The influence of a high-oxalate/low-calcium diet on calcium oxalate renal stone risk factors in non-stone-forming black and white South African subjects

Objective To evaluate the influence of a high-oxalate/ low-calcium diet on calcium oxalate stone risk factors in both black South Africans (who are la rgely immune to kidney stones) and white South Africans (in whom stones are more common). Subjects and methods Urinary and dietary variables were examined in 11 blac k and 11 white South African men. None of the subjects had had a kidney sto ne or any metabolic illness. Their normal domestic food intake was assessed using a semiquantitative food frequency questionnaire. Subjects were given a standardized high-oxalate/low-calcium diet for 3 days; 24-h urine sample s were collected before the protocol and during the final day. The samples were analysed using routine modern laboratory techniques. The urine analysi s data were used to calculate the Tiselius risk index and the relative urin ary supersaturations of calcium oxalate, uric acid and calcium phosphate. Results Urine analysis showed an intriguing anomaly; black subjects had sig nificantly higher urinary pH and oxalate values than whites (6.50 vs 6.21 a nd 0.23 vs 0.14 mmol/24 h, respectively), while their urinary citrate was l ower (1.47 vs 3.69 mmol/24 h). In addition, the Tiselius risk index and rel ative supersaturation of calcium oxalate were higher in black subjects. The se results are contrary to those which might have been reasonably expected when comparing stone-free and stone-prone groups. After the dietary protoco l, the only urinary variable which changed significantly was urinary oxalat e, which increased by 57% in whites. Conclusion Factors which are conventionally used to assess stone risk (pH, oxaluria, citraturia, relative supersaturation) are not helpful in identify ing why South African blacks are relatively immune to stones. We suggest th at relatively lower oxalate absorption rates may be a physiological feature of this racial group.