Interactions between 1,25-dihydroxyvitamin D3 and vitamin D2: effects of pharmacologic doses in normal individuals.

To determine whether the administration of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) affects the conversion of a pharmacologic dose of vitamin D2 to 25-hydroxyvitamin D (25OHD), 20 normal subjects received two separate doses of vitamin D2--one with and the other without the concomitant administration of 1,25(OH)2D3. Serum 1,25(OH)2D rose in response to 1,25(OH)2D3 administration and fell when vitamin D2 was given alone. Serum osteocalcin rose in response to 1,25(OH)2D3 administration. Serum 25OHD rose in response to vitamin D2 administration regardless of whether the subjects also received 1,25(OH)2D3. The data from this study in humans support the suggestion that the effects of 1,25(OH)2D3 on serum 25OHD concentrations are mediated through mechanisms other than impairment of production.