Contribution of dietary oxalate to urinary oxalate excretion

Background. The amount of oxalate excreted in urine has a significant impac t on calcium oxalate supersaturation and stone formation. Dietary oxalate i s believed to make only a minor (10 to 20%) contribution to the amount of o xalate excreted in urine, but the validity of the experimental observations that support this conclusion can be questioned. An understanding of the ac tual contribution of dietary oxalate to urinary oxalate excretion is import ant, as it is potentially modifiable. Methods. We varied the amount of dietary oxalate consumed by a group of adu lt individuals using formula diets and controlled, solid-food diets with a known oxalate content, determined by a recently developed analytical proced ure. Controlled solid-food diets were consumed containing 10, 50, and 250 m g of oxalate/2500 kcal, as well as formula diets containing 0 and 180 mg ox alate/2500 kcal. Changes in the content of oxalate and other ions were asse ssed in 24-hour urine collections. Results. Urinary oxalate excretion increased as dietary oxalate intake incr eased. With oxalate-containing diets, the mean contribution of dietary oxal ate to urinary oxalate excretion ranged from 24.4 +/- 15.5% on the 10 mg/25 00 kcal/day diet to 41.5 +/- 9.1% on the 250 mg/2500 kcal/day diet, much hi gher than previously estimated. When the calcium content of a diet containi ng 250 mg of oxalate was reduced from 1002 mg to 391 mg, urinary oxalate ex cretion increased by a mean of 28.2 +/- 4.8%, and the mean dietary contribu tion increased to 52.6 +/- 8.6%. Conclusions. These results suggest that dietary oxalate makes a much greate r contribution to urinary oxalate excretion than previously recognized, tha t dietary calcium influences the bioavailability of ingested oxalate, and t hat the absorption of dietary oxalate may be an important factor in calcium oxalate stone formation.