Why the Obese Get More Kidney Stones
Data suggest that higher risk of stones is associated with lower urinary pH, researchers say
Urinary supersaturation of uric acid rises with BMI, which could explain why obese individuals have a higher incidence of kidney stones, a study indicates.
Eric N. Taylor, MD, and Gary C. Curhan, MD, ScD, of the Channing Laboratory in Boston, studied 599 stone-forming and 404 non-stone-forming men from the Health Professionals Follow-up Study; 888 stone-forming and 398 non-stone-forming women in the Nurses’ Health Study I (which enrolled registered women nurses aged 30-55 years in 1976); and 689 stone-forming and 295 non-stone-forming women in the Nurses’ Health Study II (which enrolled registered women nurses aged 25-42 years). The researchers examined 24-hour urine samples from subjects with a history of kidney stones and from randomly selected controls as part of an ongoing study to compare urine composition of those with and without kidney stone disease.
Confirming prior research, Drs. Taylor and Curhan found an inverse relationship between BMI and urinary pH. In multivariate analyses, urinary pH values in men, older women, and younger women in the highest quintile of BMI were 0.17, 0.17, and 0.23 less than those in the lowest quintile of BMI, respectively, the researchers reported in the American Journal of Kidney Diseases (2006;48:905-915). As a result, urinary supersaturation of uric acid increased with increasing BMI in all study groups.
“Our results suggest that the increase in stone risk in the overweight and obese may represent an increase in the incidence of uric acid nephrolithiasis,” the authors wrote, noting that previous studies have suggested that the prevalence of uric acid stones is greater in obese than in non-obese individuals.
Urinary calcium excretion rose slightly with increasing BMI in men and stone-forming younger women (from the Nurses’ Health Study II), but the association did not persist after adjusting for urinary phosphate and sodium excretion. BMI and urinary calcium excretion were inversely related in older women (from the Nurses’ Health Study I). In all three cohorts, BMI was positively associated with urinary oxalate excretion. Due to differences in urinary volume and the excretion of inhibitors such as citrate, the urinary supersaturation of calcium oxalate did not increase across quintiles of BMI in any of the study populations.
“To the extent that urinary supersaturation predicts risk for nephrolithiasis, our data suggest that risk for calcium oxalate stone formation does not increase with increasing body size,” the investigators wrote.