updated 9/8/2006 10:29:07 AM ET 2006-09-08T14:29:07

Omega-3 fats, the “good fat” found in fish, are turning out to be more complex than some may have expected. Omega-3 fats may help protect us from heart disease, dementia and inflammation that can lead to rheumatoid arthritis, diabetes and cancer. Researchers are now finding that differences in the benefits seen in studies may reflect differences between omega-3 fats, as well as their interaction with other fats.

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EPA and DHA are omega-3 fats found in fish. ALA is the omega-3 fat found in plant foods, mostly seed and nut oils. Our bodies can convert ALA into EPA and then DHA, so researchers used to assume that eating foods with ALA provided the same benefits seen in eating fish, with its EPA and DHA. Studies now show that humans are relatively inefficient in converting ALA into EPA and DHA. A recent report shows that boosting ALA to more than 10 times the current average U.S. intake caused only small increases in blood levels of EPA and DHA.

Cancer impact still unknown
The impact of seafood’s omega-3s on cancer risk is still unclear. Their impact on heart health is more widely studied. The recommendation to eat about eight ounces (two to three servings) a week of fish rich in omega-3s — such as salmon, mackerel and sardines — leads to an average daily intake of 500 milligrams (mg) of EPA and DHA, which is associated with a lower risk of heart disease.

For those with heart disease, the recommendation for 1,000 mg EPA and DHA would require double that fish consumption and may lead some to take fish oil supplements. Omega-3-rich eggs are another source of EPA and DHA. Amounts beyond 2,000 mg EPA and DHA seem unlikely to provide added benefit, as that may be more than cells can use. Also, risk of excessive bleeding may increase if EPA and DHA intake exceeds 3,000 mg a day.

Same health benefits?
More research is needed as to whether ALA omega-3 provides the same health benefits as EPA and DHA. So far, studies are mixed as to whether omega-3s from plant foods reduce heart disease risk as much as EPA and DHA. Research has found that ALA may provide an indirect benefit by producing a healthy balance with another family of fats, the omega-6s. ALA also plays a role in reactions that influence inflammation, which may protect against some types of cancer.

For those who don’t increase fish consumption — and even for those who do — increasing sources of ALA is a good idea. Flaxseed, canola oil and walnuts each provide ALA and additional protective substances. One study estimating total omega-3 needs for optimal health suggests that if we reach the daily target of 500 mg EPA and DHA, we still need 1,700 to 3,000 mg ALA per day. On average, people take in about 1,300 to 1,700 mg of ALA per day.

Our consumption of omega-6 fats is about 10 times higher than that of ALA. Omega-6s are found in corn, soybean and other vegetable oils. Soybean oil, for example, is often listed among sources of omega-3 fat, but it contains more than seven times as much omega-6 as omega-3 fat. The enzyme that converts ALA into EPA is also used in the metabolism of omega-6s. Researchers note that if our diets included less omega-6 fats, ALA may not have to compete as much for the enzyme and its conversion to EPA and DHA could increase. We may be better off using olive and canola oils, which provide less omega-6, and getting our omega-3s from sources lower in omega-6.

Nutrition Notes is provided by the American Institute for Cancer Research in Washington, D.C.

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