Vitamin C Under Siege
A controversial study just published in the journal Science has concluded that vitamin C may help make a toxin that can damage cellular DNA.
A team of researchers led by Ian A. Blair of the University of Pennsylvania conducted test tube experiments to analyze the chemical actions of vitamin C on lipid hydroperoxide, a compound produced in the body from fat in the diet. Lipid hydroperoxide can be converted into substances called genotoxins that can damage DNA.
The researchers found that vitamin C was surprisingly efficient in converting lipid hydroperoxide into genotoxins. For this reason, they warned that extremely high doses of vitamin C might be counterproductive, if not harmful.
They cautioned, however, that because their results came from laboratory test tube experiments, similar chemical actions might not result in humans.
The above study runs counter to a growing body of research suggesting that vitamin C-one of the most popular dietary supplements-can play a role in preventing the onset of cancer and other chronic diseases. Megadoses of vitamin C were touted back in the 1970s by the late Nobel laureate Linus Pauling as a way to significantly minimize cancer risk.
Many current researchers agree with Pauling (albeit with more moderate doses), based primarily on vitamin C's antioxidant properties. Antioxidants neutralize oxygen free radicals, which are products of cell metabolism that can damage DNA.
Studies have shown that healthy adults can benefit in many ways from a daily intake of 100 to 200 mg of vitamin C. A healthy body can absorb about 200 milligrams a day; the surplus is carried away with urine.
Dr. Mark Levine of the National Institute of Diabetes and Digestive and Kidney Diseases, writing in the Journal of the American Medical Association, agreed that adults need about 200 mg of vitamin C daily. This is approximately the amount contained in five servings of fruits and vegetables.
He nonetheless cautioned that healthy people are better off eating fruits and vegetables rather than relying on supplements because absorption of the vitamin in supplements varies widely, depending on manufacturing methods and the dose taken.
The current Recommended Daily Allowance (RDA) for vitamin C is 90 mg/day for men and 75 mg/day for women. (It is recommended that smokers add an additional 35 mg/day to their respective rates.) The U.S. Department of Agriculture estimates that 39 percent of men and 43 percent of women fall short of these recommended requirements.
SOURCES:
Science, June 15, 2001; 292:5524
Journal of the American Medical Association, April 21, 1999; (281)15