Rapid aminoacidemia enhances myofibrillar protein synthesis and anabolic intramuscular signaling responses after resistance exercise

Journal Title (Medline/Pubmed accepted abbreviation): Am J Clin Nutr
Year: 2011
Volume: 94
Page number: 795-803
doi : 10.3945/ajcn.111.013722

Summary of background and research design:

Background: Protein ingestion increases aminoacidemia, which stimulates muscle protein synthesis. This effect is enhanced by resistance exercise. However, the digestion rate of proteins changes both the amplitude and timing of the aminoacidemia, which may affect muscle protein synthesis in postexercise recovery.

Hypothesis: A rapid acute rise in aminoacidemia from a protein bolus would stimulate greater rates of muscle protein synthesis and increased mTOR signaling in postexercise recovery compared with a slower sustained rise in aminoacidemia from repeated small protein boluses.

Subjects: Eight recreationally active healthy men (mean age, 21.5 ± 1 yr; height, 1.81 ± 0.02 m; weight, 80.1 ± 3.5 kg; body mass index, 24.3 ± 0.8 kg/m 2) participated in this study.

Experimental design: Randomized, cross-over

Treatments Protocol: Participants were randomized to receive either a whey bolus or pulse (10 drinks; 2.5 g every 20 min) protein drink (total: 25 g whey, 12.8 g essential amino acids [EAAs], and 3.5 g leucine) with 4% isotope-labelled phenylalanine added following an acute bout of resistance exercise (8 sets of 8 to 10 repetitions of bilateral leg extension at 10 reps maximum, determined previously, with 2-min rest between sets). All participants completed the exercise under both conditions from 8 to 72 days (mean, 30 days) apart. A bolus injection of 2.0 μmol/kg [ring-13C6]phenylalanine primed the pool before isotope infusions (rate, 0.05 μmol/kg/min) started. Muscle protein synthesis was calculated from infusions of [ring-13C6]phenylalanine (from up to 2.5 hr before exercise to 5 hr after exercise) and from muscle biopsies (at baseline [trial 1 only] and 1, 3, and 5 hr after exercise from the vastus lateralis). Blood samples (collected at baseline and after exercise: every 20 min for 2 hr and every 30 min for an additional 3 hr) were used to measure insulin, phosphorylation of mTOR signaling proteins, and amino acid levels.

Summary of research findings:
  • The area under the curve for blood levels of leucine and EAAs were similar (98% and 99%, respectively).
    • At 60 and 80 minutes, leucine and EAA concentrations were higher in the bolus group than in the pulse group (P < .05).
    • At 180, 200, and 240 minutes, leucine and EAA concentrations were higher in the pulse group than in the bolus group (P < .05).
    • The differences had a significant time × condition interaction (P < .001).
    • Baseline levels were achieved between 3 and 5 hours postexercise.
  • Insulin concentrations did not change from baseline in the pulse group, but increased in the bolus group, with higher values than the pulse group at 20, 40, and 60 minutes.
  • In both groups, muscle protein synthesis was stimulated from baseline at 1 to 3 hours (P = .026) and at 3 to 5 hours (P < .001) of recovery.
    • In the bolus group, muscle protein synthesis was greater than in the pulse group at 1 to 3 hours (P = .01) and at 3 to 5 hours (P = .001).
  • Upstream and downstream target proteins in the mTORC1 signaling pathway had increased phosphorylation after exercise under both conditions, but phosphorylation was increased to proline-rich protein-40 (PRAS40 [Thr246]), S6 kinase-1 (S6K1 [Thr389]), and ribosomal protein-S6 (rpS6 [Ser235/6]) at 1 hour after exercise in bolus group than pulse group; P < .05 for all).
    • Phosphorylation of eukaryotic elongation factor 2 (eEF2 [Thr56]; indicates increased activation) decreased in the pulse group at 1 hour after exercise compared with the bolus group (P < .05).

Interpretation of findings/Key practice applications:

Whey protein as a bolus supplement can enhance MPS during recovery from resistance exercise to a greater degree than whey protein as a more frequent, smaller-dose supplement. Because the total consumption and exposure to amino acid supplementation was similar in both conditions, the results show that the pattern of aminoacidemia was responsible for the difference in muscle protein synthesis. Interestingly, muscle protein synthesis remained increased from baseline even after amino acid levels had returned to baseline (at 3 to 5 hr), suggesting an enhancement of the effect from the resistance exercise. These results have implications for eating patterns and protein choice postexercise with regards to maximum muscle growth. However, whether it was the acute increase in leucine or EAA levels that was responsible for the changes in muscle protein synthesis is unknown from this study. Moreover, further research is necessary to evaluate if there is a threshold effect for leucine or EAAs to stimulate muscle protein synthesis.
Limitations of this study include no measurements of muscle protein breakdown or overall protein balance. Increases in muscle protein synthesis are of less benefit if muscle protein breakdown is also occurring at a greater rate.
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