Dietary oxalate and its intestinal absorption

Abstract

Dietary oxalate is currently believed to make only a minor contribution (< 20%) to urinary oxalate excretion. A recent prospective study of stone disease suggested that dietary oxalate may be a significant risk factor. This observation led us to re-evaluate the contribution of dietary oxalate to urinary oxalate excretion. Previous studies have been hampered by inaccurate food composition tables for oxalate and inadequate methods for studying intestinal oxalate absorption. This evidence as well as factors that modify oxalate absorption are reviewed. New approaches to measure food oxalate and intestinal oxalate absorption have been examined. Capillary electrophoresis appears to be well suited for the analysis of the oxalate content of food. Two individuals consumed an oxalate-free formula diet for 7 days. This diet decreased urinary oxalate excretion by an average of 67% (18.6 mg per 24 hours) compared to oxalate excretion on self-selected diets. The absence of detectable oxalate in feces by day 6 of the diet suggested that the intestinal absorption was minimal. However, an effect of the formula diet on endogenous oxalate synthesis cannot be excluded. Restoring oxalate to the formula diet increased urinary oxalate excretion and illustrates that this experimental protocol may be well-suited for studying oxalate absorption and factors that modify it. Our results suggest that the intestinal absorption of dietary oxalate makes a substantial contribution to urinary oxalate excretion and that this absorption can be modified by decreasing oxalate intake or increasing the intakes of calcium, magnesium, and fiber.