"We had expected to find a benefit of the supplements on cancer risk," said Valentina Andreeva, the study's lead author and a researcher at the University of Paris. "Instead, we found no effects in men, and some evidence of adverse effects in women."

Previous research has hinted that B vitamins might help protect people against cancer, especially colorectal cancer, though not all studies have agreed.

The original aim of Andreeva's trial was to test the effect of taking omega-3 fatty acids, vitamin B or both on cardiovascular disease in people with a history of heart attack or stroke.

To better understand whether the supplements might have additional effects, the group also collected information on how many trial participants developed cancer.

The researchers had split the study subjects into four groups: one took two vitamin-B pills a day, another group took two pills of omega-3 fats, a third group took both supplements and a fourth group took fake pills that resembled the supplements.

The B vitamins were a mixture of 3 mg of B6, 0.02 mg of B12 and 0.5 mg of folic acid.

The participants assigned to take the omega-3 fats got 600 mg a day, with the supplements containing twice as much EPA as DHA.

For about five years Andreeva and her colleagues at the French national medical research institute, INSERM, tracked cancer diagnoses among the study subjects.

More than 2,000 people finished the study, and of these, 174 developed cancer and 58 died of it.

Those in the two groups that took B-vitamin supplements had the same risk of cancer as those who took the placebo pills.

These are in line with the findings of an even larger study of heart attack survivors, which also found B vitamins to be ineffective at reducing their risk of cancer (see Reuters Health story of June 22, 2010).

Similarly, men who took the omega-3 pills had the same risk of cancer as the men who took the placebo.

However, among women who took the omega-3 pills, the risk of cancer was three-fold.

There were 21 cases of cancer in the fish-oil group, compared to eight cases in the placebo group.

Andreeva's team also found that women were more than five times as likely to die of cancer if they had taken the omega-3 pills than if they had taken the fake supplements.

"The dietary supplements that we studied are in fact active substances that, when taken over a long period of time and without a physician's advice, might have adverse effects in some populations," Andreeva told Reuters Health in an email.

But she cautioned against interpreting her statement or her group's findings as meaning the fish oil pills were to blame for the increased cancer risk.

The study's numbers are small and its design, intended to track heart disease, cannot show direct cause and effect regarding the cancers.

If anything, the group writes in the Archives of Internal Medicine, early cancers and pre-cancerous growths might have been missed when participants were recruited, and those might have been fueled in some way by the supplements.

A more recent and much larger observational study - which also cannot prove cause-effect -- found that women who took fish oil pills were a third less likely to develop breast cancer than women who didn't take the supplements (see Reuters Health story of July 8, 2010).

"Our findings must be confirmed by other studies before we could formulate or refine public health recommendations on this topic," Andreeva said.

SOURCE: bit.ly/zKpC9I Archives of Internal Medicine, online February 13, 2012.

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