NEW YORK (Reuters Health) - In South Africa, differences in the renal handling of dietary calcium and oxalate may explain why kidney stones are much less common among black populations than their white counterparts, according to a report published in the September issue of The Journal of Urology.
Previous reports have failed to explain why the South African black population is protected against kidney stones compared with other populations. One possibility, which has not been explored, is that dietary calcium and oxalate are handled differently by the kidneys of black and white individuals.
In the current study, Dr. Allen L. Rodgers and Dr. Sonja Lewandowski from the University of Cape Town, South Africa, administered five different diets to 10 healthy subjects from each race group. The diets included a low calcium diet, high oxalate diet, vitamin C-enriched diet, high salt diet, and lacto-vegetarian diet. Each diet was followed for 4 days with a 7-day wash-out period between each.
When given the low calcium diet, black subjects experienced a significant increase in urinary oxalate, a decrease in the relative supersaturation of calcium oxalate, and an increase in the relative supersaturation of brushite. In contrast, no significant renal changes were noted in white subjects given this diet.
The high oxalate diet was linked to significant, albeit different, renal changes in the racial groups, the authors note. In black subjects, the diet was associated with an increase in urinary citrate levels (p = 0.01). In white subjects, an increase in urinary pH (p = 0.01), an increase in potassium excretion (p = 0.01), and an increase in the relative supersaturation of brushite was observed (p = 0.05).
No clinically important differences were noted between the racial groups for the vitamin C, high salt, or lacto-vegetarian diets, the researchers point out.
"These results provide evidence in support of our hypothesis that South African black and white subjects may have different renal handling mechanisms for different dietary challenges and that these differences may be linked to the apparent immunity to stone formation of the former," the authors conclude.
J Urol 2002;168:931-936.
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