Back in 2002, the American Heart Association recommended that people with coronary artery disease take fish oil. Its reasoning was sound. At the time, solid research showed that eating fish or, for some people, taking fish oil, helped protect against dying of heart disease.

Since then, research has generally supported the benefits of eating more fish or taking fish oil. At the same time, studies have raised a red flag about whether people with severe angina, seriously compromised heart function, or potentially lethal rhythms in the heart's lower chambers should eat a lot of fish or take fish oil.

What's so special about fish or the oil extracted from it? In addition to being an excellent source of protein, fish tends to be rich in omega-3 fats. Our bodies need these healthful fats, but can't make them. The two omega-3s we get from seafood are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). A third type, alpha-linolenic acid (ALA), is found in many plants.

Omega-3 fats help the heart's ventricles maintain a steady beat and guard against potentially deadly erratic rhythms. They ease inflammation and help prevent the formation of dangerous clots in the bloodstream. Omega-3 fats also lower levels of triglycerides, the most common type of fat-carrying particle in the bloodstream.

Refining the prescription

A European trial tested the impact of eating oily fish at least twice a week or taking fish oil among men with easily provoked severe angina. Surprisingly, those who got more omega-3s were more likely to have died of heart disease during the several-year study.

Another note of caution came from trials of fish oil for folks prone to potentially deadly disturbed rhythms in the ventricles, notably ventricular tachycardia and ventricular fibrillation. Most people who experience one of these and live to tell about it get an implanted cardioverter-defibrillator (ICD) to detect and halt these arrhythmias. In one trial, people with an ICD who took high-dose fish oil had more episodes of ventricular tachycardia or fibrillation than those taking a placebo. In two similar trials, fish oil prevented ventricular arrhythmias in one and had no harm or benefit in another.

Experiments by Dr. Alexander Leaf and his colleagues show that omega-3 fats prevent potentially fatal rhythms by suppressing the function of hyperexcitable cells that live in scar tissue or in heart muscle that doesn't get enough oxygen. That's great when there's plenty of healthy heart tissue. But what about when these hyperexcitable cells are responsible for the lion's share of cardiac function, such as in people with severe angina or other conditions in which the heart is chronically deprived of oxygen? Stifling them with fish oil could conceivably snuff out the heartbeat, explains Dr. Leaf, emeritus professor of clinical medicine at Harvard Medical School.

Recasting recommendations

As is the case for most cardiac interventions, a single recommendation on fish and fish oil can't cover all the bases.

Yellow light. Go easy on fish and avoid fish oil if you have hard-to-control angina, severe heart failure, or an ICD. Ongoing research could modify that advice, but for now it's a prudent approach, says Dr. Leaf.

Go fish. Everyone else could benefit from eating fish, preferably fatty fish like salmon, sardines, mackerel, and trout, twice a week. It can help keep a seemingly healthy heart in good shape, and protect one that may be in trouble. Baked or broiled fish is best; deep-fried, fast-food fish doesn't count. On non-fish days, have a handful of walnuts or ground flaxseeds, use flaxseed or canola oil, or try some tofu, all good sources of alpha-linolenic acid.

Try oil. If you have one or more narrowed coronary arteries, have had bypass surgery or angioplasty, or had a heart attack, and your heart function is good, try to get 1 gram of DHA plus EPA daily. In Europe, doctors routinely prescribe fish oil for heart attack survivors. American doctors are more reluctant to follow the American Heart Association's recommendation.

High triglycerides. Taking 2 to 4 grams of EPA plus DHA a day can help control high triglycerides.

Practical matters

Even though you can buy fish oil without a prescription, talk with your doctor first before taking it.

Taking fish oil may be cheaper than eating fish, but it isn't nearly as pleasant. Gulping down the large capsules can be difficult, and some people don't like the fishy aftertaste or belches. Freezing the capsules can help with those.

Fish oil is free of mercury and other contaminants found in some fish. A prescription version, Lovaza, delivers nearly 1 gram of DHA plus EPA per capsule. Over-the-counter kinds may be cheaper, but many contain less DHA and EPA, so you need to take more capsules. Over-the-counter supplements aren't closely regulated by the FDA, so you don't always know what you are getting. For vegetarians, V-Pure capsules contain EPA and DHA extracted from algae, which is where fish get them.

The best way to get omega-3 fats, though, is to eat oily fish. It's better for you than red meat, and is far tastier than a capsule.