LONDON (Reuters) -- The benefits of taking high doses of vitamin C as a daily dietary supplement are questionable, and the practice may even be harmful, according to a team of British researchers.
Dr. Ian D. Podmore and colleagues from the University of Leicester, UK, report that vitamin C may have a deleterious effect when taken in daily doses of 500 milligrams (mg). Their findings follow a trial of 30 healthy volunteers who were given doses of 500 mg -- or about eight times the UK recommended daily allowance -- for a period of 6 weeks.
In the April 9th issue of the journal Nature, the researchers report that at a dose of 500 mg/day vitamin C has both a damaging pro-oxidant as well as a beneficial antioxidant effect on two important markers of molecular damage, 8-oxoguanine and 8-oxoadenine.
"This is the first time that anybody has ever looked at doses or at a supplementation dose in vivo, in human subjects, where we've managed to measure genetic damage using the markers as indicated, 8-oxoguanine and 8-oxoadenine," co-researcher Dr. Joseph Lunec told Reuters in an interview.
Lunec said that at 500 mg/day vitamin C has the potential of reducing oxidative stress in humans, but superimposed upon this is the pro-oxidant, deleterious effect.
"The bottom line is that the dose (500 mg) is not necessarily the optimal dose to use as a supplement," he said.
In their paper, the researchers note that the body's antioxidant defences resist oxygen radicals that have the potential of damaging DNA. However, if the balance of defences is altered, this can result in oxidative stress. This stress can give rise to biomolecular damage that may play a role in diseases such as cancer.
"Because we know DNA damage is a sequeli of oxidative stress and these are supposedly antioxidants, then theoretically vitamin C should reduce the oxidative stress," Lunec said.
"But in fact we've shown that although it does have a capacity at 500 mg to reduce that oxidative stress in humans, there is a deleterious effect superimposed upon that," he added.
Lunec said that the value of taking high doses of vitamin C supplements has been questioned in the past because it has been shown that "...as soon as you saturate your tissues you begin to excrete the vitamin C." The current study suggests that high doses may be doing damage as well.
"We shouldn't tell people to take as much vitamin C as they can to supplement their food," he said.
SOURCE: Nature (1998;392:559)
