Differences in 24-Hour Urine Composition between Black and White Women

Black women are less likely to develop kidney stones and have greater bone mass than white women. However, little is known about racial differences in urine composition. Urine pH, volume, and 24-h urinary excretion of calcium, citrate, oxalate, uric acid, sodium, potassium, magnesium, phosphate, sulfate, and creatinine of 146 black women were compared with 330 white women in the Nurses' Health Study. All participants were postmenopausal non-stone formers. ANOVA was used to compare mean urinary values. Linear regression models were adjusted for age, body mass index, dietary intake, and urinary factors. On average, black women excreted 65 mg less urinary calcium (P < 0.001), 4 mg more oxalate (P < 0.001), 9 mEq less potassium (P < 0.001), 11 mg less magnesium (P = 0.003), 120 mg less phosphate (P < 0.001), and 3 mmol less sulfate (P < 0.001) per day than did white women. The urine pH of black women was 0.11 units higher (P = 0.03) and urine volume was 0.24 L less (P = 0.001). The urinary relative supersaturations of calcium oxalate (P = 0.03) and brushite (P = 0.002) were lower in black women. No other significant differences were observed. Differences in urinary calcium and pH persisted after multivariate adjustment and after exclusion of participants who were taking thiazide diuretics or those with diabetes. In conclusion, black women excrete less urinary calcium and have a higher urinary pH than do white women. These differences are not explained by differences in age, body mass index, or diet and may account for the lower incidence of both nephrolithiasis and osteoporosis in black women.