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Dependence of Oxalate Absorption on the Daily Calcium Intake

Gerd E. von Unruh^*, Susanne Voss, Tilman Sauerbruch^* and Albrecht Hesse

*Department of Internal Medicine I and Division of Experimental Urology, Department of Urology, University of Bonn, Sigmund-Freud-Str. 25, D-53105 Bonn, Germany.

Correspondence to Dr. Gerd von Unruh, Medizinische Universitätsklinik I, Universitätsklinikum Bonn, Sigmund-Freud-Str. 25, D-53105 Bonn; Phone: ++49 228 287 5213; Fax: ++49 228 287 4323; E-mail: Gerd.von_Unruh{at}ukb.uni-bonn.de

ABSTRACT. Two to 20% of ingested oxalate is absorbed in the gastrointestinal tract of healthy humans with a daily 800 mg calcium intake. Calcium is the most potent modifier of the oxalate absorption. Although this has been found repeatedly, the exact correlation between calcium intake and oxalate absorption has not been assessed to date. Investigated was oxalate absorption in healthy volunteers applying 0.37 mmol of the soluble salt sodium [¹³C₂]oxalate in the calcium intake range from 5 mmol (200 mg) calcium to 45 mmol (1800 mg) calcium. Within the range of 200 to 1200 mg calcium per day, oxalate absorption depended linearly on the calcium intake. With 200 mg calcium per day, the mean absorption (± SD) was 17% ± 8.3%; with 1200 mg calcium per day, the mean absorption was 2.6% ± 1.5%. Within this range, reduction of the calcium supply by 70 mg increased the oxalate absorption by 1% and vice versa. Calcium addition beyond 1200 mg/d reduced the oxalate absorption only one-tenth as effectively. With 1800 mg calcium per day, the mean absorption was 1.7% ± 0.9%. The findings may explain why a low-calcium diet increases the risk of calcium oxalate stone formation.

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Copyright © 2008 by the American Society of Nephrology. Online ISSN: 1533-3450 Print ISSN: 1046-6673