DHA

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.

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.

Premature infants who are not breast-fed are often DHA-deficient.¹ 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.² Similarly, preliminary evidence shows that children with attention deficit disorder (ADD) have low DHA levels. However, no evidence demonstrates that DHA supplementation improves ADD.³ 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.⁴ 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.^{5, 6, 7, 8}

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.⁹ 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.¹⁰ However, animal researchers have reported that the oxidative damage caused by DHA alone was not prevented with vitamin E supplementation.¹¹ 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.¹² 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.¹³ 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.¹⁴ 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.¹⁵ Adding pectin or garlic when people supplement with DHA by itself has yet to be studied.

While those with heart disease and diabetes often benefit from fish oil (the primary source of DHA in the diet),^{16, 17} 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,¹⁸ though this may simply be due to small increases in weight resulting from high dietary fish oil.¹⁹ While DHA combined with EPA from fish oil consistently lowers triglycerides, it occasionally increases LDL cholesterol.²⁰

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.²¹ To date, this report has not been confirmed by other researchers. To the contrary, test tube studies report that DHA is toxic to cancer cells²² and may someday be considered as an adjunct to conventional treatment for cancer.²³ Similarly, animal studies suggest that DHA may inhibit cancer.²⁴