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),9 10 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,11 though this may simply be due to small increases in weight resulting from high dietary fish
oil.12 While DHA combined with EPA from fish oil consistently lowers triglycerides, it occasionally increases LDL cholesterol.13
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.14 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.15 However, animal researchers have reported
that the oxidative damage caused by DHA alone was not prevented with vitamin E
supplementation.16 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.17 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.18 The effect of DHA by itself on glucose levels has not been
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.19 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.20 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.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
At the time of writing, there were no well-known drug interactions
with docosahexaenoic acid.
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