The urinary response to an oral oxalate load in recurrent calcium stone formers

Abstract

Purpose: Dietary oxalate may contribute up to 50% to 80% of the oxalate excreted in urine. We studied the urinary response to an oral oxalate load in male and female idiopathic recurrent calcium oxalate stone formers with and without mild hyperoxaluria to evaluate the potential pathophysiological significance of dietary oxalate.

Materials and methods: A total of 60 recurrent calcium stone formers underwent an oral oxalate load test. Urine samples were obtained after an overnight fast. Each patient then received an oral oxalate load (5 mM. sodium oxalate dissolved in 250 ml. distilled water) and 3, 2-hour urine samples were obtained 2, 4 and 6 hours after the oxalate load. We compared the response to the oxalate load in patients with and without mild hyperoxaluria, and in male and female patients without hyperoxaluria.

Results: The peak urinary response occurred 4 hours after the oral oxalate load in all patients. Those with mild hyperoxaluria had a mean fasting urinary oxalate-to-creatinine ratio +/- SE of 0.027 +/- 0.003 and a mean peak urinary oxalate-to-creatinine ratio of 0.071 +/- 0.006. In comparison, patients with normal oxalate excretion had a fasting and peak urinary oxalate-to-creatinine ratio of 0.018 +/- 0.001 and 0.056 +/- 0.004, respectively (p <0.05). The mean 6-hour increment for urinary oxalate excretion after the oxalate load for patients with hyperoxaluria versus those with normal urinary oxalate excretion was 17.2 +/- 1.9 versus 12.1 +/- 0.98 mg. (p <0.05). In the subset of patients with normal urinary oxalate excretion mean 6-hour cumulative urinary oxalate excretion was 16.8 +/- 1.3 and 13.3 +/- 1.4 mg. in males and females, respectively (p not significant).

Conclusions: Recurrent calcium stone formers with mild hyperoxaluria have higher fasting urinary oxalate and an exaggerated urinary response to an oral oxalate load compared with recurrent calcium stone formers with normal urinary oxalate excretion. Men and women stone formers without hyperoxaluria excrete similar fractions of an oral oxalate load. Increased gastrointestinal absorption and renal excretion of dietary oxalate may be a significant pathophysiological mechanism of stone formation in patients with mild hyperoxaluria.