Obligatory nitrogen losses in pregnant rats with special reference to estimation of net protein utilization

For estimation of net protein utilization of dietary proteins during pregnancy, obligatory nitrogen losses were measured in protein-deficient rats in which pregnancy was maintained by administration of ovarian steroids. On shift from normal to protein-free diet, urinary nitrogen, expressed as mg/day or mg/100 g BW per day, decreased initially rapidly and then gradually during the first two weeks in both pregnant and nonpregnant rats. However, urinary endogenous nitrogen increased during the final week of pregnancy, whereas it continued to decrease in nonpregnant controls. The endogenous urinary nitrogen excretions during early-mid and late pregnancies were significantly higher in pregnant rats (666 mg/15 days and 234 mg/6 days, respectively) than in nonpregnant animals (585 mg/15 days and 153 mg/6 days, respectively), indicating pregnancy-induced protein hypercatabolism. The metabolic fecal nitrogen excretions in pregnant and nonpregnant rats were comparable. In pregnant rats, a protein-free diet resulted in decrease of basal energy expenditure, from 24 kcal/day on day 1 to about 15 kcal/day on days 16, 19 and 22 of pregnancy. Thus, the ratio of endogenous urinary nitrogen to basal energy expenditure increased in late pregnancy, indicating that "the law of a constant relationship of minimal nitrogen and energy output" is not applicable to the pregnant animals. We discuss which values for obligatory nitrogen loss should be used for estimating the net utilization efficiency of dietary proteins in pregnancy.