DHA -- or docosahexaenoic acid -- is one of the few truly essential fats in the human diet. DHA makes up part of the cellular membrane that surrounds and protects every cell in the brain. It gives the cell a more flexible surface, which allows chemical messages to pass through easily. Some studies, although controversial, suggest that a DHA-rich diet may lower the risk for Alzheimer's disease and help slow its progression in later stages. DHA is present primarily in the oil of fish.
Omega-3 Fatty Acids
DHA falls within a broad class of unsaturated fats known as omega-3 fatty acids. The two other omega-3 fatty acids are eicosapentaenoic acid, or EPA, and alpha-linolenic acid, or ALA; each generally has the same role. The human body can make a limited amount of DHA to incorporate directly into the cell membranes. The rest of the DHA you must obtain directly from your diet. Fish is the best source of DHA, though the DHA content varies between species. For example, 3 oz. of Atlantic salmon contains 0.95 g DHA, whereas the same amount of canned tuna contains only 0.54 g.
Dairy
Breast milk from a healthy mother contains a significant amount of DHA and should properly nourish an infant with a developing nervous system. However, infant formula does not necessarily contain any DHA at all; check the label carefully to find a brand that does. Cow milk -- and all dairy products that derive from the milk, such as cheese and yogurt -- contains a moderate amount of DHA.
Vegetarian Sources
The typical vegan or vegetarian diet is nearly devoid of DHA. However, ALA, a short-chain form of omega-3, is present in walnuts and to a lesser degree in leafy green vegetables. The human body can convert ALA into DHA, so, in theory, one is as good as another. The problem is that the body must change the short-chain version into the long-chain version. This doesn't happen very rapidly, according to Walter Willett, chair of the department of nutrition at the Harvard School of Public Health, so it's an inefficient process.
Recommended Intake
Despite its relative inefficiency, ALA is usually the main form of omega-3 in the adult diet. The European Commission recommends an omega-3 fatty acid intake equal to 2 g/day of ALA and 200 mg/day of EPA and DHA. ALA can fulfill much of the dietary need for omega-3. However, researchers are still debating whether ALA has the same heart-healthy effects as DHA and EPA, so you may want to add fish to your diet to decrease your risk of cardiovascular disease. The American Heart Association recommends people with documented cardiovascular disease consume 1 g/day of EPA and DHA. People with high triglyceride levels may need as much as 2 to 4 g/day of EPA and DHA, some of which can come from fish oil supplements, and should supplement under a physician's care.
References
- University of California; DHA-Rich Diet Protects Brain Against Alzheimer's; Elaine Schmidt; September 2004
- Oregon State University Linus Pauling Institute; Essential Fatty Acids; Jane Higdon; December 2005
- University of Maryland Medical Center; Docosahexaenoic Acid (DHA); Steven D. Ehrlich; September 2008
- "Prostaglandins, Leukotrienes, and Essential Fatty Acids"; DHA Status of Vegetarians; T.A. Sanders; August-September 2009
- U.S. News & World Report; Fish Oil Supplements, EPA, DHA, and ALA: Does Your Omega-3 Source Matter?; Sarah Baldauf; April 2009
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