The influence of dihydroxylated vitamin D metabolites on bone formation in the chick.

The ability of 1,25(OH)2D3 and of 24,25(OH)2D3 to prevent or to heal rickets in chicks was evaluated by studies of plasma biochemistry, growth plate histology, bone morphometry and microradiography, and bone mineralization. 1,25(OH)2D3 at a dose of 100 ng/day produced fewest abnormalities compared with vitamin D3-treated control chicks. Bone growth was slightly greater than vitamin D3-treated controls in chicks given a lower dose of this metabolite; the reverse was observed in chicks given a higher dose. 24,25(OH)2D3 was less effective than 1,25(OH)2D3 in preventing rickets even at doses as high as 400 ng/day. Treatment of rachitic chicks with doses of 24,25(OH)2D3 up to 300 ng/day produced no healing effect on the bone lesions, in marked contrast to the beneficial effects observed with 1,25(OH)2D3.