Requirements Estimated from the Plasma and Leukocyte Concentrations of Ascorbate

The plasma concentration of ascorbate shows a sig-moidal relationship with intake. Below about 30 mg/day it is extremely low and does not reflect increasing intake to any significant extent. As the intake rises above 30 mg/day, so the plasma concentration begins to increase sharply, reaching a plateau of 70-85 mmol/l, at intakes between 70 and 100 mg/day, when the renal threshold is reached and the vitamin is excreted quantitatively with increasing intake.

The point at which the plasma concentration increases more or less linearly with increasing intake represents a state where reserves are adequate and ascorbate is available for transfer between tissues. This corresponds to an intake of 40 mg/day and is the basis of the UK, EU, and FAO figures. At this level of intake the total body pool is about 5.1mmol (900 mg). It has been argued that setting requirements and reference intakes on the basis of the steep part of a sigmoidal curve is undesirable, and a more appropriate point would be the intake at which the plasma concentration reaches a plateau, at an intake of around 100-200 mg/day.

The US/Canadian reference intakes of 75 mg for women and 90 mg for men are based on studies of leukocyte saturation during depletion/repletion studies.

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