Obligatory Nitrogen Losses in Pregnant Rats with Special Reference to Estimation of Net Protein Utilization.

Abstract

SummaryFor 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/100g 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 preg nant rats (666mg/15days and 234mg/6days, respectively) than in nonpregnant animals (585mg/15days and 153mg/6days, respectively), indicating pregnancy-induced protein hypercatabolism. The metabolic fecal nitrogen excretions in pregnant and nonpregnant rats were compara ble. In pregnant rats, a protein-free diet resulted in decrease of basal energy expenditure, from 24kcal/day on day 1 to about 15kcal/day on days 16, 19 and 22 of pregnancy. Thus, the ratio of endogenous urinary nitrogen to basal energy expenditure increased in late pregnancy, indicat ing 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. Key Words endogenous urinary nitrogen, metabolic fecal nitrogen, en dogenous nitrogen/BMR, pregnancy induced hypercatabolism, NPU, pro tein-free diet Obligatory urinary and fecal excretions of nitrogen have been determined in various animals, including humans, to determine the minimum nitrogen require ments and/or to assess the net utilization of dietary proteins (NPU) (1). Results 493

Net Utilization of Various Levels of Whole Egg Protein in Pregnant Rats.

“…Previous reports (5,6), however, showed that pregnancy in rats fed PFD could be maintained with ovarian steroids, making it possible to estimate endogenous urinary and metabolic fecal nitrogen losses and to assess the NPUs of dietary proteins in pregnant rats (7).…”