Turning conventional wisdom on its head, new research has found that consuming omega-3 supplements may actually increase the risk of some cancers instead of reducing it.

Omega-3 supplements may slightly reduce coronary heart disease mortality and events, but slightly increase the risk of prostate cancer, according to two systematic reviews published in the British Journal of Cancer and the Cochrane Database of Systematic Reviews. "These large systematic reviews included information from many thousands of people over long periods," said lead author Lee Hooper from the University of East Anglia's Norwich Medical School in Britain. "This large amount of information has clarified that if we take omega 3 supplements for several years we may very slightly reduce our risk of heart disease, but balance this with very slightly increasing our risk of some cancers. The overall effects on our health are minimal," Hooper said.

The researchers found that both beneficial and harmful effects of omega-3 supplements are small. If 1,000 people took omega-3 supplements for around four years, three people would avoid dying from heart disease, six people would avoid a coronary event (such as a heart attack) and three extra people would develop prostate cancer. Omega-3 is a type of fat. Small amounts are essential for good health and can be found in the food that we eat including nuts and seeds and fatty fish, such as salmon.

These fats are also readily available as over-the-counter supplements and they are widely bought and used. The research team looked at 47 trials involving adults who did not have cancer, who were at increased risk of cancer, or had a previous cancer diagnosis, and 86 trials with evidence on cardiovascular events or deaths. More than 100,000 participants were randomised to consume more long-chain omega-3 fats (fish oils), or maintain their usual intake, for at least a year for each of the reviews.

They studied the number of people who died, received a new diagnosis of cancer, heart attack or stroke and/or died of any of the diseases. "The evidence on omega 3 mostly comes from trials of fish oil supplements, so health effects of oily fish, a rich source of long-chain omega 3, are unclear. Oily fish is a very nutritious food as part of a balanced diet, rich in protein and energy as well as important micronutrients such as selenium, iodine, vitamin D and calcium - it is much more than an omega 3 source," Hooper said. "But we found that there is no demonstrable value in people taking omega 3 oil supplements for the prevention or treatment of cancer. In fact, we found that they may very slightly increase cancer risk, particularly for prostate cancer," Hooper added.