That's the conclusion of a large study published in The New England Journal of Medicine.
Fish oil supplements contain omega-3 fatty acids. The American Heart Association says omega-3s have been shown to benefit the hearts of people at high risk for heart disease.
But in the new study, people with type 2 diabetes or prediabetes who took omega-3 fatty acid supplements were no less likely to die or suffer a heart attack or stroke than patients who did not take the supplements.
Last April, a review of studies involving more than 20,000 patients with heart disease found the same thing.
People in the latest study who took a fish oil capsule every day containing about 1 gram of omega-3 fatty acids had the same risk of heart attack, stroke, and death as people who did not over six years of follow-up.
Study researcher Hertzel Gerstein, MD, of McMaster University in Ontario, Canada, says the research as a whole offers no evidence to support the use of omega-3 supplements by people with diabetes or prediabetes to lower heart risk.
"We found no effect, either positive or negative," he tells WebMD. "One capsule of omega-3 fatty acid daily did not have any effect on future heart outcomes."
The study included about 12,500 patients with diabetes or prediabetes who had established heart disease or were at high risk for having a heart attack or stroke. The participants took either a 1-gram fish oil capsule a day or a placebo capsule that contained no omega-3.
The patients who took omega-3s had a reduction in triglyceride levels over six years of follow-up. Triglycerides are a type of fat found in blood. High triglycerides are linked to heart disease.
But the death rate from heart attacks and strokes was the same for patients who took omega-3s and patients who had the placebo. Also, both groups had the same rate of heart attacks, strokes, and deaths from arrhythmias and other heart-related causes.