Influence of Body Mass Index on Quantitative 24-Hour Urine Chemistry Studies in Children With Nephrolithiasis

Purpose: A high body mass index increases the risk of nephrolithiasis in adults. Despite the growing problem of pediatric obesity, little is known about the relationship between body mass index and risk of nephrolithiasis in children. We examined the association between body mass index and 24-hour urine chemistry studies in children with a history of nephrolithiasis. Materials and Methods: A total of 43 children were included in the study. We retrospectively reviewed a database of 24-hour urine chemistry studies. We calculated body mass index for each individual and cases were then stratified by percentile. The 24-hour urine chemistry studies were adjusted for daily creatinine excretion, urine volume was adjusted for age, and pH and urine supersaturations were unadjusted. Results: Body mass index percentile was below the 25th percentile in 8 cases, 25th to 49th percentile in 7, 50th to 74th percentile in 5 and 75th percentile or above in 14. On multivariate analysis the only 24-hour urine parameters with a significant relationship to body mass index were urine oxalate (negative relationship) and supersaturation of calcium phosphate (positive). As body mass index increased, urine oxalate excretion decreased and supersaturation of calcium phosphate increased. Conclusions: A high body mass index is associated with decreased urine oxalate and increased supersaturation of calcium phosphate. Given the increasing prevalence of obesity in younger patients, our findings have important clinical implications. Pediatricians and pediatric subspecialists should be aware of these findings when evaluating children with nephrolithiasis.