Slow and fast dietary proteins differently modulate postprandial protein accretion (amino acid turnoverypostprandial protein anabolismymilk proteinystable isotopes)

The speed of absorption of dietary amino acids by the gut varies according to the type of ingested dietary protein. This could affect postprandial protein synthesis, breakdown, and deposition. To test this hypothesis, two in- trinsically 13 C-leucine-labeled milk proteins, casein (CAS) and whey protein (WP), of different physicochemical proper- ties were ingested as one single meal by healthy adults. Postprandial whole body leucine kinetics were assessed by using a dual tracer methodology. WP induced a dramatic but short increase of plasma amino acids. CAS induced a pro- longed plateau of moderate hyperaminoacidemia, probably because of a slow gastric emptying. Whole body protein breakdown was inhibited by 34% after CAS ingestion but not after WP ingestion. Postprandial protein synthesis was stim- ulated by 68% with the WP meal and to a lesser extent (131%) with the CAS meal. Postprandial whole body leucine oxidation over 7 h was lower with CAS (272 6 91 mmolzkg 21 ) than with WP (373 6 56 mmolzkg 21 ). Leucine intake was identical in both meals (380 mmolzkg 21 ). Therefore, net leucine balance over th e7ha fter themeal was more positive with CAS than with WP (P < 0.05, WP vs. CAS). In conclusion, the speed of protein digestion and amino acid absorption from the gut has a major effect on whole body protein anabolism after one single meal. By analogy with carbohydrate metabolism, slow and fast proteins modulate the postprandial metabolic re- sponse, a concept to be applied to wasting situations.