While those with heart disease and diabetes have often been reported to benefit from supplementation with fish oil,^{201, 202} both groups should check with their doctor before taking more than 3 grams of fish oil per day for a few months. Elevations in blood sugar and cholesterol levels may occur in some people who take fish oil.²⁰³

The increase in blood sugar appears to be related in part to the amount of fish oil used.²⁰⁴ The impairment of sugar metabolism sometimes caused by supplementation with fish oil has been prevented by the addition of half an hour of moderate exercise three times a week.²⁰⁵

While supplementation with fish oil consistently lowers triglycerides, the effect of fish oil on LDL (“bad”) cholesterol varies, and in some people, fish oil supplementation has been reported to increase LDL levels.²⁰⁶