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Effect of Prolonged Bedrest on the Propensity for Renal Stone Formation^*

Center in Mineral Metabolism and Clinical Research, Department of Internal Medicine, Southwestern Medical School, University of Texas Health Science Center Dallas, Texas 75235
The Department of Internal Medicine, University of Texas Health Science Center, Texas Medical Center Houston, Texas 77225

Address all correspondence and requests for reprints to: Dr. Charles Y. C. Pak, Department of Internal Medicine, University of Texas Health Science Center, Southwestern Medical School, 5323 Harry Hines Boulevard, Dallas, Texas 75235.

The effect of prolonged bedrest immobilization on urinary risk factors for stone formation and on the propensity for the crystallization of calcium salts was examined in eight normal subjects. During 5 weeks of bedrest, the mean urinary calcium excretion rose during the first week and remained elevated (from 5.68 to 7.50 mmol/day). Mean urinary phosphorus excretion increased by the second week of bedrest and remained elevated (from 2.70 to 30.6 mmol/day). Urinary sodium and uric acid excretion rose slightly, as did urinary magnesium. Urinary pH, oxalate, and citrate changed slightly or not at all. Owing to these biochemical alterations, urinary saturation of calcium phosphate, calcium oxalate, and monosodium urate increased significantly during bedrest, but that of uric acid did not change. The inhibitor activity against the spontaneous nucleation of brushite (CaHPO₄ · 2H₂O) and calcium oxalate was not altered significantly by bedrest. Thus, the propensity for the crystallization of stone-forming calcium salts was enhanced by bedrest, suggesting that immobilization may confer increased risk for the formation of calcium-containing renal stones.

^* This study was supported by NASA Grant NAG 9-152 and USPHS Grants P01-DK-20543 and M01-RR-00633.

Received June 29, 1987.