Purpose: To test our hypothesis that differences in urinary calcium excretion among blacks and whites may be secondary to ethnic variations in acid (H(+)) metabolism and to prove that increases in titratable acid excretion would be found among individuals predisposed to the development of stress fractures.
Methods: We administered 8 g NH(4)Cl acutely to 11 black and 18 white healthy volunteers and measured urinary sodium, calcium, and acid excretions. We measured the Na(+)/H(+) antiporter activity using acid-loaded platelets as surrogate markers for this exchanger expressed in renal epithelial cells. We also compared differences in titratable acid excretion among a cohort of subjects with, and without, a history of stress fracture.
Results: NH(4)Cl-induced increases in titratable urinary acid correlated with changes in the renal excretion of calcium and sodium, and stimulated acid excretion correlated with basal acid loss. Despite comparable changes in plasma pH, whites, when compared to blacks, had much greater basal acid excretion and NH(4)Cl-induced acid excretion. Whites also had much greater baseline calcium excretion rates when compared to blacks. Following acid loading, whites continued to exhibit greater calcium excretion rates than blacks. Acid loading significantly decreased sodium excretion in whites but not in blacks. Blacks also had significantly attenuated Na(+)/H(+) exchange activity. In a cohort of resting, athletic students, we found enhanced basal H(+) and phosphate excretion among subjects who experienced stress fractures during their rigorous physical training when compared to those individuals who did not.
Conclusion: Blacks may have a greater endogenous buffering capacity than whites, or the reported ethnic differences in sodium and calcium excretion rates between blacks and whites may be secondary to racial variations in renal H(+) excretion. We conclude that both ethnic differences in bone mineralization and bone integrity in athletes are mediated by heritable differences in titratable acid excretion.