Large vitamin C doses may raise risk of cancer

High doses of vitamin C could raise rather than lower the risk of cancer by triggering a biological change in the body that damages DNA, new research suggests.

The findings go a long way to explaining why, contrary to expectations, studies have failed to find any benefits from vitamin C supplements, a favourite with stars such as Geri Halliwell, in the fight against cancer.

The work, which is published today in the journal Science, also backs up thediscovery two years ago by a team at Leicester University, who found that taking very high doses of vitamin C ­ up to 500 milligrams daily ­ appeared to increase the level of chemicals in the blood associated with DNA damage.

The latest research, by scientists from the University of Pennsylvania, Philadelphia, is based on a series of tests involving the equivalent of a daily intake of vitamin C of 200mg. In Britain the Government's recommended daily allowance is 60mg although some health food and pharmaceuticals companies prescribe daily doses of up to 1,000mg to treat "passive smoking" and the common cold. One 200 millilitre (7 fl oz) glass of orange juice provides about 50mg of vitamin C.

The American study shows that vitamin C is capable of inducing the production of DNA-damaging chemical agents called genotoxins. DNA mutations caused by these compounds have been found in a variety of tumours.

Dr Ian Blair, from the university's Centre for Cancer Pharmacology, who led the research, said: "It's possible that vitamin C isn't working in cancer prevention studies because it's causing as much damage as it's preventing, but that's really speculation ... What we can say is that vitamin C doesn't work when you expect it to."

Dr Wendy Doyle, spokeswoman for the British Dietetic Association, said the findings reinforced the message that vitamins should be obtained from natural sources rather than supplements.

"I would say people should continue to eat five portions of fruit and vegetables a day. These foods don't just contain vitamin antioxidants but also other plant chemicals that are health promoting. There's no substitute for a good balanced diet."

She strongly advised against the American "fad" followed by Halliwell of injecting vitamins. Shooting vitamins straight into the bloodstream bypassed the body's natural regulatory mechanism, which enables it to take what it needs. Dr Doyle said: "It's going to put extra strain on the kidneys, which are having to get rid of what could be toxic substances."

When functioning as expected, vitamin C is a powerful antioxidant that helps to disarm free radicals ­ highly reactive and destructive molecules created naturally in the body. They are also associated with many age-related ailments such as heart disease, cancer and arthritis.

Some experts in the past have recommended dietary supplements of vitamin C for treating and preventing cancer, while many people routinely take high doses in the belief that it can ward off colds.

The American team now plans to see whether vitamin C produces substantial amounts of genotoxins in intact cells, and whether they generate cancer-causing mutations.