Depletion and repletion kinetics of vitamin C in humans.

Depletion and repletion profiles of vitamin C were determined in young and elderly males and females. Plasma vitamin C concentrations were determined upon entry into the study, once each week during a 5-wk period of dietary restriction (10 mg/day) of the vitamin and twice each week during a 2.5-wk period of supplementation with 500 mg/d. The rate constant (K) and the half-life of the depletion phase (t1/2) were determined. Other values calculated included the mean plasma vitamin C concentration in the four subject groups at entry, depleted and supplemented states of vitamin C nutriture. A t test indicated that none of the calculated values differed due to age. In contrast, K was significantly smaller in men, resulting in a longer t1/2. The entry and depleted plasma vitamin C concentrations did not differ due to gender whereas repleted plasma vitamin C concentrations were higher in women than in men. Data analysis indicated that plasma depletion of vitamin C is a first-order process. In addition, t1/2 was inversely related to entry vitamin C levels at concentrations below approximately 85 mumol/L whereas above this concentration t1/2 was approximately constant and averaged 14.2 d. This implies that a conservation effect may be operative at low plasma vitamin C concentrations.