Are you one of those who daily pop omega-3 fatty acid supplements like fish oil for its health benefits? Beware, they may be rancid pills, and not healthy as claimed, suggests a new study.

Derived from plants (algae) as well as seafood, omega-3 supplements sometimes labelled as fish oil are often taken because of research that suggests they may provide health benefits.

Rancidity is measured by the amount of oxidation of the oil in the supplement. As supplements become more oxidised, the nutritional benefits delivered to the consumer are reduced.

"Our study suggests many of these supplements are not fresh and thus may not provide a potential health benefit," said Leigh A. Frame, Associate Professor of clinical research and leadership at George Washington University in the US.

Higher levels of omega-3 have been associated with a wide range of benefits to multiple organs, including the brain and eyes, but the most common reason for its popularity with consumers is to help the cardiovascular system.

The researchers conducted six years of tests on 72 of the most popular brands of omega-3 supplements, using the recommended rancidity limits that are voluntarily set by GOED, a global trade group that represents omega-3 manufacturers.

The results, published in the Journal of Dietary Supplements, found a total of 45 per cent of flavoured and unflavoured supplements tested positive for rancidity, with 32 per cent of flavoured supplements testing positive and 13 per cent of unflavored pills.

However, added flavouring can mask the rancidity of supplements.

While the oxidation is present in the flavoured supplements, indicating there is a positive level of rancidity, Frame said the amount of oxidation isn't known because of the flavouring compounds themselves.

"Flavouring is potentially masking the freshness of omega-3 supplements in two ways," said lead author Jacob Hands, a medical student at the varsity’s School of Medicine and Health Sciences.

If fish oil supplements are fresh, they won't have a fishy taste or smell, he explained. "Flavouring can mask those aspects but also makes it difficult for researchers to determine the level of oxidation and the quality of the supplement."

While there are third-party companies who test vitamins and supplements to verify a label's purity of its substances, even brands that are tested cannot be guaranteed freshness, Frame explained.

Both Frame and Hands suggest exercising caution with flavoured fish oil supplements at this point due to the uncertainty of how the flavouring may affect their freshness and, thus, any potential health benefits.