Calcium metabolism in the young adult male as affected by level and form of phosphorus intake and level of calcium intake.

A 60-day human metabolic study was conducted to measure polyphosphate hydrolysis and to compare the effects of supplements of phosphorus from ortho- and polyphosphates as well as supplements of both calcium and orthophosphates on calcium metabolism. The experiment was arranged in a 4 x 4 latin square design with eight subjects and four 15-day dietary periods. During its passage through the digestive tract, the polyphosphate supplement was 80.5 +/- 5% hydrolyzed to orthophosphate. Calcium absorption was significantly lower when the polyphosphate supplement was given than when the orthophosphate supplement was given. Both forms of phosphate caused a reduction in fractional renal tubular reabsorption of calcium, but only the orthophosphate supplement improved calcium balance. Calcium equilibrium was achieved, however, only when supplements of both calcium and orthophosphate were given. Both phosphorus supplements caused an increase in urinary cyclic AMP, indicating an increase in parathyroid hormone (PTH) secretion, but bone resorption as measured by urinary hydroxyproline was not affected by either phosphate supplement. The combined supplement of calcium and orthophosphate, however, caused decreases in the excretion of both cyclic AMP and hydroxyproline, suggesting a decrease in PTH-mediated bone resorption.