UPDATE: The study behind this story has since been retracted by its authors at the University of Canterbury. They admitted making errors "in the calculation of the amounts of EPA and DHA in five of the fish oil supplement capsules which means that we have underestimated the doses".
"All are now within 15 percent of label value. This, in turn, affects our assessment of compliance with health claims; however, they remain variable. Our assessment of mercury risk is unchanged.
"We apologise for this error."
Newshub's original story is below.
One of the world's most popular supplements has again been put to the test.
A study out of Canterbury University revealed 60 percent of fish oil supplements didn't have the advertised amount of Omega 3 fatty acids in them to reap advertised health benefits.
Fish oil is one of the most popular supplements on the market for its well-known Omega 3 health benefits.
"It's very popular, it seems to sell quite well people come in on a returning basis for it - it seems to work for them so once they've taken it, they realise that it's better for your health," Addington Pharmacy's Ray Sefton says.
But it is now under the microscope, not because it doesn't work, but for the claims made on the labels.
According to the new study, you might not be getting what you're paying for.
The study tested the 10 most popular fish oil brands on the market.
It found only four of the brands had the correct amount of Omega 3 fatty acids in them as advertised on the label.
Julia Rucklidge, professor at Canterbury University, says the study doesn't mean fish oils don't work, but some don't have as much EPA and DHA in them - the important fatty acids found in fish oil to improve heart, brain and joint health.
If there's not enough, there are no real benefits.
However, Natural Health Products New Zealand says the study is deeply flawed. It argues there were fundamental calculation errors in the omega test, which skewed the results.
Consumer New Zealand isn't impressed with the latest findings.
"The disappointing thing is that consumers continue to be duped when they're buying these products and they are quite expensive," Consumer NZ chief executive Sue Chetwin says.