Taking fish oi l suppleme nts has no benefit in preventing or treating Type 2 diabetes, a large review of randomized clinical trials has found.

Some observational studies have suggested a positive effect for the supplements, which are high in omega-3 fatty acids, and they are widely promoted for diabetes control and prevention.

But this new analysis, commissioned by the World Health Organization and published in BMJ, included 83 trials involving 121,070 subjects with and without diabetes, mostly testing omega-3s against placebo. Omega-3s had little or no effect on the likelihood of a diabetes diagnosis, on average blood glucose levels over time, or on fasting insulin. There was some weak evidence that omega-3 doses above 4.4 grams a day could be harmful for people with diabetes.

The effects of two other polyunsaturated fatty acids — alpha-linoleic acid (ALA) and omega-6 fatty acids — were unclear. The researchers found no effect of either increasing total consumption of all types of polyunsaturated fatty acids, or of adjusting the ratio of omega-3s to omega-6s.