By Janet Raloff

Web edition: November 20, 2008

Diets rich in fish oil offer a number of health benefits, from fighting heart disease to boosting immunity. However, many noxious contaminants preferentially accumulate in fat. These include pesticides, brominated flame retardants, dioxins, and some related compounds known as polychlorinated biphenyls. So there’s been some concern that if a fish was pulled from polluted waters, its fat might be polluted too. And those pollutants could end up an unwanted bonus in commercial fish-oil supplements.

A new survey of some 154 different fish-oil capsules sold by 45 different companies now confirms that some supplements are remarkably dirty and others quite pure. In general, PCBs and a breakdown product of DDT were the major pollutants in fish-oil supplements. Adrian Covaci of the University of Antwerp, Belgium, and his colleagues reported their findings today at the Society of Environmental Toxicology & Chemistry meeting here in Tampa.
 
The mean concentrations detected were 15 nanograms per gram of oil for the PCBs and about two-thirds that amount for the DDT metabolite. Mean concentrations of polybrominated biphenyls, a widely used class of flame retardants, were much lower — down around 2 ng/g. That’s good news. Such trace amounts tend to be only about 3 to 12 percent as much of these pollutants as people would have received had they downed a meal of fish.

On the other hand, not all supplements were clean. And Covaci’s group linked the purity of the fish-oil capsules to several general factors. For instance — and no surprise — supplements made from the oil of fish caught in cleaner waters (hint: in Europe that would not be the Baltic) tended to have lower concentrations of toxic pollutants.

Perhaps a little less intuitive, oil derived from smaller species, such as anchovies, typically harbored fewer pollutants than did oil from their far bigger cousins — cod or salmon. Explains Covaci: Smaller fish tend to be shorter lived, so “they will have had less time to accumulate pollutants.”

Some companies go through a number of advanced chemical-stripping processes to remove fish-oil pollutants. And Covaci’s group confimed that the products from these suppliers tend to be quite clean.

The rub: Most companies don’t advertise how they processed their oils, and some don’t identify the source of their fish, much less what species they had tapped for its oil.

One surprise: Many companies that had labeled their supplements as being certified free of dioxins or methyl mercury actually turned up positive for those toxic contaminants. So they were still higher — more tainted — than the really clean supplements. Companies get away with advertising their fish-oil capsules as being free of these pollutants, Covaci explained to me, because pollutant values in them fall below concentrations allowed under European Union food-safety regs.

So much for truth in advertising.