London - Fish-oil supplements credited with a range of health benefits could trigger prostate cancer.
Experts found that omega-3 fatty acids may raise the risk of the most lethal form of the disease by more than 70 percent.
Researchers warned against omega-3 pills and recommended eating just one or two meals of oily fish a week.
Fish-oil supplements are said to protect against heart attacks and strokes, stave off arthritis, boost brain power and prevent behavioural disorders in children.
However, scientists found that those with the highest levels of omega-3 in their blood were 71 percent more likely to develop fast-growing, hard-to-treat prostate tumours.
They were also more likely to contract the slower, less deadly form of the disease, with the overall prostate cancer risk raised by 43 percent.
The team from the Fred Hutchinson Cancer Research Centre in Seattle warned: “There is really no evidence that taking dietary supplements is beneficial to health and there is increasing evidence that taking high doses is harmful.”
Dr Alan Kristal said the levels of omega-3 linked to the increased cancer risk would be reached by taking just one supplement a day or three or four meals of fish such as salmon and mackerel each week.
Of mealtimes, he said: “There are good things in fish, so the message is moderation. It is probably not bad for you, and it tastes good.”
However, he stressed that when compared to fatty acids received by eating oily fish, the amount consumed via pills was “huge”.
It is unclear how fish oil could trigger tumours, but omega-3 may restrict the immune system, or damage DNA.
It is also unclear whether it helps tumours to grow and spread.
The finding came amid a wider research project of more than 2 000 men, examining whether supplements of vitamin E and the mineral selenium can help prevent prostate cancer – the most common cancer in British men, killing more than 10 000 a year.
Selenium provided no benefit, and vitamin E increased the odds of contracting the disease.
Kristal said: “As we do more and more of these studies – and I have been involved in them most of my career – we find high doses of supplements have no effect or increase the risk of the disease you are trying to prevent.”
Several recent studies have called omega-3s benefits to the heart into question, too. One, which examined 20 studies involving almost 70 000 people, found that those who took omega-3 were no less likely to die of a heart attack or stroke than others.
“There is not really a single example of where taking a supplement lowers chronic disease risk.” - Daily Mail