat, which tends to raise blood cholesterol. The fat of an Arctic seal or whale is much less saturated. It's more akin to fish oil, which is rich in monounsaturated and polyunsaturated fatty acids, which tend to lower blood cholesterol.

That much has been known since the early 1950s, when detailed analyses of the Eskimo diet began to appear in scientific journals. But when physicians advocated fish in cholesterol-reducing diets, their preference was for lean varieties, such as cod and flounder. So-called fatty fish like salmon were assigned the same pariah status as lard and egg yolk. Shellfish, considered high in cholesterol, were also frowned upon.

Today, research originating with studies of the Eskimos in the late 1970s has now cast an entirely new light on fish oils and their effects on body chemistry. Concurrent investigation has also brought shellfish back to the cholesterol-conscious consumer's menu.

Fat Fish and Thin Blood

During the 1970s, Danish researchers observed that the Eskimos of northwest Greenland exhibited a mild bleeding disorder: They bruised easily and tended to experience nosebleeds. Subsequent investigation revealed that their platelets—blood cells involved in clotting—were less sticky than those of Europeans or Americans, and didn't clump together as readily. This finding suggested that the Eskimos' apparent resistance to coronary disease might be linked to the difference in their platelets. It may be that less-sticky platelets are less likely to form a clot, or "thrombus," that could cause a heart attack by blocking a coronary artery.

Whatever is affecting platelet function may also be serving to protect Eskimos from coronary disease. That hidden factor, it now appears, may be the Eskimos high intake of certain fatty acids in their diet.

All fats, solid or liquid, are composed of various combinations of fatty acids. The types and amounts of acids present are what make one fat different from another. One group of polyunsaturated fatty acids—called the omega-6 group—is abundant in land plants. Another—the omega-3 group—is more plentiful in sea plants, especially in cold waters.

Omega-3 fatty acids typically remain liquid at extremely low temperatures. As it turns out, this is providential for both fish and sea mammals, particularly those that inhabit the Arctic. Omega-3 fatty acids are critical for maintaining the flexibility of the animals' cell membranes—which enables seals and fish to move about in their characteristic floppy way, rather than stiffly or not at all.

Fish are a good source of omega-3 fatty acids because they eat sea plants—or eat other fish that do. Generally, the colder the water and the oilier the fish, the greater the omega-3 content.

Like other fatty acids, the omega-3 variety enter the membranes of many body cells, including the platelets. The two omega-3 acids most common in fish oil are eicosapentaenoic acid and docosahexaenoic acid, or EPA and DHA. Research now indicates that sufficient intake of EPA and DHA can make platelets less "sticky"—and less likely to form clots. This effect on platelets is not confined to the Eskimo diet. Japanese fishermen who consume large amounts of fish daily also exhibit reduced platelet stickiness. Like the Eskimos, these fish-eaters experience a very low rate of coronary disease.

Part of the recent research has served to retire former misconceptions about fish. Fatty fish, for example, were supposed to raise blood cholesterol. They don't. Studies at the Oregon Health Sciences University showed that experimental diets high in fish oil and salmon (a fatty fish) actually lower the level of cholesterol in the blood somewhat. Such diets also dramatically reduce the blood level of triglycerides—fatty substances that are suspected of playing a role in coronary disease.

Moreover, the term fatty applied to fish is something of a misnomer. In the total universe of fat things, fish tend to be rather svelte. Overall, oily fish—salmon, mackerel, bluefish, lake trout, sardines, herring, and the like—have only about one-fifth to one-half the average fat content of lean beef.

Shellfish are very low in fat, often having less than the white meat of chicken. But they were long thought to be high in cholesterol and therefore inappropriate for people with elevated blood cholesterol. The first clue that this might not be so came in the late 1960s, when scientists at Oregon Health Sciences University tested the effects of shellfish in a dietary study. Participants who ate approximately a pound of shrimp, lobster, and crab daily for several weeks experienced little rise in blood cholesterol. When the diet shifted to mollusks—oysters, scallops, and clams—the participants actually showed a slight decrease in blood cholesterol.

The results were so unexpected that the researchers decided to shelve the study. When they finally published it in 1982, they explained that they had not submitted the data initially because they had not understood it. But recent insights had made some sense of the puzzling results.

Among those insights was a more accurate measure of the cholesterol in shellfish. Previous analytical techniques had identified marine substances similar to cholesterol as cholesterol itself. As a result, the cholesterol content of many shellfish had been greatly exaggerated.

More recent analyses by the U.S. Department of Agriculture have revised the numbers sharply downward, especially for mollusks. The formerly maligned sea scallop, for example, has not only about half the fat of white-meat chicken, but less cholesterol as well. Even some popular crustaceans score reassuring marks. The cholesterol level of crab and lobster is roughly comparable to the dark meat of chicken and is much lower in fat. Shrimp tends to be higher in cholesterol than other shellfish and most meat, but only moderately so.

Confirmation From Recent Findings

The succession of favorable reports reached a peak in 1985 when the New England Journal of Medicine published three studies supporting the possible benefits of seafood. Prominently featured was a report from the Netherlands concluding that fish consumption might lower the risk of fatal heart attack.

Some 850 men in the town of Zutphen had been followed for 20 years to assess various risk factors in heart disease. A major finding was that fish consumption was associated with a reduced rate of fatal coronaries. Men who ate roughly 7 to 11 ounces of fish weekly had less than half the coronary death rate of men who ate no fish at all. Men who consumed more than 11 ounces of fish a week—about a pound, on average—experienced virtually the same reduction in risk as those who ate 7 to 11 ounces. There was no extra advantage to the higher level of intake.

Even when other common risk factors such as hypertension, smoking, and elevated blood cholesterol were taken into account, the favorable association with fish intake persisted. The Dutch researchers concluded that "as little as one or two fish dishes per week may be of preventive value in relation to coronary heart disease."

In its implications for U.S. public health, the Dutch study was more provocative than the findings among Eskimos or Japanese fishermen. The Japanese consumed more than half a pound of fish daily. Eskimos may have eaten the equivalent of nearly a pound a day. But the Dutch ate fish in amounts that Americans might well get used to eating. One or two fish meals a week is well within range of a health-conscious public. Even though the Dutch findings only suggest—rather than prove—that fish is good for the heart, there's still ample room for more fish as part of a well-balanced American diet.

Fish Oil Capsules

Besides urging more patronage at the fish counter, public-health officials generally agree on another issue. They want people to eat fish rather than swallow fish-oil capsules.

Wide press coverage of the research on fish oil helped create a popular item in the food-supplement market. Capsules of fish oil containing marine fatty acids—especially EPA and DHA—were suddenly peddled to people hoping to gain an edge on heart disease, arthritis, or other assorted maladies.

Fish oil is admittedly more than just snake oil with gills. Large doses of marine fatty acids given both as salmon oil and as fish-oil capsules have shown promise for lowering excessive levels of triglycerides in the blood. There's also evidence that such doses may have an anti-inflammatory effect by inhibiting certain biochemical processes in white blood cells. But research is preliminary and contradictory, and little is known about actual clinical benefits.

Also unknown is the extent of adverse effects—which may include diarrhea and delayed blood clotting. (People on aspirin therapy or taking anticoagulants should avoid fish-oil capsules.) One certain side effect is cost: The manufacturers' "recommended daily dose" of fish-oil supplements can run you anywhere from $10 to $40 or more per month—and even that would not be enough to approach the level of fatty acids found to be beneficial in clinical studies.

There's no proof as yet that taking a daily fish-oil capsule or two will do anything more than grease your intestines. Pending further clinical trials, most researchers advise eating fish instead.


Meanwhile, the possible dark side to a high-fish diet needs to be explored. Inhibited platelet function isn't necessarily a sign of bristling health. Changing the body's fatty-acid makeup may have effects, good and bad, that are still uncharted. For example, there's some evidence that Eskimos and Japanese fishermen experience higher death rates from brain hemorrhage than Europeans or Americans do. Some scientists suggest that the difference could be related to inhibition of platelet function. Some people are allergic to fish, becoming ill from eating even a small amount.

For most people, however, there isn't much to worry about in a couple of fish meals a week.

Most problems arise when fish come from contaminated waters. If you're fond of raw shellfish, for example, it's difficult to avoid some risk of hepatitis or food poisoning. If shellfish come from an area where outbreaks have been traced to them, avoid eating any raw. Always steam clams for at least four to six minutes, not just until the shell opens.

Eating raw fish, such as preparations of Japanese sushi or sashimi, also involves a risk, mainly from an intestinal parasite known as the fish tapeworm. Although ocean fish usually present no problem, lake fish and Alaskan salmon are sometimes infected. Accordingly, if you prepare raw fish yourself, stick to ocean varieties. In restaurants, inquire about the source of fish before ordering.

Commercially sold fish come under federal regulations that strictly limit them to extremely low levels of mercury, pesticide residues, or other contaminants. Those regulations don't apply to sport fish, however. If you eat sport fish frequently, try to avoid those from lakes or streams known to have pollution problems.

Current limits on the amount of mercury in commercial fish are designed to protect fetuses, which are most vulnerable to mercury toxicity. Even so, it's possible for a pregnant woman to exceed the permissible level for mercury intake—and transmit the contaminant to the fetus—if she eats a lot of fish, particularly large predators such as swordfish, tuna, red snapper, freshwater trout, and northern pike.

Luis Treacy

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