[Seasonal variations of vitamin D metabolites in man]

Studies concerning the influence of seasons and ultraviolet radiations on the metabolism of 1,25-dihydrocholecalciferol (1,25 (OH)2D), the active form of vitamin D, have given conflicting results. In the present study serum concentrations of the 3 main vitamin D metabolites--25-hydroxyvitamin D, 24,25-dihydroxyvitamin D and 1,25-dihydroxyvitamin D--were measured by radiocompetitive assay (a) monthly during one year in 7 normal subjects, and (b) before, during and after 4 weeks of "whole body" exposure to ultraviolet radiations in 11 other subjects. In study (a) parallel changes in 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D concentrations were observed during the year, with a rise in the summer; there were no significant monthly or quarterly changes in 1,25-dihydroxyvitamin D concentrations. In study (b), 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D concentrations rose by 150% and 200% respectively after 4 weeks' exposure to ultraviolet radiations and again, there were no significant changes in 1,25-dihydroxyvitamin D concentrations. These results confirm that when synthesis of the substrate (25-hydroxyvitamin D) increases, synthesis of 1,25-dihydroxyvitamin D is inhibited by a regulatory process which does not apply to 24,25-dihydroxyvitamin D synthesis.