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.1 In part, this may be because some DHA is converted to EPA in the body.2 Unlike EPA, however, DHA may not reduce excessive blood clotting.3
DHA appears to be essential for normal visual and neurological (nervous system) development in infants.4 5 However, DHA supplementation did not affect the development of visual acuity in formula-fed infants in a double-blind trial.6 Nevertheless, other double-blind research links DHA supplementation in premature infants to better brain functioning.7 The effects of DHA on the nervous system may well extend beyond infancy. Young adults given 1.5–1.8 grams DHA per day showed less evidence of aggression in response to mental stress, compared with people in the control group in a double-blind trial.8
DHA supplementation in healthy young men has been shown to decrease the activity of immune cells, such as natural killer (NK) cells and the cells that regulate inflammation responses in the body.9 The anti-inflammatory effects of DHA may be useful in the management of autoimmune disorders; however, such benefits need to be balanced with the potential for increased risk of infections.
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 within the first year of life. 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.10
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 (refer to the individual health concern for complete information):
Who is likely to be deficient? Premature infants who are not breast-fed are often DHA-deficient.11 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.12 Similarly, preliminary evidence shows that children with attention deficit disorder (ADD) have low DHA levels. However, no evidence demonstrates that DHA supplementation improves ADD.13 Preliminary evidence suggests that people with a variety of rare but related congenital diseases (Zellweger’s syndrome, neonatal adrenoleukodystrophy, and infantile Refsum’s disease) may be DHA-deficient, and may even benefit from DHA supplementation.14 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.15 16 17 18
How much is usually taken? Most healthy people do not supplement with fish oil containing DHA or vegetarian sources of DHA. The level of DHA given to premature infants who are not breast-fed should be determined by a pediatrician. Much of the research in adults has been based on 1–3 grams per day of DHA from fish oil, although higher levels have been taken when isolated DHA from microalgae sources is used.
Because cod liver oil contains large amounts of vitamin A and vitamin D, women who are or who could become pregnant should consult a doctor before taking cod liver oil. Adults should make sure the total amount of vitamin A and vitamin D from cod liver oil and other supplements does not exceed 25,000 IU (7,500 mcg) per day for vitamin A (15,000 IU per day for those over age 65) and 800 IU per day for vitamin D, unless they are being supervised by a doctor.
Are there any side effects or interactions? While those with heart disease and diabetes often benefit from fish oil (the primary source of DHA in the diet),19 20 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,21 though this may simply be due to small increases in weight resulting from high dietary fish oil.22 While DHA combined with EPA from fish oil consistently lowers triglycerides, it occasionally increases LDL cholesterol.23
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.24 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. Some evidence indicates that vitamin E may be protective against oxidative damage caused by fish oil.25 However, animal researchers have reported that the oxidative damage caused by DHA alone was not prevented with vitamin E supplementation.26 The level of oxidative damage caused by DHA has not been shown to result in significant health problems.
Some evidence suggests that adding vitamin E to EPA/DHA may prevent the fish oil-induced increase in serum glucose.27 Similarly, the impairment of glucose tolerance sometimes caused by the omega-3 fatty acid has been prevented by the addition of half an hour of moderate exercise three times a week.28 The effect of DHA by itself on glucose levels has not been adequately studied.
People who take fish oil containing EPA and DHA and who also take 15 grams of pectin per day have been reported to have reductions in LDL cholesterol.29 This suggests that pectin may overcome the occasional problem of increased LDL cholesterol from fish oil supplementation. The LDL cholesterol-raising effect of EPA and DHA may also be successfully prevented by taking garlic supplements (or presumably adding garlic to the diet) along with EPA and DHA.30 Adding pectin or garlic when people supplement with DHA by itself has yet to be studied.
According to a report in a Japanese medical journal, three people at high risk for colon cancer developed a variety of cancers after one to two years of supplementation with DHA.31 To date, this report has not been confirmed by other researchers. To the contrary, test tube studies report that DHA is toxic to cancer cells32 and may someday be considered as an adjunct to conventional treatment for cancer.33 Similarly, animal studies suggest that DHA may inhibit cancer.34
At the time of writing, there were no well-known drug interactions with docosahexaenoic acid.
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