Dietary protein intake and urinary excretion of calcium: a cross-sectional study in a healthy Japanese population.

To evaluate whether habitual excess protein intake is a significant risk factor for calcium loss, we studied the relation between urinary excretion of calcium and protein intakes, in 349 male and 406 female Japanese aged 20-79 y. The subjects were apparently healthy, free-living, and consuming diets of their own choosing. We divided the subjects into two groups: those aged 20-49 y and those aged 50-79 y. In each group, we observed a significant positive correlation between daily urinary excretion of calcium and protein intake. Calcium excretion also correlated positively with daily urinary excretion of urea. Multivariate analyses revealed that in each age group the relation between calcium excretion and urea excretion remained significant even after sex, age, body weight, urinary sodium excretion, and calcium intake were adjusted for. The correlation of calcium excretion with animal protein intake was significantly positive in both sexes and in each age group whereas that with plant protein was not. We observed a significant positive correlation between daily calcium excretion and daily urinary excretion of sulfate. The correlation in 50-79-y old subjects remained significant even after sex, age, body weight, sodium excretion, and calcium intake were adjusted for. Our findings suggest that excess protein, especially that rich in sulfur-containing amino acids, in habitual diets may augment calcium excretion in the urine, at least in the elderly.