Fish oil is now the third most widely used dietary supplement in the United States, after vitamins and minerals, according to a recent report from the National Institutes of Health.
At least 10 percent of Americans take fish oil regularly, most believing that the omega-3 fatty acids in the supplements will protect their cardiovascular health.
But there is one big problem: The vast majority of clinical trials involving fish oil have found no evidence that it lowers the risk of heart attack and stroke.
From 2005 to 2012, at least two dozen rigorous studies of fish oil were published in leading medical journals, most of which looked at whether fish oil could prevent cardiovascular events in high-risk populations. These were people who had a history of heart disease or strong risk factors for it, such as high cholesterol, hypertension or Type 2 diabetes.
All but two of these studies found that compared with a placebo, fish oil showed no benefit.
And yet during this time, sales of fish oil more than doubled, not just in the United States but worldwide, said Andrew Grey, an associate professor of medicine at the University of Auckland in New Zealand and the author of a 2014 study on fish oil in JAMA Internal Medicine.
“There’s a major disconnect,” Grey said. “The sales are going up despite the progressive accumulation of trials that show no effect.”
In theory at least, there are good reasons that fish oil should improve cardiovascular health. Most fish oil supplements are rich in two omega-3 fatty acids that can have a blood-thinning effect, much like aspirin, that may reduce the likelihood of clots. Omega-3s can also reduce inflammation, which plays a role in atherosclerosis. And the Food and Drug Administration has approved at least three prescription types of fish oil, Vascepa, Lovaza and a generic form, for the treatment of very high triglycerides, a risk factor for heart disease.
But these properties of omega-3 fatty acids have not translated into notable benefits in most large clinical trials.
Some of the earliest enthusiasm for fish oil goes back to research carried out in the 1970s by the Danish scientists Dr. Hans Olaf Bang and Dr. Jorn Dyerberg, who determined that Inuits living in northern Greenland had remarkably low rates of cardiovascular disease, which they attributed to an omega-3-rich diet consisting mainly of fish, seal and whale blubber. Dr. George Fodor, a cardiologist at the University of Ottawa, outlined flaws in much of this early research, and he concluded that the rate of heart disease among the Inuit was vastly underestimated. But the halo effect around fish oils persists.
The case for fish oil was bolstered by several studies from the 1990s, including an Italian study that found that heart attack survivors who were treated with a gram of fish oil daily had a drop in mortality, compared with patients taking vitamin E. These findings prompted groups like the American Heart Association to endorse fish oil about a decade ago as a way for heart patients to get more omega-3s in their diets.
“But since then, there has been a spate of studies showing no benefit,” said Dr. James Stein, the director of preventive cardiology at University of Wisconsin Hospital and Clinics. Among them was a clinical trial of 12,000 people, published in The New England Journal of Medicine in 2013, that found that a gram of fish oil daily did not reduce the rate of death from heart attacks and strokes in people with evidence of atherosclerosis.
“I think that the era of fish oil as medication could be considered over now,” said the study’s lead author, Dr. Gianni Tognoni of the Institute for Pharmacological Research in Milan.