Urinary oxalate levels are not affected by dietary purine intake or allopurinol

Absence of effect of allopurinol on oxalate excretion by stone patients on random and controlled diets

After 1 year of allopurinol treatment in 36 patients with a history of uric acid and/or calcium oxalate lithiasis and hyperuricosuria, we observed that in addition to the desired decreases in uric acid there were apparently significant decreases in urinary oxalate levels: 37 +/- 3 mg. per day (mean +/- standard error) before therapy and 31 +/- 4 mg. per day after a mean decrease of 16% (p less than 0.05) with an equivalent decrease in the supersaturation (calcium oxalate) of the urine. However, the decrease in oxalate could have been related to changes in dietary habits rather than to any specific effects of allopurinol on oxalate metabolism. Therefore, we recruited 26 of the patients for a study in which dietary factors were controlled. Each participant was assigned to 1 of 3 diet groups: low or high protein, or a customary diet. Each patient collected a urine specimen while on allopurinol and again after the medication was discontinued. With analytical procedures that we ascertained to be free of any significant methodological bias, we observed no significant changes in urinary oxalate excretion that could be attributed to allopurinol. There were significant differences in oxalate excretion on versus off allopurinol between the low and high protein groups, with higher oxalate levels found for the latter group. Our results indicate that allopurinol does not have a specific effect on oxalate metabolism or oxaluria.

The effect of decreasing the concentration of urinary urate on the crystallization of calcium oxalate in undiluted human urine

The effect of lowering the urate concentration, using uricase immobilized on to nylon tubing, on the nucleation, growth and aggregation of calcium oxalate crystals in undiluted human urine was examined. The median urate concentration was significantly (p less than 0.01) reduced from 2.8 to 0.55 mmol/l., but this had no reproducible effect on the metastable limits, the volume of crystalline calcium oxalate deposited and the size of the crystals and aggregates produced from the 10 urine samples examined. It was concluded that further studies aimed at testing the effects of a raised urinary urate concentration on the crystallization of calcium oxalate should be made a matter of high priority.