Studies on the metabolism of calcium and phosphorus and on the availability of these elements from milk and from an inorganic source

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Abstract
In 3 balance studies of Ca and of P derived from milk or from CaHPO42H2O the Ca of milk was always found to be more available than that of CaHPO42H2O in young [male] rats. In the presence of extra P (as Na2HPO412H2O) sub-optimal quantities of Ca were retained from spray-dried milk with an efficiency of 98.1% and from CaHPO42H2O, 96.5%, a significant difference. Under such optimal conditions the daily loss of Ca was 0.3 mg. per rat, 1/3 in the feces and 2/3 in the urine. The retention of P was never so efficient, even in the presence of extra Ca (as CaCO3). The minimum daily loss was 1.3 mg. per rat, nearly 90% of it in the feces. This relatively inefficient retention of P at sub-optimal levels was believed to be due to fixation of some of it by intestinal bacteria. There was no evidence of active excretion of either Ca or P by the large intestine.