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.
Benefits Unclear for People at Lower Risk
More than half (60%) of the study participants had already experienced a heart attack or stroke when they entered the study. Most of these patients were taking drugs such as statins, ACE inhibitors, and aspirin to lower their risk for a second heart attack or stroke.
American Heart Association president Donna K. Arnett, PhD, says it remains to be seen if people who aren't on optimal prevention treatments benefit from taking fish oil.
It is also not clear if people with a lower risk for heart attack and stroke benefit, she says.
At least three studies in low-risk people are under way. Results from the first of these studies should be published within the next few months.
"Until the results from these trials are known, we can't really say if omega-3 supplementation is beneficial for people at lower risk," Arnett tells WebMD.
Eating Fish, Omega-3 Foods Good for Heart
Gerstein says the latest findings do not contradict studies showing that eating more fatty fish and other omega-3-rich foods is good for the heart.
"People who eat a diet rich in omega-3 fatty acids eat fewer foods that may be harmful, such as red meat," he says. "Eating more fish and less red meat may be better for health, but it may not be the omega-3."