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Comparison of the Hypercalcemic Action of Vitamins D₂ and D₃ in Chicks and the Effect on Tetracycline Fixation by Bone¹

Department of Radiation Biology and Biophysics, University of Rochester School of Medicine and Dentistry, Rochester, New York

Doses of crystalline vitamins D₂ and D₃ ranging from 3 IU/day vitamin D₂ and 40 IU/day vitamin D₃ to 40,000 IU/day of each vitamin were administered daily to chicks fed a rachitogenic diet and the effects were noted on body weights, serum calcium and phosphorus, tetracycline uptake and bone microradiographic appearance. Although body weight and bone weight data indicated that vitamin D₃ was approximately 10 times more toxic than vitamin D₂ (about the same as the antirachitic potency difference), relative toxicity ratios could not be estimated from the effects on serum calcium level, or on bone ash. At all dose levels except at 40,000 IU/day, the vitamin D₃ gave a higher percentage of bone ash than vitamin D₂. No tetracycline was taken up by bones of chicks given 40,000 IU/day of vitamin D₃ indicating no new bone deposition and a high degree of resorbing activity in these bones.

Manuscript received 22 April 1965.