Vitamin C Supplements May Do as Much Harm as Good

From the WebMD Archives

June 14, 2001 - Debate has been raging for years over the health benefits of vitamin C. Proponents claim that megadoses of the powerful antioxidant do everything from preventing the common cold to curing cancer. But hard evidence has been lacking. Now, data show that while vitamin C does protect against DNA damage in some circumstances, it may just stimulate it in others.

Within cells, the continuous breakdown of oxygen results in byproducts called free radicals. These highly reactive ions eagerly bond with other molecules, setting off chemical reactions and forming new, sometimes dangerous, compounds. If they affect DNA, the resulting mutations can lead to tumors.

Antioxidants such as vitamin C deactivate free radicals, rendering them harmless. Because people who eat a lot of vitamin C-rich foods tend to be particularly healthy, the thinking has been that taking vitamin C supplements should be beneficial.

But "there's very little evidence that vitamin C supplementation does anything," says study leader Ian A. Blair, PhD, professor of pharmacology and director of the Center for Cancer Pharmacology at the University of Pennsylvania. "Data that show a reduction of risk of heart disease and cancer come from dietary studies. None show that you get the same [benefit] from supplementation."


In a series of test-tube experiments, Blair's team took a closer look. They added vitamin C to several substances commonly found in the human body and watched what happened. Though vitamin C did neutralize several of the resulting free radicals, one reaction stood out as potentially problematic.

Linoleic acid is the major polyunsaturated fatty acid in sunflower and safflower cooking oils, as well as in human plasma. Sometimes, linoleic acid gets broken down in the body into a substance called lipid hydroperoxide. If certain metal ions are present, these lipid hydroperoxide substances are further broken down to create "genotoxins" -- substances harmful to DNA. In the experiment, vitamin C promoted this dangerous transformation even more than metal ions.

"So [vitamin C] does some good things, and it does some bad things," Blair tells WebMD.

Just don't jump to the wrong conclusion, Blair cautions. The findings do not mean that ingesting vitamin C will cause cancer. Rather, "this is just one more of a whole series of studies that show you should eat your fruits and vegetables and not worry about vitamin C supplementation."

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