Calcium Metabolism in Ponies Fed Varying Levels of Calcium

Abstract

Calcium metabolism of young, growing ponies fed diets containing 1.5, 0.8 and 0.15% calcium was studied using combined balance and kinetic methods. Variations in calcium intake produced large differences in excretion and retention but had no effect on the level of plasma calcium or on the size of the exchangeable pool. Intestinal absorption, renal excretion and calcium removal from bone responded to the dietary level of calcium to maintain calcium homeostasis. The rates of intestinal absorption and the removal of calcium from the skeleton increased while renal excretion decreased in response to low intake. Opposite responses occurred on a high intake. In contrast to the rate of removal, the rate of deposition of calcium in the skeleton was insensitive to calcium intake and retention. Endogenous fecal calcium excretion was unaffected by dietary calcium. The calcium requirement for horses was estimated by assuming that the endogenous fecal excretion was the major obligatory loss. The horse must absorb about 2.5 g of calcium/day per 100 kg in order to balance the obligatory loss. A dietary level of about 5 g/day per 100 kg is necessary assuming 50% absorption. A similar estimate of the calcium requirement was obtained by extrapolating retention to zero absorption.