Cecal nitrogen metabolism and amino acid absorption in the rabbit

Nitrogen balance and weight changes were determined in two groups of rabbits fed diets containing all natural protein or part nonprotein nitrogen from urea. The rabbits were fed the experimental diets for a 10-week period starting at 7 weeks of age. No significant differences in weight gains or nitrogen retention were found between groups. Total nitrogen, ammonia, urea, and free amino acid levels were determined in the contents of several segments of the gastrointestinal tract of the rabbits fed both diets. Significantly less total nitrogen, ammonia, and free amino acids were found in the lower colon compared with the cecal contents of both groups. Subsequent studies with (C)alanine revealed considerable fermentation of alanine in both the cecum and upper colon contents, but there was little absorption into the blood. It was concluded that the disappearance of nitrogen that occurred between the cecum and colon was in the form of ammonia, and that the value of urea as a nitrogen source resulted primarily from tissue synthesis of dispensable amino acids and coprophagy.