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 largely immune to kidney stones) and white South Africans (in whom stones are more common).


METHODS
Urinary and dietary variables were examined in 11 black and 11 white South African men. None of the subjects had had a kidney stone 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 samples were collected before the protocol and during the final day. The samples were analysed using routine modern laboratory techniques. The urine analysis data were used to calculate the Tiselius risk index and the relative urinary supersaturations of calcium oxalate, uric acid and calcium phosphate.


RESULTS
Urine analysis showed an intriguing anomaly; black subjects had significantly higher urinary pH and oxalate values than whites (6.50 vs 6.21 and 0.23 vs 0.14 mmol/24 h, respectively), while their urinary citrate was lower (1.47 vs 3.69 mmol/24 h). In addition, the Tiselius risk index and relative supersaturation of calcium oxalate were higher in black subjects. These results are contrary to those which might have been reasonably expected when comparing stone-free and stone-prone groups. After the dietary protocol, the only urinary variable which changed significantly was urinary oxalate, which increased by 57% in whites.


CONCLUSIONS
Factors which are conventionally used to assess stone risk (pH, oxaluria, citraturia, relative supersaturation) are not helpful in identifying why South African blacks are relatively immune to stones. We suggest that relatively lower oxalate absorption rates may be a physiological feature of this racial group.
Lewandowski
S
S
Department of Chemistry, and Nutrition and Dietetics Unit, University of Cape Town, Cape Town, South Africa.
Rodgers
A
A
Schloss
I
I
D001741:Black or African American; D044383:Black People; D002129:Calcium Oxalate / Q000008:administration & dosage* / Q000652:urine; D002136:Calcium, Dietary / Q000008:administration & dosage*; D004032:Diet; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007669:Kidney Calculi / Q000208:ethnology / Q000209:etiology* / Q000652:urine; D008297:Male; D012307:Risk Factors; D013019:South Africa / Q000208:ethnology; D044465:White People
D002136:Calcium, Dietary; D002129:Calcium Oxalate