The Antirachitic Activity of Various Forms of Vitamin D in the Chick

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Abstract
Crystalline vitamin D3 assayed by the U.S.P. antirachitic vitamin D test on rats has a potency of 0.025 γ per unit or 40,000,000 units per gram. This is identical with the potency of crystalline vitamin D2. The minimal protective daily dose for chicks fed a rachitogenic diet was found to be 2.5 I.U. of cod liver oil, 2.5 I.U. of crystalline vitamin D3, 2.7 I.U. of irradiated cholesterol, 120 I.U. of Viosterol (A.R.P.I. process) and 85 I.U. of crystalline vitamin D2. The antirachitic ratio of effectiveness of cod liver oil to Viosterol (A.R.P.I. process) rat unit for rat unit in the chick is 1:48. The ratio for vitamin D3 to vitamin D2 is 1:34. The reasons for the variations in potency of vitamin D preparations when tested on rats and chicks are discussed.