Observations on Protein Digestion In Vivo

Abstract

The rate of disappearance of nitrogen from the stomach and the small intestine of “trained” rats consuming purified diets (containing all the proximate principles) containing 30 or 50% of gelatin, and 30% of casein, soybean protein, zein, gliadin, egg albumin, beef proteins, pork proteins or an amino acid mixture has been determined. The slowest rate of stomach-emptying was noted when the diet contained 30% of pork proteins whereas the most rapid rate of stomach-emptying was found with the 30% gliadin diet. The shapes of the stomach-emptying curves varied from almost exponential (gliadin diet) to nearly linear (gelatin diet). The percentage of nitrogen in the stomach contents (mg N/mg dry weight × 100) remained nearly constant during the early hours of the experimental period studied when the diet contained casein, soybean protein, beef proteins, pork proteins, or gliadin; decreased slightly when the diet contained gelatin or egg albumin, and possibly for that containing amino acids; and increased greatly when the diet contained zein. More nitrogen accumulated in the small intestines of rats fed diets containing zein and gelatin than in the intestines of those fed casein; a small accumulation was found when the diet contained egg albumin, pork or beef proteins and no accumulation above that for rats fed casein was observed when the diet contained the amino acid mixture, soybean protein or gliadin. Heat treatment of meat proteins appeared to render the meat less digestible. The conclusion is drawn that it is not advisible to generalize from results obtained with a single protein about the effect of the protein content of the diet on the rate of stomach-emptying and intestinal accumulation of nitrogen.