Levine says adults need about 200 mg of vitamin C daily, approximately the amount contained in five servings of fruits and vegetables a day. "Our work reinforces the health message that healthy people should be eating five servings of a variety of fruits and vegetables every day. You'll get adequate vitamin C and you have the potential benefit of preventing disease, especially certain cancers, " Levine explains. Scientists do not know if the protection comes from vitamin C alone or from an interaction of vitamin C with other substances in foods. The U.S. Department of Agriculture and the National Cancer Institute both advocate a diet rich in fruits and vegetables.
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. Daily doses of 200 mg of vitamin C from supplements do not decrease the incidence of certain kinds of cancer.
The current Recommended Daily Allowance (RDA) for vitamin C is 60 mg/day, a number originally set in 1980 and reviewed in 1989 by the Food and Nutrition Board, a part of the National Academy of Sciences. The RDA is based on the amount of vitamin C needed to prevent scurvy, a potentially fatal disease marked by fatigue and bleeding. "We need to think not just about deficiency, which has been the starting point for recommended daily allowances," Levine says, "but about a variety of other scientific factors, such as how much vitamin C is concentrated in blood and tissue when a person takes a specific amount, how much is excreted in urine, what the beneficial and adverse effects are."
When the subjects in Levine's study received 30 mg, most reported feeling tired and irritable. At 200 mg, blood plasma had more than 80 percent maximal concentration of vitamin C and tissues were completely saturated. At doses of 500 mg and higher, there was excess vitamin C that was completely excreted in urine. At 1,000 mg, some volunteers showed high levels of oxalate and uric acid in their urine, which might lead to kidney stones.
According to a 1991 National Survey of Health and Nutrition, half of all American men consume less than 84 mg of vitamin C daily in their food. Half of U.S. women consume 73 mg a day from food. For 20 to 30 percent of U.S. adults, food provides less than the current RDA of 60 mg. These surveys did not include vitamin C intake from supplements, which are taken by 40 to 50 percent of the U.S. population.
The Food and Nutrition Board is planning a broad revision of dietary intake guidelines. In 1998, the Board developed new reference values for nutritional intake in addition to the RDA, including estimated average requirements (EAR), adequate intake (AI), and tolerable upper intake level (UL).
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