Vitamin D metabolites in normal subjects during one year. A longitudinal study.

The serum concentrations of 25-hydroxycholecalciferol (25OHD3), 25-hydroxyergocalciferol (25OHD2), 24,25-dihydroxyvitamin D (24,25(OH)2D), and 1,25-dihydroxyvitamin D (1,25(OH)2D) were measured in 10 normal subjects every 2 months for 1 year. Parallel seasonal variations were found in serum 25-hydroxycholecalciferol and 24,25-dihydroxyvitamin D reaching maximum values in June. Moreover, a highly significant correlation between changes in these two metabolites was observed (r = 0.89, P less than 0.001). On the other hand, the mean serum 1,25-dihydroxyvitamin D concentration remained constant throughout the year. Our data add further evidence to the tight regulatory mechanism of serum 1,25-dihydroxyvitamin D and the lack of regulatory mechanism of serum 24,25-dihydroxyvitamin D.