Q: What is the difference between flaxseed oil and fish oil? Can I take them interchangeably?

A: Both fish and flax oils contain compounds called omega-3 fatty acids, but the easiest way to reap the reputed health benefits of omega-3s is to eat fish or take fish-oil supplements. Flaxseed oil contains alpha-linolenic acid (ALA), the so-called "parent" compound from which all other omega-3 fatty acids are derived. Fish such as mackerel, salmon, and trout (and oil capsules made from them), on the other hand, contain the omega-3 fats eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

Researchers of omega-3 fats have looked at the effects of EPA and DHA in the prevention and treatment of heart disease, cancer, and certain inflammatory conditions. Results indicate that EPA and DHA, not their parent fat ALA, are beneficial in this area. The body can convert ALA to EPA and DHA, but not very efficiently. Although many studies have found that if you increase your intake of ALA, you also increase levels of EPA in your body, the increase is modest at best, as the body uses most of the ALA in flax for energy rather than converting it into EPA. Almost none is made into DHA. In contrast, taking even small amounts of preformed EPA and DHA increases body stores of these two fats considerably. If you dislike fish, taking flaxseed oil and preformed DHA supplements (derived from algae) is an alternative. But you will have to take in significantly more flaxseed oil than EPA itself to get the intended effect.

This month's "Ask The Expert" is written by Dan Lukaczer, ND, director of clinical services at the Functional Medicine Research Center, a division of HealthComm International Inc., in Gig Harbor, Washington.