Vitamin C has been long touted as a healthful and wholesome dietary supplement, because it can protect DNA against certain kinds of damage. It now appears to also have an unsavory chemical side effect. In the 15 June issue of Science, researchers report that vitamin C can also spur the formation of molecules known to scramble DNA.
Antioxidants such as vitamins C and E can disarm the extremely destructive molecules called free radicals, so many people believed that antioxidant supplements would help prevent cancer. This has not proved to be true in human trials, however. Now researchers led by Ian Blair of the University of Pennsylvania in Philadelphia may have an explanation.
In the process of destroying free radicals, vitamin C turns into what's called a vitamin C radical. If certain metal ions are nearby, vitamin C radical can turn compounds called lipid hydroperoxides into genotoxins, which switch bases around in DNA, disrupting its delicate code. However, these metal ions are rare in human blood.
Something had to be forming the genotoxins, though: Even healthy people show characteristic genotoxin-induced DNA damage. Blair and his colleagues decided to see if vitamin C alone would do the trick. Working in solutions carefully cleaned of all relevant metal ions, the researchers showed that the equivalent of a 200-milligram vitamin C supplement could trigger the formation of the suspect genotoxins.
It's not clear how big a risk this reaction poses, however. The vitamin C concentrations the researchers used aren't much higher than those found in normal human blood. But not all people have high levels of genotoxins' raw material, lipid hydroperoxide. Blair and his colleagues intend to look more closely at two patient populations that do: Women with breast cancer and children taking certain drugs for leukemia.
The prevalence of these genotoxins and the DNA damage they cause has been puzzling cancer biologists for years, because the relevant metal ions are rare in blood and the conditions aren't quite right. "Nobody had a clue where they were coming from," says Larry Marnett, a biochemist at Vanderbilt University in Nashville, Tennessee. The new explanation, that vitamin C might be the trigger, "makes a whole lot of sense chemically."