Fish has long been a staple of healthy eating, since it’s packed with omega-3 fatty acids and antioxidants that can help protect against heart disease and cancer. In fact, experts are so convinced of the benefits of the omega-3s in fish that health officials recently recommended Americans eat more of it — about 8 ounces, or two to three servings, of fish a week — in its latest revision of the Dietary Guidelines for Americans.
But what about fish oil capsules? Doctors have also believed that taking omega-3s as supplements can offer a similar protective benefit to the heart. But a new study published in the Archives of Internal Medicine throws the theory into doubt: according to the analysis of 14 controlled trials in which nearly 20,500 patients with a history of heart disease were randomly assigned to take omega-3 supplements or placebo, those taking the fatty acid pills had about the same rates of heart disease, death from heart attacks, congestive heart failure and stroke as those on placebo.
To date, the studies on omega-3 fatty acids and recurrent heart problems have been contradictory: some have shown that heart patients taking omega-3 supplements had a lower risk of heart attack and heart-related death than those not taking them, but others have shown no such benefit. The difference may have to do with how some of the earlier studies were set up, says one of the current study’s co-authors, Seung-Kwon Myung from Seoul National University. In some trials, the participants knew they were taking omega-3 supplements, and they might have had a bias toward seeing a benefit because of this knowledge. (Myung did not include these types of unblinded trials in his analysis.) “I think the beneficial effects of omega-3 supplementation shown in those trials are not reliable,” says Myung.
(MORE: Brain Food: Eating Fish May Lower Your Risk of Alzheimer’s)
Still, Myung’s findings don’t necessarily mean that omega-3s — the study looked at the fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) — are useless when it comes to preventive health. Indeed, the American Heart Association (AHA) recommends that both heart patients and those who don’t yet have heart disease eat fatty fish at least two times a week, and if they can’t consume that much fish, then to boost their omega-3 intake with supplements. According to the AHA, studies show that omega-3 fatty acids can decrease the risk of abnormal heartbeats, keep triglyceride levels down and inhibit the build up of atherosclerotic plaques in the heart’s blood vessels.
It’s possible, says Myung, that natural sources of omega-3 fatty acids may be more potent than supplements. “I recommend heart patients (as well as healthy people) not to take omega-3 fatty acid supplements because there is no evidence of those beneficial effects against cardiovascular disease,” he says. “However, I recommend at least two servings per week of fish because it has been reported that fish consumption has the preventive effect for cardiovascular disease based on the previous observational studies.”
(MORE: Fish Oil During Pregnancy Fights Colds Among Newborns)
It’s also possible that Myung and his colleagues failed to see a strong positive effect from omega-3 supplements among people with pre-existing heart disease because these patients may need a higher level of omega-3s to see benefit. The researchers looked at a range of doses of EPA and DHA, but perhaps a scarred, damaged heart that has survived a heart attack or angina is affected differently by omega-3 fatty acids than an intact and healthy heart.
As Harvard researchers Drs. Frank Hu and JoAnn Manson also point out in a commentary accompanying the new study, it’s possible too that drugs like statins may mask the benefit from fish oils because the medications are so much more powerful. That may also explain why older trials have tended to show a fish oil benefit, while newer ones have not.
Either way, there’s no downside to eating more fish, which happens to be a good source of protein that’s far lower in saturated fat than red meat. So while it may not help heart patients avoid another heart event, it probably won’t hurt them either.
SPECIAL: How Not to Get Sick
MORE: Exercising Your DNA
Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.