Studies on the mechanism of protein-induced hypercalciuria in older men and women.

A human metabolic study was conducted to observe the effect of level of protein intake on urinary calcium, calcium absorption and calcium balance in older adults and to further study the mechanisms of protein-induced hypercalciuria. An increase in protein intake from about 47 to 112 g while maintaining calcium, magnesium and phosphorus intakes constant caused an increase in urinary calcium and a decrease in calcium retention. Glomerular filtration rate was increased and fractional renal tubular reabsorption was decreased by the increase in protein intake; total renal acid, ammonium and sulfate excretions more than doubled, whereas urinary sodium decreased by 38%. The changes in urinary calcium were positively correlated with the increase in total renal acid and sulfate excretion as well as with the decrease in fractional renal tubular reabsorption of calcium. Thus, the data indicate that protein-induced hypercalciuria is due to an increase in glomerular filtration rate and a decrease in fractional renal tubular reabsorption of calcium, the latter of which may be caused by the increased acid load on the renal tubular cells.