Forty years ago, your family doctor might have recommended a spoonful of cod liver oil daily, "for your health." Common wisdom at the time was that cod liver oil is loaded with vitamin D in a form your body can use. Most milk is fortified with vitamin D now, so cod liver oil has fallen out of favor. (The nasty taste didn't help.)

But fish oil has been making a comeback. Advertisements for fish oil tout health benefits for conditions as varied as arthritis, heart disease, and colitis. What's the truth? When you're considering whether to add fish oil to your diet, follow your heart.

Researchers noticed that certain social groups that eat fish regularly are much healthier than Americans, especially in the incidence of heart disease. Nutritionists began looking at Greenland Eskimos (Inuit) in the 1950s to determine why they were so healthy, and their efforts finally began to focus on the amount and type of fat the groups were consuming.

It turns out that certain kinds of fish are a rich source of a type of fat known as omega-3 fatty acids. The beneficial compounds in fish oil are called eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These compounds have the ability to affect characteristics of blood cells, making platelets less "sticky," which reduces the risk of blood clots. This is good news for practically everyone; blood clots cause most heart attacks and 80 to 90 percent of strokes.

In addition, omega-3 fatty acids can have a positive effect on heart rhythm. The EPA and DHA reduce the risk of a condition called ventricular fibrillation. This rhythm disturbance can cause a fatal heart attack. A recent Italian study published in the journal Lancet looked at 11,000 men for more than three years. They found that a daily dose of 1 gram of EPA/DHA reduced the risk of death by almost 20 percent, with the risk of cardiovascular-related death being reduced by 30 percent. The researchers attributed the benefit to fish oil's effect on heart rhythm.

An added bonus is that omega-3 oils are also good for your lipid profile, that is, the amount and ratio of triglycerides to cholesterol in your bloodstream. Triglyceride levels are especially critical for women and for people with diabetes. A high triglyceride level in these people puts them at increased risk for cardiovascular problems.

Something fishy?

Fish oil has many benefits, but there are a couple of side effects you should be aware of. First is the chance that fish oil could cause a slight increase in your LDL cholesterol level. If your total cholesterol and your ratio of HDL to total cholesterol are within normal ranges, this shouldn't be a concern, but if your cholesterol is high or has not been determined, consult with your physician before adding fish oil to your diet.

The second area is cause for a bit more attention. Because fish oil reduces the "stickiness" of your blood, making clots less likely, it can also make you bleed more easily. Some patients even dropped out of clinical trials because of nosebleeds. Another consequence of easy bleeding is a type of stroke called a hemorrhagic stroke. The low rate of heart disease among the Inuit is combined with a higher risk of stroke. If you are taking vitamin E or a blood thinner, such as aspirin or the prescription medication warfarin (Coumadin), check with your physician before increasing your consumption of fish oil.

How much is enough?

You don't need to become a seal, eating a diet of all-fish-all-the-time. One or two servings of fatty fish a week seems to convey the health benefits. If you don't like fish, consider taking a daily supplement of 2-3 grams. Look for a supplement that contains both DHA and EPA; you get slightly different benefits from each. Regardless of how you choose to get your omega-3 fatty acids, your heart will be hooked.

Fish vary greatly in their oil content. Here are the ones to look for:

Anchovies and salmon (Atlantic or pink) are your best bets. The other varieties have a higher omega-3 content, but they also contain high levels of undesirable fat. Sardines, for example, contain another 12 grams of fat along with the omega-3 fatty acids. Atlantic salmon contains only another 4.2 grams, anchovies another 3.5 grams, and pink salmon another 2.5 grams.