Observations on Protein Digestion In Vivo

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Abstract
The amount of nitrogen in the stomach and intestinal contents of rats was determined at intervals after the animals had consumed a “nitrogen-free” meal or meals containing the equivalent of 22% of protein from casein, gelatin, zein or an amino acid mixture. When the meal contained free amino acids or gelatin, food intake was depressed, stomach-emptying pattern was altered and the moisture content of the stomach contents was increased compared with values for the casein control group. More nitrogen was present in the intestinal contents of rats fed diets containing zein or gelatin than in those of rats fed casein or amino acids. The greatest part of the extra nitrogen was in an insoluble fraction when the diet contained zein and in a trichloroacetic acid-soluble fraction when the diet contained gelatin. Only small amounts of the nitrogen in the intestinal contents of rats fed the amino acid, casein, or “nitrogen-free” diets were in insoluble or trichloroacetic acid-precipitable fractions. The peptides from the intestinal contents of rats fed the gelatin diet were estimated to contain about twice as many amino acids as those from rats fed the other diets. The amount of nitrogen in the soluble fraction of the intestinal contents of rats fed a diet containing protein or amino acids which was precipitable with trichloroacetic acid was small but was consistently greater than that observed when a “nitrogen-free” diet was fed.