Dietary intakes and urinary excretion of calcium and acids: a cross-sectional study of women in China.

The relationship between dietary intakes and urinary calcium was examined in a cross-sectional survey of 764 middle-aged and elderly women with markedly different dietary patterns and lifestyles. Urinary calcium was correlated positively with urinary acids, including titratable acid (r = 0.46, P < 0.0001), ammonia (r = 0.42, P < 0.0001), and sulfate (r = 0.52, P < 0.0001). Urinary excretions of calcium and acids were correlated positively with intakes of animal and nondairy animal protein but were correlated negatively with plant-protein intake, possibly because of the alkaline nature of plant foods. Further analyses showed that urinary calcium and acids were associated positively with acid-forming foods and were associated negatively with plant foods. These results indicate that under free-living conditions urinary calcium excretion is likely determined by the acid-base status of the total diet, including among other factors the contribution of sulfur amino acids to urinary acid production.