Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol

Background: The cholecalciferol inputs required to achieve or maintain any given serum 25-hydroxycholecalciferol concentration are not known, particularly within ranges comparable to the probable physiologic supply of the vitamin. Objectives: The objectives were to establish the quantitative relation between steady state cholecalciferol input and the resulting serum 25-hydroxycholecalciferol concentration and to estimate the proportion of the daily requirement during winter that is met by cholecalciferol reserves in body tissue stores. Design: Cholecalciferol was administered daily in controlled oral doses labeled at 0, 25, 125, and 250 mug cholecalciferol for approximate to20 wk during the winter to 67 men living in Omaha (41.2degrees N latitude). The time course of serum 25-hydroxycholecalciferol concentration was measured at intervals over the course of treatment. Results: From a mean baseline value of 70.3 nmol/L, equilibrium concentrations of serum 25-hydroxycholecalciferol changed during the winter months in direct proportion to the dose, with a slope of 0.70 nmol/L for each additional 1 mug cholecalciferol input. The calculated oral input required to sustain the serum 25-hydroxycholecalciferol concentration present before the study (ie, in the autumn) was 12.5 mug (500 IU)/d, whereas the total amount from all sources (supplement, food, tissue stores) needed to sustain the starting 25-hydroxycholecalciferol concentration was estimated at approximate to96 mug (epsilonapproximate to3800 IU)/ d. By difference, the tissue stores provided approximate to78-82 mug/d. Conclusions: Healthy men seem to use 3000-5000 TU cholecalciferol/d, apparently meeting > 80% of their winter cholecalciferol need with cutaneously synthesized accumulations from solar sources during the preceding summer months. Current recommended vitamin D inputs are inadequate to maintain serum 25-hydroxycholecalciferol concentration in the absence of substantial cutaneous production of vitamin D.