The researchers suggest that consumers should switch to vegetable oil-based products.
Sales of fish oils have seen strong growth in recent months on the back of evidence showing that omega 3 fatty acids, found in high quantities in certain fish, may reduce the risk of heart disease, improve mental health and also fight ageing.
But fish oils are also prone to contamination by organic chemicals. Last month researchers found that farm-raised salmon contain more contaminants than wild salmon, which they attribute partly to the fish oils added to salmon feed.
Writing in the April 7 edition of theJournal of Agricultural and Food Chemistry, the researchers describe an analysis of 21 fish and vegetable oil dietary supplements available in the UK. Levels of polybrominated diphenyl ethers (PBDEs) ranged from about 15-34 nanograms per gram of fat, while the range was 0-13 only four years ago.
"This is a relatively large increase," said author Dr Miriam Jacobs from the University of Surrey in Guildford, UK. "The extensive use of these chemicals in recent years means that they can get into places where they shouldn't be, such as the marine environment."
PBDEs have been found in a number of unexpected places recently, from human breast milk in the United States to peregrine falcon eggs in Sweden. Used in the textile and electronics industries, the chlorine compounds take a long time to degrade and since they are fat soluble, often end up in fatty tissues of fish and other aquatic species.
The study, which compared the results to an analysis carried out on the same brands purchased eight years ago from the same retailers, found that levels of pesticides and polychlorinated biphenyls (PCBs) had not risen significantly over this period.
But supplements based on vegetable oil and whole body fish oil showed little or no contamination throughout the current and previous studies. "It suggests that a consumer can reduce her or his intake of the persistent organic pollutants by changing to a formulation that contains less cod liver oil, and that contains a proportion of vegetable oil sources of omega 3 fatty acids," Jacobs said.
Vegetable oils contain short-chain fatty acids, which are generally thought to offer less health benefit than the long-chain fatty acids from fish oils. Researchers have shown, however, that humans can metabolise and produce long-chain fatty acids from short-chain vegetable sources. But Norwegian company Pronova Biocare, which claims to be the leading supplier of concentrated fish oils, said vegetable oils are a limited source of omega-3 fatty acids.
"The omega-3 fatty acids in vegetable oils are only alpha-linolenic acid (ALA) while the beneficial EPA and DHA are not present at all. The human body can, to some extent, convert ALA to EPA and DHA, but there is increasing documentation to show that this conversion is very slow and inadequate," said a company spokesperson.
Pronova sources its crude oil from the eastern Pacific waters where the concentration of such pollutants is lower than elsewere. It also uses molecular distillation to remove such contaminants.
"Ourresults show that the process is very effective also for removal of flameretardants. All our crude oils, regardless of the concentration ofpollutants, are run through this process," added the spokeswoman.
The company says previous findings on chemical contamination of fish products have not impacted its sales, and demand is still surging.The fish oil market for human consumption is now thought to be around7,000 to 8,000 tons, with up to 90 per cent of this used in the supplementindustry and the rest in functional foods.
Jacobs notes that better testing for PBDEs may help eliminate them in the future. "We hope these findings will stimulate further research into the newer pollutants to better protect the public and the environment. Regulatory authorities conduct food-monitoring programmes for dioxins and PCBs, but far less so for other contaminants."
The research was initially published on 26 February on the journal website.