Effects of food restriction and body weight loss on metabolic fecal nitrogen excretion in the rat.

Metabolic fecal nitrogen was measured in young adult female rats fed synthetic diets before and after weight losses had been caused by restricted food intake. When food intake was decreased from 5.5 to 2.4 g or from 6.7 to 2.3 g per 100 g body weight per day, metabolic fecal nitrogen per gram food intake increased by 48 and 59%, respectively. However, weight losses did not affect metabolic fecal nitrogen excretion significantly until some 40% loss of weight had been obtained. At this point metabolic fetal nitrogen excretion was observed to increase by 11%. These results indicate that the 6% increase in apparent digestibility of crude protein in a stock diet which occurs during food restriction is due to a genuine increase in absorption of dietary nitrogen.