Category: Clinical Stones: Medical Management
Introduction & Objective : Urinary oxalate levels are affected by both dietary and endogenous components. Prior studies have demonstrated the positive correlation between weight/body mass index (BMI) and urinary oxalate excretion. Our objective was to determine if this association is secondary to increased endogenous oxalate synthesis.
Methods : Healthy subjects, between 18 and 65 years old, with variable BMI were recruited and separated into lower BMI (33) cohorts. Every subject had a normal serum comprehensive metabolic panel. Subjects consumed a low oxalate controlled diet containing 16% protein, 30% fat, 54% carbohydrate, 1000 mg calcium, and 30 mg oxalate which was devoid of vitamin C and calcium supplements. The extremely low oxalate diet dramatically limits the contribution of dietary oxalate to the urinary oxalate pool and thus allows an accurate estimation of urinary oxalate derived from endogenous oxalate synthesis. Subjects remained on this diet for 3 days. 24-hour urine collections were performed on the last two days. Urinary oxalate was measured by ion chromatography coupled with mass spectroscopy. Statistical analysis included Chi-squared, correlation and linear regression analysis, and student t-test.
Results : There were 5 subjects in the higher BMI group (3 female and 2 male; mean age 41±15 years; mean BMI 36.7±4.2) and three in the lower BMI group (2 female, 1 male; age 40±7 years; mean BMI 24.7±2.7). Subjects in the higher BMI cohort had a significantly greater excretion of urinary oxalate (36±7 vs 23±5 mg/day, p=0.021) compared to the lower BMI cohort. Urinary oxalate excretion (mg/day) was positively correlated with waist circumference (r=0.94, p=0.0006), waist to hip ratio (r=0.75, p=0.03), and BMI (r=0.9, p=0.002).
Conclusions : These results demonstrate a positive correlation between urinary oxalate derived from endogenous oxalate synthesis and BMI as well as other measures of obesity. This also provides an explanation for the association between stone risk and obesity.
Kyle Wood– Assistant Professor, University of Alabama at Birmingham, Birmingham, Alabama
John Knight– Assistant Professor, University of Alabama at Birmingham, Birmingham, Alabama
Ross Holmes– Professor, University of Alabama at Birmingham, Birmingham, Alabama
Dean Assimos– Professor, University of Alabama at Birmingham, Birmingham, Alabama
University of Alabama at Birmingham
Kyle David Wood MD is an Assistant Professor at the University of Alabama at Birmingham. He has completed an Endourology Fellowship. His clinical practice is focused on the medical and surgical management of complex kidney stones. He was an AUA Research Scholar and recently recieved a K08 NIH mentored grant. His research interest is on endogenous oxalate synthesis and its relationship to kidney stone disease.
University of Alabama at Birmingham
Dr. Dean George Assimos is currently the Anton J. Bueschen Professor and Chairman of the Department of Urology at the University of Alabama at Birmingham School of Medicine. He obtained a BS degree in biology from Purdue University in 1974 and an MD degree from Loyola University of Chicago in 1977. He subsequently completed his urology residency at Northwestern University in 1983. He was a fellow in intra-renal surgery and kidney stone disease at Wake Forest University in 1984 after which he was an A.U.A. research scholar at the same institution from 1984-1986. He has dedicated his career to the care of patients afflicted with complex renal stone disease including medical and surgical management. He has conducted clinical, translational and basic science research focusing on factors which impact urinary oxalate excretion. He and his research colleagues have received steady funding from the NIH for these endeavors. They have demonstrated the important contribution of dietary oxalate to the urinary oxalate pool, defined the importance of interactions between dietary calcium and oxalate, and characterized mechanisms of gastrointestinal oxalate absorption and endogenous oxalate synthesis. He is the author of numerous papers and chapters pertaining to these subjects. He has served as a member of the A.U.A Nephrolithiasis Guidelines and was the Chair of this committee. Dr. Assimos is on the editorial board of several urologic journals. He is also committed to urologic education participating as a speaker and course director for the A.U.A. and other organizations. He has received several prestigious honors including the American Urological Care Foundation Distinguished Scholar Alumnus Award, the Ralph Clayman Mentor Award from the Endourological Society, and the AUA Hugh Hampton Young Award. Dr. Assimos is the past president of the Southeastern Section of the A.U.A.