Studies on the Biological Value of Inorganic Phosphates

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Abstract
A method for comparing the availability to chicks of phosphorus from different sources has been described. It involves feeding graded increments of the phosphates in a diet very deficient in phosphorus and determining the resulting increases in bone ash after 4 weeks. For compounds of moderate or high availability, a purified type of diet very low in phosphorus was used. For studying poorly available compounds, a diet with satisfactorily low phosphorus content was formulated from ingredients of a practical type. Pure beta-tricalcium phosphate was used in all experiments as a standard of comparison and arbitrarily assigned a biological value of 100. The chemically pure orthophosphates, mono-, di-, and tricalcium phosphate, sodium acid phosphate and potassium acid phosphate, were highly available. The acid salts, monocalcium phosphate and potassium acid phosphate, were better utilized than the other pure orthophosphates. Feed grade materials of excellent availability were average samples of dicalcium phosphate, defluorinated phosphate and domestic steamed bone meal. Other bone products of slightly lower availability were spent bone char, bone ash and imported bone meal of unknown history. None of the pyrophosphates or metaphosphates was satisfactory, although significant amounts of phosphorus were utilized from calcium acid pyrophosphate and vitreous metaphosphate. Among the untreated rock products, only Curacao Island phosphate showed a satisfactory degree of availability. Florida land pebble phosphate was appreciably better utilized than brown phosphate rock from Tennessee. Waste pond phosphate, the “colloidal” phosphate of commerce, was studied in the purified type of diet and also the practical type of basal ration. Excessive mortality occurred with the purified diet containing colloidal phosphate. In the practical diet, colloidal phosphate was approximately 25% as effective as an equivalent amount of phosphorus from beta-tricalcium phosphate in increasing the ash content of bones. However, it had no significant beneficial effect on growth.