We Love Fish

Cardiologists have long known that eating fish helps protect against heart disease. What they don't know is why fish are beneficial. For years they figured it was a simple question of substitution: folks who replace red meat with fish are naturally cutting down their intake of saturated fat, which the body easily converts into artery-choking plaques. But a growing body of evidence collected over the past 30 years suggests there's something special about fish. In particular, fish contain nutrients called omega-3 fatty acids (especially abundant in species like sardines, salmon and mackerel) that seem to promote cardiovascular health.

One group convinced of the benefits is the American Heart Association, which released new dietary recommendations earlier this month. For the first time, the A.H.A. has recommended that everyone eat two 3-oz. servings of fatty fish a week. But the A.H.A.'s expert panel wasn't ready to declare that taking omega-3 all by itself, in pill form, will protect your heart. It's just too easy to get more omega-3 than you need from pills, and the panel was worried that an excess could trigger serious side effects, such as internal bleeding.

Now it's the Food and Drug Administration's turn to consider the question. As the result of a lawsuit brought by alternative-medicine advocates, the FDA is supposed to decide before the end of this week whether the manufacturers of omega-3 pills and fish oils can advertise the fat's heart benefits. If the FDA agrees, omega-3 will join a select group of nutrients, including psyllium, soy and whole oats, that is cleared for similar health claims. The agency was keeping mum in advance about which way it was leaning, but the evidence provided some clues about how it might rule.

First some background. Omega-3 fatty acids belong to a group of compounds known loosely as polyunsaturated fats. (Omega-6 fatty acids make up the other major type of polyunsaturated fats. More on that later.) These fats serve as the raw material for a whole host of essential structures in the body, from brain cells to molecules that regulate inflammation, blood pressure and blood clotting. Since our bodies cannot manufacture their own supply of omega-3s, we have to get them from the food we eat, mostly from fish but also from such plant sources as flax, soybeans and walnuts. Indeed, many European countries supplement their infant formula with various omega-3 fatty acids.

What does science tell us? The earliest studies simply reported that populations that traditionally eat a lot of fish--think Greenland Eskimos, Native Americans of the Northwest and the Japanese--have relatively low rates of heart disease. Then laboratory analyses showed that omega-3 fatty acids lower the risk of clots developing in the blood--a common trigger for a heart attack--while reducing the level of triglycerides, another fatty compound that has been linked to heart disease, and decreasing the number of irregular heartbeats. All pretty good circumstantial evidence, but not enough to support a health claim.