Calcium Metabolism in Ponies Fed a High Phosphorus Diet

Calcium metabolism was studied by combined metabolic balance and kinetic methods in four young Shetland ponies fed a diet containing 0.4% calcium and either 0.2% or 1.2% phosphorus in a replicated 2 × 2 Latin square experimental design. Phosphorus retention and plasma phosphorus concentration were greater when the ponies were fed the high phosphate diet. The high phosphate intake decreased calcium absorption, urinary excretion and retention but increased total and endogenous fecal calcium excretion. However, the ponies were in positive calcium balance when fed either the basal or high phosphate diet. The turnover of bone, estimated from the rates of deposition of calcium in the skeleton and removal of calcium from the skeleton, increased in three of the four ponies in response to the high phosphate diet. Plasma calcium concentration decreased when the ponies were fed the high phosphate diet. However, the more rapid turnover of bone calcium observed was not associated with consistent changes in the exchangeable calcium pool, the compartments of the pool or the rates of exchange.