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Fish oil supplements on Canadian shelves can oxidize and go rancid, which some experts say means they should not be consumed. But it's not always easy for consumers to tell which products have degraded.

Fish oil supplements have become a booming $200-million a year market in Canada, as many consumers try to increase their intake of omega-3 fats to benefit their cardiovascular and brain health.

Oils can degrade

Omega-3 fatty acids are found in a variety of fatty fish such as salmon.

Fish oil capsules

Canadians spend $200 million a year on fish oil supplements, looking to increase their intake of omega-3 fats to benefit their cardiovascular and brain health. (CBC)

"Fish oil is a polyunsaturated fatty acid; it has multiple double bonds. So it's very vulnerable to oxygen, light and other conditions," says Preston Mason, a biochemist at Harvard Medical School.

"Fish oil that has oxidized above the maximum limits is highly unlikely to have any health benefit, and in fact, such oxidized lipids contribute to cardiovascular diseases and should be avoided," Mason says.

Some experts say that oxidized oils are more difficult for the body to process, and could be especially difficult for people who have a history of heart problems. But little research has been done on oxidized fish oil.

Eating fish a better option: expert

While evidence shows that eating fish is associated with some health benefits, experts are divided on the benefits of taking a fish oil supplement.

David Agus

'Buy that fish at the fish market, you've got your own quality metric,' says Dr. David Agus, a bestselling author and professor of medicine. (CBC)

Dr. David Agus, bestselling author of The End of Illness and a professor of medicine at the University of Southern California, says this is another reason to get nutrients from food.

"Very simply, whenever you expose fish oil to air and to light, there starts to be degradation, which leads towards rancidity," he says. "Whereas buy that fish at the fish market, you've got your own quality metric: You poke it, you smell it and you ask when it came in and you know if it's good or not."