How much vitamin C is enough? Is it the 60 milligrams a day - the amount in half a cup of fresh orange juice - that is the current Recommended Dietary Allowance (RDA), the 30 to 40 milligrams that some nutritional biochemists think it should be, the hundreds of milligrams that millions of Americans now take as a daily supplement or the thousands of milligrams that the late Dr. Linus Pauling believed would protect against serious illnesses, including cancer?
A detailed new federally sponsored study, by far the most comprehensively done to date, says none of the above. The study, directed by Dr. Mark Levine and published Tuesday in The Proceedings of the National Academy of Sciences, found that the “optimal” daily intake of vitamin C was more like 200 milligrams, although only about 10 milligrams are needed to prevent vitamin C deficiency.
The researchers, at the National Institutes of Health, also concluded that daily doses above 400 milligrams “have no evident value” and that amounts of 1,000 milligrams (1 gram) or more, which many people now take as daily supplements or on occasion to prevent or treat illness, could be hazardous. Beyond a dose of about 400 milligrams, the study showed, the body’s ability to absorb vitamin C sharply declines and excess vitamin is excreted.
Unlike previous studies used to establish recommended amounts, this one looked beyond the levels needed to prevent scurvy.
“This means Linus Pauling was all wrong, at least with respect to healthy people,” Levine remarked in an interview. “He had the best of intentions, but he did not have the science to support his hypothesis.”
Industry sources estimate that 30 to 40 percent of Americans now take vitamin C supplements, and that about 1 in 5 supplement users take over a thousand milligrams a day.
Although the 200-milligram level is more than three times the currently recommended amount, it is a level that can still be readily obtained from foods, especially if one follows the latest federal advice to eat five or more servings a day of fruits and vegetables. For example, one would exceed the 200-milligram level by consuming four ounces of orange juice, half a cup of cooked broccoli, one baked potato and one kiwi fruit.
But the most recent national survey indicated that less than a third of Americans consumed five or more servings of fruits and vegetables a day. This suggests that unless significant improvements are made in people’s eating habits, it would be necessary to take supplements or fortify commonly eaten foods with vitamin C for most of the population to consume 200 milligrams each day.