CORVALLIS, Ore. - An extensive review of research on potentially harmful effects of vitamin C supplementation has largely put to rest concerns that it may have damaging "pro-oxidant" effects in the human body, a question that had been raised by some laboratory studies in recent years.
The new report by scientists in the Linus Pauling Institute at Oregon State University will be published in June in the FASEB Journal, a professional publication of the Federation of the American Society of Experimental Biology.
"Some in-vitro studies in the laboratory have found that vitamin C can have undesirable reactions with metal ions such as copper or iron in the presence of an oxidant such as hydrogen peroxide," said Anitra Carr, a research associate with the Linus Pauling Institute. "Some scientists, including one study that got international attention last year, have hypothesized that the same thing may be possible in animals.
"But after carefully examining 44 different in-vivo, or human and animal studies done in this area, which included both our own research and that of others, it's become clear that what happens in the test tube or a laboratory solution is not the same as what happens in the human body," Carr said.
According to Carr and Balz Frei, co-author of the report and director of the Linus Pauling Institute, in a laboratory setting it's been clearly shown that vitamin C can react with certain metals to produce free radicals, which are unstable molecules that in turn can interact with lipids, proteins or DNA to cause cellular damage and mutations.
"The whole idea of taking anti-oxidant vitamins is to reduce oxidation, not increase it," Carr said. "Studies have linked oxidized proteins to aging, oxidized DNA to cancer and oxidized lipids to heart disease. This damage is often slow and cumulative, so even a small reduction in oxidative damage over a long period of time can have very positive health effects."
Vitamin C is one of the most commonly used nutritional supplements in the world, largely for its known anti-oxidant effects, and an increasing number of studies are finding that a diet high in vitamin C or supplementation with it may have value in problems ranging from cancer to angina pectoris and heart disease, the LPI scientists said.
The suggestion that vitamin C might actually cause degenerative health problems, rather than reduce them, caused a considerable stir when it was brought up, Carr said.
But according to the LPI researchers, a large body of work now done in this area tends to prove just the opposite - that in animal and human bodies, vitamin C is clearly an anti-oxidant and should have beneficial health effects.
Reasonably high levels of vitamin C supplementation also do not appear to be a problem, the OSU researchers said, contrary to some other suggestions that modest levels of vitamin C supplements might not have unhealthy effects but larger doses could.
To find out what is actually going on in real organisms, as opposed to the test tube, most researchers look at what they call "biomarkers" of oxidative damage - biological keys that help tell what is going on during the metabolic process.
In numerous studies done with vitamin C supplementation mostly with humans and some other animals, 38 different studies found a reduction of oxidative biomarkers and 14 showed no change, Carr said.
There were six studies which showed some increase in oxidative biomarkers with vitamin C supplementation, the OSU researchers found. But in each of those studies, there was at least one significant problem with the use of inappropriate biomarkers, methodologies, study systems or poor experimental design, they said.
"We conclude that in human bodies vitamin C does not interact with metals such as iron to become a pro-oxidant," Carr said. "It appears that in the body, such metals are strongly bound to proteins and are not really free to bind with vitamin C to form free radicals."
This research was supported by the National Institutes of Health.
OSU's Linus Pauling Institute is an international leader in the study of vitamins, phytochemicals and other micronutrients in foods, and their potential roles in disease prevention and therapy.