Renal response to lithogenic and anti-lithogenic supplement challenges in a stone-free population group.

Objective: In South Africa, urolithiasis is extremely rare in the black population, but is common in the white population. The objective of this study was to investigate the individual effects of 5 different dietary and supplemental challenges (high dietary calcium, calcium supplement, vitamin B6 supplement, L-glutamine supplement, and L-cysteine supplement) on the urinary risk factors for calcium oxalate urolithiasis in subjects from both race groups.

Design: Complete Latin Square design.

Setting: University research laboratory.

Subjects: Subjects were recruited from the student cohort of the University of Cape Town (10 male subjects from each race group). Selection criteria were no history of renal or metabolic diseases, and no chronic or acute medication. Subjects served as their own controls.

Intervention: After 7 days on a self-selected standardized diet, a 24-hour baseline urine sample was collected. A second 24-hour urine sample was collected after 5 days on the prescribed dietary or supplemental challenge. These were analyzed for biochemical and physicochemical risk factors. Additionally, 24-hour dietary recall questionnaires were recorded at baseline and after the 5-day test period, and were analyzed using a food analysis program. Statistical analysis of variance was performed on all of the data.

Main Outcome Measures: Urine composition, relative supersaturation of urinary salts, calcium oxalate metastable limit, and Tiselius risk index.

Results: None of the protocols altered any of the urinary biochemical or physicochemical risk factors in black subjects. In white subjects, the calcium diet significantly increased urinary potassium (P =.0001) and decreased the relative supersaturation of brushite (P =.035); the calcium supplement significantly decreased the Tiselius risk index (P =.014); vitamin B6 supplement significantly decreased urinary calcium (P =.016), urinary phosphate (P =.027), and the relative supersaturation of brushite (P =.004); L-glutamine supplement significantly decreased relative supersaturation of calcium oxalate (P =.01); L-cystine supplement significantly decreased urinary calcium (P =.031) and the Tiselius risk index (P =.013).

Conclusions: Because none of the challenges had an effect on the urinary risk factors in black subjects, it is speculated that a renal or gastrointestinal homeostatic adjustment occurs in this group, thereby keeping urinary concentration of substances in balance.