A team at Leicester University has shown that taking 500 milligrams or more of vitamin C daily increases blood levels of chemicals which signal that cell DNA is being damaged. They reckon that when taken in such volumes, vitamin C stops working as an antioxidant - a chemical that mops up the highly reactive "free radical" oxygen atoms that can damage cell membranes and their DNA - and instead becomes a pro-oxidant, enhancing such damage.
Cumulatively, such free radical damage is thought to be a major cause of illnesses such as cancer, rheumatoid arthritis and heart disease because it damages the cell's machinery beyond repair. Yet health food shops and pharmaceuticals manufacturers such as Roche sell vitamin C for a wide range of treatments including "passive smoking" and the common cold, prescribing daily doses of up to 1000 milligrams. The normal recommended daily allowance is just 60 milligrams.
"Roche and others say a gram a day is the amount to take, while supermarket shelves suggest 500 milligrams a day," said Professor Joseph Lunec of the chemical pathology department at the university. "But extrapolating from our figures, that would do damage." He commented that "unless you're very severely deficient in vitamin C, you don't need to take those sizes of dose. Yet some people do, for years." In the experiment, whose results are published today in the science journal Nature, a group of 30 men and women were given daily vitamin C for six weeks and an inert placebo for another six weeks. Blood samples were taken at three-weekly intervals during the 12 weeks, and again seven weeks later when all the vitamin C had been washed out of their bodies. The markers for oxidative damage consisted of potentially harmful modifications to specific molecular components of DNA.
The scientists found that while levels of one marker were reduced by vitamin C, another marker for DNA damage known as 8-oxoadenine was significantly increased.
The research team said a fine balance normally existed between the action of oxidant free radicals and the body's defences against them.
Wynnie Chan, a nutrition scientist at the British Nutrition Foundation which promotes healthy eating, said the findings were a reminder that vitamin supplements are no substitute for a balanced diet.
She said: "It is true that people take high doses of vitamin C not only for its antioxidant properties but also because they think it helps prevent colds. People should really try to get their nutrients from a varied and balanced diet and not rely on the magic effect of supplements."
The finding follows a row over the government's decision to limit sales of vitamin B6. Last month doses above 10 milligrams became available only through pharmacists - subject to a ceiling of 50 mg. Larger amounts would require a doctor's prescription. The reason for the imposed limit was that experts say that in high doses the vitamin can cause neurological symptoms, such as tingling and numbness, and possibly permanent nerve damage. But the Government received 100,000 letters from women who used it to relieve water retention and pre-menstrual tension over the plan, which was based principally on a 1987 study.Reuse content