Protein digestion and absorption in the rat.

1. A randomly labelled 14C protein was synthesized in order to investigate the site and rate of digestion and absorption of dietary protein in the rat. 2. A liquid test meal consisting of protein and a non-absorbable marker, 51CrCl3, was administered to rats which were then sacrificed at intervals up to 4 hr after ingestion of the meal. Analysis of intestinal contents showed that as gastric emptying proceeded, the meal moved rapidly to the distal two thirds of the small intestine. 3. Protein digestion and absorption occurred predominantly in this area over a period of 1-2 hr. 4. Amounts of endogenous protein present in the small intestine never exceeded amounts of exogenous protein during maximum absorption of exogenous protein (0-1 hr). At later time periods (2-4 hr), however, more endogenous than exogenous protein was detected in the intestinal lumen. 5. It is concluded that the digestion and absorption of dietary protein is a rapid process, taking place in the distal two-thirds of the small intestine. Endogenous protein levels do not exceed exogenous protein levels until after the bulk of exogeneous protein is absorbed.