Calcium utilization: effect of varying level and source of dietary protein.

Dietary protein exerts a significant calciuretic effect. A twofold increase in protein at constant levels of calcium and phosphorus intakes causes a 50% increase in urinary calcium. The protein-induced hypercalciuria results primarily from decreased fractional renal tubular reabsorption of calcium associated with catabolism of excess sulfur amino acids and the resultant urinary excretion of acid and sulfate. A protein-induced elevation in glomerular filtration rate also contributes to the calciuresis. Dietary phosphorus also modifies the calciuretic effect of proteins, as it increases renal tubular reabsorption of calcium and thereby exerts a hypocalciuretic effect. Consequently, a soy-based diet was able to maintain calcium balance at a calcium intake of 457 mg/day in spite of a protein intake of 90 g, presumably due to the lower level of sulfur amino acids in the soy diet and to the 1450 mg phosphorus which accompanied the soy protein.