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.