Changes in bone turnover in young women consuming different levels of dietary protein

Although high protein diets are known to increase urinary calcium excretion and induce negative calcium balance, the impact of dietary protein on bone turnover and fractures is controversial. We therefore evaluated the effect of dietary protein on markers of bone turnover in 16 healthy young women. The experiment consisted of 2 weeks of a well balanced diet containing mode rate amounts of calcium, sodium, and protein followed by 4 days of an exper imental diet containing one of three levels of protein (low, medium, or hig h). On day 4, serum and urinary calcium, serum PTH, 1,25-dihydroxyvitamin D , serum osteocalcin, bone-specific alkaline phosphatase, and urinary N-telo peptide excretion were measured. Urinary calcium excretion was significantl y higher on the high than on the low protein diet. Secondary hyperparathyro idism occurred on the low protein diet. Urinary N-telopeptide excretion was significantly greater during the high protein than during the low protein intake (48.2 +/- 7.2 vs. 32.7 +/- 5.3 nM bone collagen equivalents/mM creat inine; P < 0.05). There was no increase in osteocalcin or bone-specific alk aline phosphatase when comparing the low to the high diet, suggesting that bone resorption was increased without a compensatory increase in bone forma tion. Our data suggest that at high levels of dietary protein, at least a p ortion of the increase in urinary calcium reflects increased bone resorptio n.