Relationships between cholecalciferol metabolism and growth in chicks as modified by age, breed and diet.

Fast-growing heavy (White Rock) chicks, fed a vitamin D-deficient diet, exhibited a higher activity of kidney 25-hydroxycholecalciferol-1-hydroxylase (1-hydroxylase) than slow-growing light (White Leghorn x Rhode Island Red) chicks fed the same diet. 1-Hydroxylase and duodenal calcium-binding protein (CaBP) declined with age. Feeding of low energy diets with or without vitamin D resulted in a slower rate of growth and reduced 1-hydroxylase activity and CaBP concentration. Severe dietary restriction of either calcium or phosphorus resulted in a lower growth rate as well as a duodenal CaBP as compared to a moderate mineral restriction. The severe dietary calcium restriction also resulted in a lower 1-hydroxylase activity than that resulting from a moderate restriction. The results clearly indicate that high 1-hydroxylase activity and a high intestinal CaBP are associated with a high growth rate.