Dear Doctors Column August 10, 2009

Question:

I've heard the buzz about fish oil being good for you, but what's the big deal? I don't like fish very much. Does eating a little fish really make that big a difference?

Answer:

Yes, including fish (or fish oil) in your diet is a big deal. Here's why: It prevents and treats cardiovascular disease, and in some cases it may prevent death or hospitalization. Extensive research consistently has shown the omega-3 polyunsaturated fatty acids contained in fish oil are beneficial to everyone, not just those with heart disease. 

Research has found omega-3 fatty acids may reduce the risk of abnormal heart rhythms, lower triglycerides, produce a slight reduction in blood pressure, help prevent plaque build-up in arteries (atherosclerosis), and decrease inflammation.

Studies also suggest that these fats promote healthy vision and brain development in infants whose mothers consume fish during pregnancy or while nursing. Most researchers conclude that these effects are modest but can add up to significant benefits when people make fish a regular part of their diet.

In several large trials, omega-3 fatty acids lowered heart disease risk by as much as 30%. Scientists also found omega-3s helped prevent hardening and narrowing of the arteries (atherosclerosis) and a wide range of abnormal heart rhythms (arrhythmias), especially in atrial fibrillation.

One omega-3 pill (about 850 mg) a day reduced deaths in patients with heart failure up to 9%, according to one study. Researchers are working to determine the amount of fish oil most helpful to patients with heart failure.

How much?

Experts in the American Heart Association (AHA) and other organizations recommend at least 500 mg per day, which is about two fatty-fish meals a week. Those with known coronary heart disease or heart failure should get 800 to 1000 mg a day (about four or five fish meals a week).

Fish are an excellent source of high-quality protein and many types of seafood are rich in nutrients such as selenium, antioxidants, and omega-3s.

Seafood with the highest levels of omega-3 fatty acids, primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), include salmon (canned, farm-raised, and wild), blue crab, and tuna (fresh or canned, both the white, or albacore, and chunk light varieties).

Seafood is not the only source of omega-3 fatty acids. Tofu, soybeans, walnuts, canola oil, and leafy green vegetables such as spinach also are good sources and are packed with other valuable nutrients. These non-seafood sources contain another type of omega-3 called alpha linoleic acid (ALA) - an essential amino acid people must get from their diet. ALA breaks down into EPA and DHA, but the overall evidence for heart health benefits from ALA is much weaker than it is for EPA and DHA, so to be sure you're reaping the most benefit for your heart you may want to stick with fatty fish until more study is done.

Most people don't eat enough fish consistently to reach the recommended levels, so supplements are a good option. You can get your omega-3 fatty acid quota entirely from supplements, or, if you eat some fish, you can take a supplement every other day. An added benefit of supplements is that they usually have had impurities, such as mercury, removed.

If you use an over-the-counter supplement, be sure to read the label to ensure you are getting the recommended amount of DHA and EPA. The actual concentration of DHA and EPA varies from manufacturer to manufacturer, and a 1-gram pill may contain other types of fish oil.

Mercury in Fish

It is confusing to read about the many health benefits of fish and then hear that seafood can contain unhealthy levels of mercury and other contaminants. Almost all fish and shellfish contain trace amounts of mercury. These levels are not dangerous for most healthy adults but can potentially harm the developing nervous systems of unborn babies, infants, and young children.

Mercury occurs naturally and also is produced by industrial pollution. It accumulates in streams and oceans, where it converts to methylmercury, the type of mercury that can harm a fetus or young child.

Large, predatory fish that have long life spans have the highest levels of methylmercury; these include swordfish, shark, king mackerel, and tilefish. Tuna, depending on type, also can contain significant levels of mercury. Among canned varieties albacore tuna has more than three times the mercury of chunk light tuna. Fresh tuna steaks are typically yellowfin or bigeye (both also are called ahi tuna), and sushi is usually made from albacore or bluefin tuna. All contain higher levels of mercury than canned chunk light tuna. To learn more, visit the AHA's Web page on mercury and omega-3 levels in different types of fish.

Two scientific reports, one from the Institute of Medicine (IOM), a group charged with providing the federal government with scientifically informed analysis and independent guidance, and an analysis published in the Journal of the American Medical Association reached the same conclusion: For most teenagers and adults, the benefits of a diet that includes fish outweigh potential health risks.  

The IOM recommends the following:

• Women who are pregnant or may become pregnant or who are breastfeeding can safely eat up to 12 ounces (about two servings) of seafood a week, including one 6-ounce serving of albacore tuna. They should avoid swordfish, shark, king mackerel, and tilefish. Children younger than 12 years also should follow these guidelines.
• Healthy adolescents and adults, including women who are not pregnant and are not planning a pregnancy, may reduce their risk of heart disease by eating two servings of fish per week.
• Adults at risk for coronary heart disease may benefit from regularly eating seafood, especially varieties that are high in omega-3s.
• People who eat more than two servings of seafood a week should eat a variety of fish and shellfish to reduce the risk of exposure to contaminants from a single source.

The bottom line: Fish is an excellent source of high-quality protein and other nutrients, and research shows that these benefits outweigh the risks posed by mercury and contaminants present in seafood. However, if you prefer to supplement or just don't like fish, you'll reap the same heart benefit from omega-3 fatty acid supplements.