Slow and fast dietary proteins differently modulate postprandial protein accretion

Abstract

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 intrinsically 13C-leucine-labeled milk proteins, casein (CAS) and whey protein (WP), of different physicochemical properties 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 prolonged 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 stimulated by 68% with the WP meal and to a lesser extent (+31%) with the CAS meal. Postprandial whole body leucine oxidation over 7 h was lower with CAS (272 +/- 91 micromol.kg-1) than with WP (373 +/- 56 micromol.kg-1). Leucine intake was identical in both meals (380 micromol.kg-1). Therefore, net leucine balance over the 7 h after the meal 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 response, a concept to be applied to wasting situations.

Figures

Figure 1

Protocol design of the different studies with the labeled (13C-WP and 13C-CAS studies) and unlabeled (UL-WP and UL-CAS studies) milk protein fractions.

Figure 2

(A) Plasma leucine concentrations, (B) enrichments of the i.v. infused 2H3 tracer, and (C) enrichments of the orally administered 13C tracer, after a labeled WP meal (13C-WP study) and a CAS meal (13C-CAS study). ∗, Statistical differences between the two protein meals (P < 0.05). The dashed lines at the top of each panel indicate a significant difference from baseline (P < 0.05) within each study.

Figure 3

(A) Total leucine rate of appearance, (B) exogenous leucine rate of appearance, and (C) endogenous leucine rate of appearance after labeled WP ingestion (13C-WP study; open triangles) and labeled CAS ingestion (13C-CAS study; closed circles). ∗, Statistical differences between the two protein meals (P < 0.05). The dashed lines at the top of each panel indicate a significant difference from baseline (P < 0.05) within each study.

Figure 4

(A) Total leucine oxidation after unlabeled milk protein ingestion (UL-WP and UL-CAS studies), (B) exogenous leucine oxidation after labeled protein ingestion (13C-WP, open triangles; 13C-CAS, closed circles), (C) and Total NOLD (i.e., protein synthesis) from UL-WP and UL-CAS studies. ∗, Statistical differences between the two protein meals (P < 0.05). The dashed lines at the top of each panel indicate a significant difference from baseline (P < 0.05) within each study.