Recommendations for vitamin C intake are under revision by
the Food and Nutrition Board of the National Academy of Sciences. Since
1989 when the last recommended dietary allowance (RDA) of 60 mg was
published, extensive biochemical, molecular, epidemiologic, and
clinical data have become available. New recommendations can be based
on the following 9 criteria: dietary availability, steady-state
concentrations in plasma in relationship to dose, steady-state
concentrations in tissues in relationship to dose, bioavailability,
urine excretion, adverse effects, biochemical and molecular function in
relationship to vitamin concentration, direct beneficial effects and
epidemiologic observations in relationship to dose, and prevention of
deficiency. We applied these criteria to the Food and Nutrition
Board's new guidelines, the Dietary Reference Intakes, which include 4
reference values. The estimated average requirement (EAR) is the amount
of nutrient estimated to meet the requirement of half the healthy
individuals in a life-stage and gender group. Based on an EAR of 100
mg/d of vitamin C, the RDA is proposed to be 120 mg/d. If the EAR
cannot be determined, an adequate intake (AI) amount is recommended
instead of an RDA. The AI was estimated to be either 200 mg/d from 5
servings of fruits and vegetables or 100 mg/d of vitamin C to prevent
deficiency with a margin of safety. The final classification, the
tolerable upper intake level, is the highest daily level of nutrient
intake that does not pose risk or adverse health effects to almost all
individuals in the population. This amount is proposed to be less than
1 g of vitamin C daily. Physicians can tell patients that 5 servings of
fruits and vegetables per day may be beneficial in preventing cancer
and providing sufficient vitamin C intake for healthy people, and that
1 g or more of vitamin C may have adverse consequences in some
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