The dietary vitamin C may be considerably lower than the calculated amount in the food ingested, largely because of its destruction by heat and oxygen and its loss in cooking water. On the other hand, the mean total intake of vitamin C may also be considerably higher because (1) supplements of vitamin C are ingested by 35% of a representative U.S. adult population (Stewart et al., 1985), (2) food composition tables used in the U.S. Department of Agriculture surveys provide the L-ascorbic acid content only and do not include the biologically active dehydroascorbate, and (3) ascorbic acid is added to some processed foods because of its antioxidant or other properties (NRC, 1982).
Vitamin C status is usually evaluated from signs of clinical deficiency, plasma (or blood) levels, or leukocyte concentrations. It has also been evaluated from isotopic estimates of body stores.
Clinical signs of scurvy, including follicular hyperkeratosis, swollen or bleeding gums, petechial hemorrhages, and joint pain, are associated with plasma (or serum) vitamin C values of less than 0.2 mg/ dl, leukocyte concentrations of less than 2 µg/108 cells, and a body pool size of less than 300 mg (Hodges et al., 1969, 1971; Sauberlich, 1981). To eliminate clinical signs of scurvy in several groups of male subjects, vitamin C intakes ranging from 6.5 to 10 mg/day were required (Baker et al., 1971; Bartley et al., 1953; Hodges et al., 1969, 1971).
The dietary allowances for vitamin C must be set, somewhat arbitrarily, between the amount necessary to prevent overt symptoms of scurvy (approximately 10 mg/day in adults) and the amount beyond which the bulk of vitamin C is not retained in the body, but rather is excreted as such in the urine (approximately 200 mg/day). Between these limits, body stores vary directly with intake, albeit not linearly. Since vitamin C is poorly retained in the body in the absence of continuous intake, the RDA has traditionally been set at a level that will prevent scorbutic symptoms for several weeks on a diet lacking vitamin C. Observed depletion rates in a small group of well-nourished adult men with a body pool of approximately 1,500 mg were exponential and averaged 3.2% daily (range, 2.2 to 4.1% in nine subjects), which would yield a body pool of vitamin C of 300 mg (the amount below which scorbutic symptoms can occur) in about