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Fish oil is the third most widely used supplement in the United States, with around 10 percent of Americans taking it regularly — often for the perceived benefit to cardiovascular health.

But there are two big problems when it comes to fish oil: Researchers still cannot definitely say whether it actually lowers the risk of heart attack and stroke. And even if they could, there can be big differences between the fish oil that is prescribed to patients by their physicians, and some versions of the supplement that are available over-the-counter.

Preston Mason of the Harvard Medical School explains the distinction in the above scene from the new FRONTLINE documentary, Supplements and Safety, which premieres tonight on most PBS stations (check local listings).

It comes down to oxygen, says Mason. Fish oil is extracted as a byproduct from oily fish like anchovies. As the fish are crushed, they’re exposed to air, meaning the oil becomes oxidized. That can turn the smell of fish oil rancid, which is one thing, but more worrisome is that oxidized fish oil contains oxidized lipids, which can trigger changes inside human cells that lead to health problems like cardiovascular disease.

Prescription-grade fish oil needs to be approved by the Food and Drug Administration, notes Mason, and because the approval process comes with stringent production standards, the risk of oxidation runs low. However, in Mason’s work and in other previously published studies, researchers have found that the type of fish oil that’s sold in stores often has high levels of oxidation.

Even if oxidation weren’t an issue, some say there is still no conclusive evidence that the supplement can ward off a heart attack. Among those experts is the University of Aukland’s Andrew Grey, who in a 2014 study in JAMA Internal Medicine reviewed seven years worth of fish oil research published in top medical journals. Grey found that in all but two studies, researchers could not show that fish oil was any more effective in preventing cardiovascular events in high-risk populations than a placebo.

“I think for cardiovascular disease, one has to say, there is no compelling evidence that taking fish oil protects against the first heart attack or a second heart attack,” Grey told FRONTLINE in an interview for Supplements and Safety. “And so, people who are advised to do that, or are doing it, are wasting their time. And their money.”

Adam Ismail, executive director of the GOED fish oil trade association questions that conclusion, telling FRONTLINE, “There’s certainly ample evidence that it helps things like reducing blood pressure, reducing your risk of coronary health.” It’s hard to argue, Ismail added, that the omega-3s found in fish oil “aren’t important for how your heart functions.”

Still, until a clearer link can be established, researchers like Grey and Mason say that eating fish may just be the better option.

Content retrieved from: http://www.pbs.org/wgbh/frontline/video/how-beneficial-is-fish-oil/.