Stimulation of Bone Resorption by Increasing Dietary Protein Intake in Rats Fed Diets Low in Phosphorus and Calcium

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Abstract
The effect of three levels (3.3, 11, and 33%) of dietary protein on bone resorption was studied in young rats fed diets deficient in P and Ca. Plasma P decreased and plasma Ca rose with an increase in dietary protein and returned to near normal values 8 h after food withdrawal. Urinary Ca rose linearly with increasing levels of dietary protein while urinary P was insignificant in the animals fed 11 and 33% protein. Assuming a molar ratio of Ca/P of 8.13/6 in young bones, estimates of the amount of P liberated from bone were calculated from urinary Ca and evaluated at 0.07,1.05, and 3.29 mg/100 g body weight per 8 h for the animals fed 3.3, 11, and 33% protein, respectively. Histological sections of the tibia revealed a progressive widening of the epiphyseal plate. Correlations between Ca excretion, levels of dietary protein, and width of epiphyseal plate were linear. Plasma P and Ca variations and Ca and P excretions in thyroparathyroidectomized and thyroxin replaced and in parathyroidectomized rats fed diets deficient in P and Ca containing 33% protein were similar to those of sham-operated controls. From this study we concluded that dietary protein independently of parathormone or calcitonin determines the magnitude of the physiological and histological changes observed in growing animals fed diets inadequate in P and Ca.

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