Requirements Estimated from Maintenance of the Body Pool of Ascorbate

An alternative approach to estimating requirements is to determine the fractional rate of catabolism of total body ascorbate; an appropriate intake would then be that required to replace losses and maintain the body pool.

Clinical signs of scurvy are seen when the total body pool of ascorbate is below 1.7mmol (300 mg). The pool increases with intake, reaching a maximum of about 8.5mmol (1500 mg) in adults— 114 mmol (20mg)/kg body weight. The basis for the 1989 US RDA of 60 mg was the observed mean fractional turnover rate of 3.2% of a body pool of 20 mg/kg body weight/day, with allowances for incomplete absorption of dietary ascorbate and individual variation.

It has been argued that a total body pool of 5.1 mmol (900 mg) is adequate; it is threefold higher than the minimum required to prevent scurvy, and there is no evidence that there are any health benefits from a body pool greater than 600 mg. The observed body pool of 8.5 mmol in depletion/repletion studies was found in subjects previously consuming a self-selected diet, with a relatively high intake of vitamin C, and therefore might not represent any index of requirement. Assuming a total body pool of 5.1 mmol and catabolism of 2.7%/day, allowing for efficiency of absorption and individual variation gives a reference intake of 40 mg/day.

Because the fractional turnover rate was determined during a depletion study, and the rate of ascorbate catabolism varies with intake, it has been suggested that this implies a rate of 3.6%/day before depletion. On this basis, and allowing for incomplete absorption and individual variation, various national authorities arrive at a recommended intake of 80 mg.

The rate of ascorbate catabolism is affected by intake, and the requirement to maintain the body pool cannot be estimated as an absolute value. A habitual low intake, with a consequent low rate of catabo-lism, will maintain the same body pool as a habitual higher intake with a higher rate of catabolism.

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