What does it do? Omega-3 fatty acids, such as docosahexaenoic acid (DHA), belong to the class of nutrients called essential fatty acids. DHA has been shown to reduce levels of blood triglycerides. High triglycerides are linked with heart disease in most, but not all, research. DHA alone appears to be just as effective as fish oils (which contain both DHA and EPA) in beneficially lowering triglyceride levels in people at risk for heart disease. In part, this may be because some DHA is converted to EPA in the body. Unlike EPA, however, DHA may not reduce excessive blood clotting.

DHA appears to be essential for normal visual, intelligence and ability to memorize development in infants. However, DHA supplementation did not affect the development of visual acuity in formula-fed infants in a double-blind trial. Nevertheless, other double-blind research links DHA supplementation in premature infants to better brain functioning. The effects of DHA on the intelligence and ability to memorize may well extend beyond infancy.

DHA supplementation in healthy young men has been shown to decrease the activity of immune cells, such as natural killer (NK).

DHA deficiency plays an important role in a group of congenital diseases called peroxisomal disorders, which damage the protective covering (myelin) around nerves. Although rare, the worst of these disorders (i.e., Zellweger's syndrome) is life-threatening. Daily oral supplementation of 100-600 mg of DHA has been shown to increase blood levels of DHA, to protect myelin, and to improve the signs and symptoms of these potentially devastating disorders.

Where is it found? Cold-water fish, such as mackerel, salmon, herring, sardines, black cod, anchovies, and albacore tuna, are rich sources of DHA and EPA. Similarly, cod liver oil contains large amounts of DHA and EPA. Certain microalgae contain DHA and are used as a vegetarian source of this nutrient in some supplements. Most fish oil supplements contain 12% DHA.

DHA has been used in connection with the following conditions:

• Childhood intelligence
• High blood pressure
• High triglycerides
• Rheumatoid arthritis
• Depression
• Lupus
• Psoriasis
• Angina
• Chronic obstructive pulmonary disease (COPD)
• Diabetes
• Dysmenorrhea (painful menstruation)
• Migraine headaches
• Osteoarthritis

Who is likely to be deficient? Premature infants who are not breast-fed are often DHA-deficient. A link has appeared between DHA deficiency and Alzheimer's disease; however, no evidence at this time indicates that supplementation with DHA will help Alzheimer's patients. Similarly, preliminary evidence shows that children with attention deficit disorder (ADD) have low DHA levels. However, no evidence demonstrates that DHA supplementation improves ADD. Preliminary evidence suggests that people with a variety of rare but related congenital diseases (Zellweger's syndrome, neonatal adrenoleukodystrophy) may be DHA-deficient, and may even benefit from DHA supplementation. Many doctors believe the diets of most people eating a Western diet do not provide optimal amounts of omega-3 fatty acids.

At least four studies have reported a reduced blood level of omega-3 fatty acids in people with depression.

How much is usually taken? Most healthy people do not supplement with fish oil containing DHA or vegetarian sources of DHA. Much of the research in adults has been based on 1-3 grams per day of DHA from fish oil.

Are there any side effects or interactions? While those with heart disease and diabetes often benefit from fish oil, such people should check with their doctor before taking more than 3 or 4 grams of fish oil per day for several months. Elevations in blood sugar have sometimes been reported, though this may simply be due to small increases in weight resulting from high dietary fish oil. While DHA combined with EPA from fish oil consistently lowers triglycerides, it occasionally increases LDL cholesterol.

Fish oil is easily damaged by oxygen, so small amounts of vitamin E are often included in fish oil supplements to prevent such oxidative damage. Doctors often recommend that people who supplement with fish oil or DHA take vitamin E supplements to protect EPA and DHA within the body from oxidative damage. The level of oxidative damage caused by DHA has not been shown to result in significant health problems.

To the contrary, test tube studies report that DHA is toxic to cancer cells and may someday be considered as an adjunct to conventional treatment for cancer. Similarly, animal studies suggest that DHA may inhibit cancer.