Many popular fish oil supplements are rancid, tests show. Here's how to protect yourself.

Fish oil supplements have come under fire recently after research found many brands make claims about heart health that aren't backed up by clinical trial data. Now there's even more bad news for these supplements: Many consumers are getting fish oil pills that are rancid.

Nearly 10% of American adults take fish oil supplements for a range of health issues, making this a potential problem for plenty of people.

But why are many fish oil supplements going bad, and how can you tell?

What the study says

The multiyear study found that the vast majority of fish oil supplements tested have oxidized and are rancid by the time they get to consumers. That can cause the supplements to have a fishy odor, which they're actually not supposed to have, as well as reduce their health benefits.

What are the key findings?

The study, published in the Journal of Dietary Supplements, analyzed 72 over-the-counter omega-3 supplements sampled from 2014 to 2020. (Fish oil is a dietary source of omega-3 fatty acids.)

The supplements were assessed by type (liquid, soft gel, spray), source (algae, calamari, fish, krill, mussels) and third-party certifications, before the study's researchers analyzed how well the supplements held up.

The researchers discovered that 68% (or 23 out of 34) of the flavored supplements and 13% (five out of 38) of the unflavored ones exceeded the TOTOX (total oxidation value) upper limit set by the Global Organization for EPA and DHA, which measures oxidation in the supplements — meaning they had oxidized and were rancid.

The researchers also pointed out that there was a "statistically significant" change in flavor in pills that were rancid.

"One of my concerns is high-quality supplements and how to get them," study co-author Leigh Frame, associate director of the Resiliency and Well-being Center at the George Washington University School of Medicine and Health Sciences, tells Yahoo Life. "Fish oil was an obvious one to study — oil goes rancid over time and there's a real risk of it not being fresh."

What experts think

The findings are a "serious concern," nutritionist Gina Keatley, co-owner of Keatley Medical Nutrition Therapy, tells Yahoo Life. "Consumers trust these products for their health benefits. It's essential for the industry to prioritize transparency and consumer safety."

Diane Rigassio Radler, a dietitian and director of the Institute for Nutrition Interventions at Rutgers University, tells Yahoo Life it's possible that "maybe nothing" will happen if you ingest rancid fish oil supplements. But, she adds, "the research is not all benign."

One study of 54 healthy participants who took oxidized (rancid) fish oil, high-quality fish oil or high-oleic sunflower oil found that those in the rancid fish oil group experienced significant (and unfavorable) effects on lipoproteins — particles made of protein and fats that carry cholesterol through your bloodstream to your cells — compared with those in the high-quality fish oil group, who saw a beneficial effect on lipoproteins.

Another study of 52 women who were randomly assigned to get oxidized or nonoxidized fish oil supplements for 30 days found that those in the rancid group did not see any significant impact on cholesterol and blood pressure. The nonoxidized group, by comparison, had lowered blood pressure and total cholesterol.

"Consumers generally take supplements for a health benefit, and so if the active compound — omega-3 in this case — is compromised, they may not get the health benefit," Radler says.

Best-case scenario, says Frame, "these supplements don't work because they don't provide that beneficial antioxidant property." But, she says, there's also a risk they may increase inflammation in the body. Bodily inflammation has been linked to a slew of health issues, including heart disease, type 2 diabetes and some forms of cancer.

Keatley agrees. "Consuming rancid or oxidized fish oil supplements may not provide the intended health benefits. In fact, it may lead to digestive discomfort, such as burping, indigestion or diarrhea," she says. "Over time, the consumption of oxidized fats may contribute to cellular damage and inflammation."

Why it matters

Dietary supplements are a largely unregulated industry in the U.S., and because of that, it can be tricky to find a quality product. "Supplements can come to the consumer market without documented efficacy and safety profiles," Radler points out.

Fish oil typically has a shelf life of one to two years, but it can vary based on the formula, added ingredients and storage conditions, Keatley says. (Fish oil supplements stored in hot conditions are more likely to go rancid quickly than those kept in cool locations, Frame notes.)

How can you tell if your fish oil is bad? Frame says it will have a fishy odor when you break it open. "If you take it, you may also have a fishy burp," she says, although she notes that it can be difficult to tell this if the company has added flavorings to its supplement.

"The best advice is to make sure that you're buying high-quality supplements from a reliable source," Frame says. "That sounds straightforward, but it can be difficult on Amazon, where a lot of people are buying their supplements from. If you're buying them from a warehouse, it's hard to say how they were kept."

If you buy supplements online, Frame says it's important to make sure you're buying from a reputable manufacturer and, ideally, one that uses third-party testing. "If a supplement tastes unusually sour or off, it's best to avoid it," Keatley says. "Checking the expiration date and ensuring proper storage conditions can help maintain the supplement's quality. Finally, don't discount actual fish, which have all the oil you need and more."