The digestion and absorption of protein in man

Abstract

1. The absorption of the amino acid components of two protein-containing test meals has been studied in six healthy volunteers. One meal contained 15 g of milk protein and the other contained 15 g of gelatin. In a control experiment a meal was given which contained a negligible amount of protein.2. The subjects were intubated with a single lumen tube; then each meal was swallowed and intestinal residues were obtained from known levels. The amino acid composition of the intestinal contents was compared with that of the original meal. Correction was made for net water shifts by reference to a non-absorbable marker compound (polyethylene glycol 4000).3. The results showed that at least 70–75% of the milk protein test meal had been absorbed when the sampling holes were 230 cm from the nose. It is suggested, however, that most, if not all, of the meal had been absorbed when the sampling holes were 140 cm from the nose.4. Amino acids were absorbed at rates proportional to their concentrations in the meal.5. Gelatin, a protein known to be relatively resistant to enzymic hydrolysis, was poorly absorbed from the region of the small intestine under study.6. Estimates of the amount of endogenous protein secreted in response to the test meals ranged from z to 8 g, equivalent to 13–53% of the protein containing test meals.7. The absorption of certain amino acids, e.g. the dicarboxylic amino acids, was more rapid than was expected; glutamic and aspartic acids are absorbed slowly from a mixture of amino acids, both in vitro and in vivo.