Effect of Vitamins D2 and D3 on Serum Calcium and Phosphorus in Rachitic Chicks

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Abstract
The experimental schedule presented in this paper provides 3 independent quantitative criteria by which antirachitic effectiveness can be evaluated. Crystalline vitamins D2 and D3 were given orally to chicks fed a rachitogenic diet, and a variety of parameters were evaluated after 28 days. Chicks fed the rachitogenic diet exhibited low serum calcium concentrations and percentage bone ash, but had significantly higher serum phosphorus levels than chicks fed a standard chick diet or treated adequately with either form of vitamin D. Their serum phosphorus response was therefore unlike that reported for most previously studied experimental animals. Dose-response curves based on the 3 parameters indicated a flatter response to vitamin D2 than to vitamin D3 and therefore any efficacy ratio calculated from data of this nature would be a function of the degree of healing taken as an endpoint. When based on CD50 (curative dose giving a response midway between animals on rachitogenic and standard chick diets), the vitamin D3-to-D2 efficacy ratio was estimated at about 8:1-to-11:1. Pronounced subperiosteal rarefaction was observed in the microradiographs of tibia from chicks fed the rachitogenic diet.