There's something fishy going on in the billion-dollar fish oil supplement industry.
A new study has found that the bulk of supplements sold in Australia and New Zealand are almost a third lower in omega-3 fatty acids than their labels claim.
Only three out of 32 fish oil supplements analysed by NZ scientists contained the same concentrations of fatty acids as listed on the label.
The supplements had on average 68 per cent of the claimed content.
Fish oils are among the most popular dietary supplements in the world, linked to helping prevent everything from cardiovascular disease to mental illness.
The University of Auckland's Professor Wayne Cutfield said the research team found how much each supplement cost made no difference to the quality of the product.
"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." Prof Cutfield said.
Although the researchers did not publish the brand names of the fish oil supplements that were analysed, Prof Cutfield said almost half were encapsulated, labelled and marketed from Australia.
Most fish oil products sold globally, including Australia, are sourced from deep sea fish from the west coast of South America.
The researchers found the majority of the supplements tested were considerably oxidised, in other words the oils were on the road to becoming rancid.
The active ingredients in omega-3 are fragile and prone to oxidisation, which could occur on the long trip from South America, said Prof Cutfield.
"Exposure to light, air and increasing temperatures above freezing, increases the likelihood that they will degrade and become oxidised."
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.
For the consumer the best advice is to protect their fish oil by storing it sealed in the fridge and out of direct light, he said.
But he advised those who want to ensure good levels of omega-3 to simply eat fish.
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."