Effect of Protein-Induced Calciuria on Calcium Metabolism and Bone Status in Adult Rats

Abstract

⁴⁵Ca-labeled adult male rats were fed diets high in protein to determine long-term effects on calcium metabolism and bone status. Factors influencing renal excretion of calcium were examined for their involvement in protein-induced hypercalciuria. Control rats were fed a 6% casein diet. Test diets contained 6% casein plus 24% protein as lactalbumin, beef, casein, soy, egg white or gelatin. All diets were equal in Mg, P, and Ca. Collections made during the 20-week feeding regimen indicated a transient but marked calciuria (≥200% of control) occurring at or prior to days 56–59 by rats fed the lactalbumin, egg white, gelatin (P ≤ 0.001) and 30% casein (P ≤ 0.01) diets. Soy and beef diets were not calciuric. At days 56–59, rats fed lactalbumin, 30% casein, soy and egg white exhibited significantly depressed urinary specific activity of calcium (P ≤ 0.001), and all rats fed test diets produced higher fecal endogenous calcium, suggesting an increased absorption. No compositional differences indicative of bone resorption were present in the femur or mandibles of any rat fed test protein, dismissing bone as the source of calciuria. End-products of protein metabolism known to chelate calcium or compete with its renal reabsorption were significantly correlated with urinary calcium; these included sulfate, oxalate and sodium.