Kidney Week

Abstract: SA-PO169

Contribution of Endogenous Oxalate Synthesis to Urinary Oxalate Excretion

Session Information

• Vascular Calcification, Nephrolithiasis, Bone
  November 05, 2022 | Location: Exhibit Hall, Orange County Convention Center‚ West Building
  Abstract Time: 10:00 AM - 12:00 PM

Category: Bone and Mineral Metabolism

• 402 Bone and Mineral Metabolism: Clinical

Authors

• Fargue, Sonia, The University of Alabama at Birmingham School of Medicine, Birmingham, Alabama, United States

Sonia Fargue,

• Knight, John, The University of Alabama at Birmingham School of Medicine, Birmingham, Alabama, United States

John Knight,

Background

Urinary oxalate excretion is a known risk factor for calcium oxalate kidney stones and dietary oxalate absorption can contribute to 50% of urinary oxalate. In the absence of adequate dietary control, the rate of endogenous synthesis of oxalate is unknown. Using isotope tracer methodology, we determined the turnover rate of oxalate in healthy volunteers and compared it with urinary excretions on a controlled diet.

Methods

A primed, continuous infusion of ¹³C₂-oxalate was administered for 5 hrs in the fasted state to 16 healthy adults (7M/9F) between 24 and 56 years of age and BMI 23-44 kg/m² after 2 days of equilibration on a low oxalate, normal calcium fixed diet. Blood and urine were collected for analysis of oxalate by Ion Chromatography coupled with Mass Spectrometry and the rate of oxalate turnover rate calculated using urine ¹³C₂-oxalate mole percent enrichments at steady-state. Two 24-hr urines and 5 hourly urines in the fasted state were collected after dietary equilibration. Body composition was assessed by impedance.

Results

Mean 24-hr urinary oxalate excretion on the fixed diet was 20 ± 4 mg oxalate/day (range 12-28 mg/day), or 13 ± 3 mg oxalate/g creatinine. Projected 24-hr urinary oxalate was 17 ± 4 mg/day using fasted hourly collections. Isotopic equilibration was achieved within 3 hrs for most subjects. Mean urine enrichment with ¹³C₂-oxalate at steady-state was 19 ± 5%. Mean plasma enrichment at steady-state was 19 ± 1% in a subset of subjects analyzed. Average recovery rate of ¹³C₂-oxalate infused was 100 ± 10%. Oxalate turnover rate was 107 ± 26 nmol/hr/kg and endogenous oxalate synthesis 17 ± 4 mg/day (range 11-24 mg/day, 19 ± 4 and 16 ± 4 mg/day for males and females, respectively). There was a positive correlation between lean body mass and oxalate endogenous synthesis.

Conclusion

The range of oxalate synthesis rates spanned a two-fold interval even under fasting conditions and the use of fixed diet. The main factor associated with oxalate synthesis was lean body mass. There was good agreement with 24-hr and fasting hourly urinary oxalate excretions under these conditions. Future studies in oxalate kidney stone formers will address whether endogenous synthesis of oxalate is increased in this population and what factors underlie oxalate turnover.

Funding

• NIDDK Support