Less than 10 per cent of fish oil supplements sold in Australia contain the amount of omega-3 fatty acids their labels claim, with many on the way to becoming rancid, a new study has found.

As a result, researchers have advised people wanting to boost their omega-3 levels to simply eat fish.

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The University of Auckland study revealed only three out of the 32 fish oil supplements examined contained the same concentration of fatty acid listed on the label.

The supplements only contained on average 68 per cent of the claimed content.

Fish oil is one of the world’s most popular dietary supplements, and is linked with helping prevent cardiovascular disease and aid brain function.

Researchers also found the majority of supplements tested were considerably oxidised, which meant they were becoming rancid.

The active ingredients in omega-3 are fragile and prone to oxidisation, which could occur after being sourced, University of Auckland professor Wayne Cutfield said.

“Exposure to light, air and increasing temperatures above freezing, increases the likelihood that they will degrade and become oxidised,” Mr Cutfield said.

The effects on humans of long-term exposure to oxidised oils has not been studied, he said.

It has been suggested that very oxidised fish oil may promote the formation of fatty deposits in the arteries and thus the opposite of what the consumer is expecting.

Mr Cutfield advised those who want to ensure good levels of omega-3 to simply eat fish.

The study also found the supplement’s cost had no impact on the quality of the fish oil.

“You might think that a more expensive fish oil is less likely to be degraded, that is not the case, there is no relationship with price,” he said.

The University of South Australia’s Peter Clifton called for tighter regulations on dietary supplements in Australia.

“Clearly the bulk fish oil producers have been deceiving the public and the encapsulators about the EPA and DHA [eicosapentaenoic and docosahexaenoic] content of their oil,” said Prof Clifton, who also works at the Baker IDI Heart and Diabetes Institute.

“For those members of the public trying to get an anti-inflammatory or triglyceride-lowering effect from fish oil the reason the oil may not be working for them may be under-dosing, despite taking the recommended number of capsules.”

– with AAP