Crystalluria in marathon runners. IV. Black subjects.

Crystal sizes (scanning electron microscopy) and distributions (Coulter Counter) as well as 24-h urinary sodium (Na) and calcium (Ca) excretions (flame atomic absorption) were determined in a group of black South African runners immediately after a marathon and again 3 weeks later. White runners and black and white control subjects were included in the study. Particle volume-size histograms for black controls and black runners were identical. There was no significant difference in the Na excretion of all the groups. However, while urinary Ca excretion was significantly raised in white runners relative to white controls, Ca excretion in black runners was unchanged relative to their controls. It is postulated that the lower rates of urinary Ca excretion may result from lower rates of Ca resorption from bone in response to the cyclical loading of running in black marathon runners. The results of this study suggest that black marathon runners are not prone to the same increased risk of renal stone formation as are white runners.