In fact, according to a report from independent testing company Labdoor, people’s fish oil supplements may be fishier than they’d like. The company claims to have discovered that a significant number of supplements sold on the market are heavy in oxidized oils, which can make them taste rancid and smell rotten, and potentially affect their quality as well.
The lab tested 54 top-selling fish oil brands marketed in the U.S. and available elsewhere in the world, collected between 2014 and 2018. About 10 percent of the brands had levels of oxidized oil high enough to be considered rancid. according to voluntary legal standards, the report found. Of these offenders, some had levels 11 times higher than recommended.
“It was quite common,” Dan Mark, Labdoor’s director of research, told The Guardian earlier this year. “For us, they would start to smell and feel.”
Fish oil is rich in omega-3 fatty acids, which are also a type of dietary fat known as polyunsaturated fat. Fats are essential to a healthy diet, but polyunsaturated and monounsaturated fats are thought to be the best fats to consume on a regular basis, rather than other types such as trans fats and saturated fats. It is less certain whether fish oil and omega-3 fatty acids specifically provide additional health benefits. Some data have suggested that regularly eating seafood rich in these oils is linked to a modestly lower risk of cardiovascular problems, for example, while other data have found no comparable benefit from simply taking fish oil supplements. Regardless of the mixed data, many people take fish oil supplements, and it’s estimated to be a multi-billion dollar industry.
The report is far from the first evidence to suggest that a significant proportion of fish oil products are rancid, although estimates based on industry studies are lower. Flavorings have often been added to these products to mask any odor or taste, which could also disguise the degree of oxidation in a product. But aesthetics aside, too much oxidation can be bad in other ways. Limited data has suggested that oxidized fish oil simply does not offer the same potential benefits as fresh fish oil and could even have a negative effect on blood cholesterol levels.
Oxidation is not the only potential problem with these products. The Labdoor report and other studies have shown that, like other types of supplements, fish oil can be dosed inconsistently, with some pills containing very different amounts of omega-3 fatty acids than listed. Even assuming that fish oil is worth taking, this inconsistency could mean users aren’t getting enough of the product to actually benefit. As a whole, the supplement industry is barely regulated towards drugs and other drugs. In addition, as the Guardian also reported, the fish oil industry contributes to ongoing environmental problems such as overfishing.
This article has been updated since it was first published.