Utilization of ascorbic acid
Active absorption of ascorbic acid has been demon-trated in the jejunum and ileum in humans.21,22 Uptake of ascorbic acid into the mucosal cell depends on the presence of sodium in the lumen. A membrane sodium carrier also binds ascorbic acid and, while sodium enters dIe cell by facilitated diffusion, ascorbic acid is carried along regardless of its intracellular concentration. Energy is required in the expulsion of sodium to the serosal fluid" to maintain its normally low intracellular concentration and, as a result, for the absorption to continue. In the absence of sodium, absorption of ascorba te proceeds at a slow rate by simple diffusion. The mechanism of ascorbate absorption resembles that of actively absorbed sugars and amino acids but is not shared by them.23 The efficiency of absorption decreases \vith increased intakes. From single doses of up to 100 mg 800/0 to 90% is absorbed compared with about 55% from a dose of 1000 mg (1 g), about 35% from 4 g, and less than 200/0 from doses exceeding 10 g.4,24,25 Secretion of water into the gut with high intakes of ascorbic acid appears to be responsible for the diarrhea that is frequently associated with ingestion of megadoses of vitamin C.22 The amount of ascorbic acid in different tissues varies; adrenal and pituitary tissue, brain, pancreas, kidney, liver, and spleen have relatively high concentrations; blood cells contain more than blood serum. The level of circulating ascorbic acid responds to changes in the intake of the vitamin. It also reflects the adequacy of tissue ealthy adult men approximates 1500 mg on an average daily intake of 45 mg to 75 mg. 26,28 When no ascorbic acid is ingested, about 3% (2.20/0-4.10/0) ofthe reserves are depleted daily; 13,28 as a result, 45 mg (34 mg-61.5 mg) of absorbed ascorbic acid per day would be needed to maintain the body pool at tha t level. Clinical symptoms of scurvy appear in 30 to 45 days when the body pool drops below 3.00 mg.13
Higher intakes (200 mg) have been shown to increase the total body pool to 2300 mg to 2800 mg.29 Such in takes maintain the maximum plasma concentration, which in most people is between 1.1 mgand 1.4 mgper dl, depending on the renal threshold. No benefits are known to accrue from maintenance of maximum tissue reserves. Approxima te ranges for blood ascorbic acid levels and body pool sizes that correspond to different levels of intake.
Human requirement
Elaborate studies have been made to determine human requirements for ascorbic acid at different ages, under different conditions of environment, during physical exertion, in fevers, and in infections. The amount necessary to prevent frank symptoms of scurvy in humans is far less (10 mg) than that recommended for an optimum state of health.
In the 1980 edition of the Recommended Dietary Allowances,30 the Food and Nutrition Board increased the recommended intake for adults to 60 mg per day from the 45 mg recommended in 1974. This intake level was calculated to maintain an average ascorbate body pool of 1500 mg at a daily catabolic rate of3% to 40/0 and 850/0 efficiency of absorption. Daily intake of 35 mg was recommended for infants during the first year oflife. This amount would be obtained by a breast-fed infant who consumed 850 ml of milk per day. However, 100 mg per day was recommended for the newborn during the first week of life to protect against transient tyrosinemia, which is especially common in premature infants. During pregnancy and lactation, daily intakes of80 mg and 100 mg, respectively, were recommended.
The joint FAO/WHO Expert Committee se.t somewhat lower recommendations: 30 mg for adults (men and women over 13 years), 50 mg during pregnancy and lactation, and 20 mg for infants and children up to 13 years 01d.31
These intakes in adults are usually associated with plashealth.ma ascorbate levels of:> 0.4 mg per dl and normal health. It should be noted that these recommendations were established more than a decade ago. Significant new information about ascorbic acid metabolism has since become available. Considering the possible but quantitatively unknown effects of individual variation, emotional and environmental stress, age, smoking, and oral contraceptive agents and other drugs, the Food and Nutrition Board concluded that higher intake levels were desirable and easily attainable from the average diet in the United States.
