Calcium and phosphorus metabolism in pregnant rats ingesting a high protein diet

In both nonpregnant and pregnant rats as dietary protein increased, increases in urinary calcium were observed. In nonpregnant rats, urinary calcium excretion was significantly increased as the level of dietary protein increased from 16 to 24 to 32%, but in the pregnant rats increases in urinary calcium as protein intake was raised from 24 to 32% were not observed. The present data indicated that the increase in urinary calcium was probably a result of a shift of the endogenous excretion of calcium from the feces to urine and not from intestinal absorption of dietary calcium or bone resorption of calcium. Responses of parameters indicative of intestinal absorption either were inconsistent or were not affected as protein intake increased. A high protein diet had no effect on the amount of calcium lost from the bones. Although the results were inconsistent with regard to calcium retention, a positive calcium balance was achieved by all rats fed the three different levels of dietary protein. Urinary phosphorus excretion increased with increased protein intake, but the response was modified by the physiological state. In pregnant rats, the increase from 16 to 24% dietary protein had a greater influence on urine phosphorus than the increase from 24 to 32%; this pattern was reversed in the nonpregnant rats.