The Ministry of Primary Industries (MPI) and Medsafe are now investigating the findings, while the industry body for producers of omega-3 supplements has reacted with surprise.
Fish oil supplements are commonly used in New Zealand.
The 2008/09 New Zealand Adult Nutrition Survey showed that 48 per cent of adults used supplements in the year prior to the study, with 31 per cent being regular users.
Oil supplements - although not just omega-3 - were the most frequently used (16 per cent).
The tested products cost between 7c and $1 per gram; bottles of 60 capsules of the supplement typically ranged between $40 and $60 in pharmacies.
Consumers take fish oil supplements for many reasons, but particularly because it has shown promising effects of lowering risk of heart disease and inflammation, and improving brain power.
The research, published in the major journal Nature, also looked at oxidation levels of the products, as omega-3 fatty acids are known to be unstable and can break down when exposed to light, heat and oxygen.
They found that over half had oxidised to a level higher than the recommended limit - and this had nothing to do with the best-before date, price, or country they came from.
The vast amount of oils were sourced from South American deep sea fish, however the researchers deliberately chose not to name the brands and which passed or failed their testing.
When all of the measures the researchers used to evaluate the oxidation levels were applied, only 8 per cent were in line with international recommendations.
Study co-author Professor Wayne Cutfield said while this did not imply that the products were dangerous, more investigation was needed to establish the potential impacts of the degradation.
"Many of us buy items that contain properties that are slightly less than what is claimed, and that is not such a big deal; but that it's degraded is not what we would expect," he said.
"It would help if there were tougher or more rigorous regulations, in terms of requirements to demonstrate what is actually in the product and whether it's degraded, or whether it's potentially a health risk or not."
Last night, an MPI spokeswoman said the ministry was "working closely" with Medsafe on the requirements of the Dietary Supplements Regulations 1985 and how they apply to these products.
Kevin Krail, executive director of the Omega-3 Centre, said the findings had come as a surprise.
"We have seen studies like this before and the most recent results typically show these omega-3 fish oil nutritional supplements are of high quality and meet international standards," he said.
"What we will be doing is getting our scientists in touch with the study collaborators and closely examining the test method and analysis."
But Consumer NZ chief executive Suzanne Chetwin said she wasn't surprised by the research and believed more monitoring was needed around supplement claims.
"It is concerning when these claims are being made as people think they are eating healthily, but don't know they are taking much smaller quantities than they think," she said.
"The regulations are probably there, but there needs to be far more scrutiny."
Professor Murray Skeaff, a Professor in Human Nutrition at the University of Otago, said If the results are accurate, then fish oil supplements in New Zealand could be added to a long list of dietary supplements for which there were serious discrepancies between what the manufacturer claimed the consumer was getting and what was actually in the supplement.
"Of additional concern is that the vast majority of the fish oil supplements contain amounts of oxidised fats that exceed recommended levels, in other words the oils are on the road to becoming rancid if not already so," he said.
"The researchers should publish the brand names of the fish oils supplements that were analysed so that consumers may be able to identify the supplements of highest quality."
Dr Matt Miller, a marine lipid chemist at Plant and Food Research, said the results were "troubling" but not surprising, as there had been other comparable studies conducted around the world with similar results.
"However, there is still an overwhelming body of scientific evidence about the nutritional benefits of omega 3 supplementation. It is well understood that omega 3 is a very bioactive compound and the double edged sword is that it is prone to oxidative degradation."
Dr Miller, who is also president of the Australasian section of the America Oil Chemist Society, said the high levels of oxidation shown in some products were indicative of the long journey the oil has to take to get to the pharmacy.
"As indicated in the article most of the fish oil available in New Zealand, whether capsulated here or in another country, comes from the anchovy or sardine fishery off the coast of Peru," he said.
After catch, it was rendered, sometimes refined and sent by ship around the world where it was then refined and capsulated by local companies.
"Although care is taken, this long journey from ocean to consumer provides ample opportunity for oxidation, a degradative chain reaction, to occur," he said.
"As discussed in the report, it is uncertain whether the level of oxidation in the study poses a health risk, however, for the consumer the best advice is to protect their fish oil by storing it sealed in the fridge and out of direct light."
Plant and Food Research was working with several New Zealand companies, such as Seadragon Marine Oils and the Sealord Group Ltd, to investigate the potential for NZ industry to supply high quality marine oil products.
New Bill needed to ensure label claims are accurate: industry group
The findings have prompted a call by industry umbrella group Natural Products NZ (NPNZ) for the "rapid" passing of a bill that would help to ensure dietary supplements contain what they say they do.
The Natural Health and Supplementary Products Bill, which awaits its third reading in Parliament, would regulate the manufacturing and selling of natural health products in this country, said NPNZ, which represents the country's natural products, functional foods, complementary medicines, cosmeceuticals and nutraceuticals industries.
The bill would also strengthen regulation around which ingredients and health benefit claims will be permitted or not permitted, and what product information must be provided.
"The bill is an important piece of legislation, which we believe will benefit consumers, the natural health and supplementary products industry and New Zealand as a whole," the group's executive director Alison Quesnel said.
The new regulation was needed due to consumers' growing demand for alternative options to maintain health and assist disease prevention.
"New Zealand therefore needs a modern regulatory environment that recognises consumers' right to information about natural health products and their functions, and the right to access products that are safe and effective."
Ms Quesnel said the findings also came as a surprise to her, and that further investigation was needed in relation to the research.
"Despite the potential questions that the research raises it is important to remember there is still an overwhelming body of scientific evidence about fish oil's nutritional for brain, heart and joint health."
Fishy claims
What was the research?
The study by University of Auckland researchers has revealed that of 36 fish oil products sold in Auckland and marketed online, most nationwide contained considerably less of the vital omega-3 fatty acids claimed by their labels. The products contained an average of just 68 per cent of the claimed content - and more than two thirds of supplements tested contained less than 67 per cent. Two products only included only a third of what was on the label.
What else did they discover?
They found that over half had oxidised, or broken down, to a level higher than the recommended limit - and this had nothing to do with the best-before date, price, or country they came from. When all of the measures the researchers used to evaluate the oxidation levels were applied, only 8 per cent were in line with international recommendations. The key properties in the supplements are considered highly fragile and can break down on exposure with air, light or heat. The scientists suggest more work is needed to better understand the health implications of oxidised fish oil, which remained "unclear".
Why is this important?
Up to 30 per cent of Kiwis take dietary health supplements, and omega-3 products are considered one of the most, if not the most, popular.
Consumers take fish oil supplements for many reasons, but particularly because it has shown promising effects of lowering risk of heart disease and inflammation, and improving brain power. Buyers need to know whether the amounts of fatty acids in the products are as claimed on the label, and by what rate they may have degraded before sale.
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