Enhanced amino acid sensitivity of myofibrillar protein synthesis persists for up to 24 h after resistance exercise in young men


Journal Title (Medline/Pubmed accepted abbreviation): J. Nutr..
Year: 2011
Volume: 141
Page numbers: 568-573
doi (if applicable): 10.3945/jn.110.135038

Summary of Background and Research Design

Background: Rates of protein synthesis are highest immediately after muscle-fatiguing resistance exercise with high volumes of contractile activity in conjunction with consumption of quality protein. It has been observed that protein synthesis rates remain above baseline for longer periods of time- up to 24-48 hrs but it has not yet been studied how energy intake affects protein synthesis in this later time frame. Also, it is unknown if the type of exercise [higher intensity (size of the weights as a percentage of the 1-repetition maximum, 1RM) or higher contractile volume (repetitions × load)] affects the later response of protein synthesis to exercise.

In this study, the authors investigate the following parameters that report on rates of muscle synthesis:

  • Rate of synthesis of mixed muscle protein – includes all types of muscle protein
  • Rate of synthesis of the myofibrillar muscle protein fraction – comprised of actin and myosin, the main muscle fibers responsible for muscle contraction
  • Rate of synthesis of the sarcoplasmic muscle protein fraction – analogous to the cytoplasm of a non-muscle cell. Within the sarcoplasm there are some cellular organelles and their proteins (ex. mitochondria) and soluble proteins such as myoglobin (binds oxygen).
  • Cell signaling events in the mTOR signaling pathway, which is one of the key pathways in protein synthesis and cellular growth. The phosphorylation of proteins such as Akt, mTOR, 4E-BP1, and p70S6K all help drive the process of translation (i.e., synthesis of new proteins) forward.

 

Hypothesis: 1) The rate of protein synthesis would be greater 24 hrs after resistance training than at baseline (resting conditions). 2) The rate of protein synthesis would be greater if exercise was performed to failure (high intensity) rather than if the same work load was achieved with smaller weights and more reps.

Subjects: Recreationally active men (n = 15), age = 21 ± 1 yrs.

Experimental design: randomized, counterbalanced. Subjects participated in 2 of 3 conditions for unilateral leg extensions:
  1. 4 sets of 90% of their 1RM to failure (90FAIL)
  2. 30% of their 1RM to match the work they achieved during 90FAIL (30WM)
  3. 30% of their 1RM to failure (30FAIL)

Treatments and protocol: First, the subjects determined their 1RM for unilateral leg extensions for both legs. Participants did not partake in resistance exercise for at least 3 days before the measurement of resting rate of protein synthesis or within 24 hours of the exercise treatments. On the morning of exercise, subjects consumed an Ensure Plus standardized meal (450 kcal) 2 hrs before they arrived at the lab. They then accomplished the resistance training exercise. After exercise, they consumed about 600-712 kcal at the cafeteria. The participants returned to the laboratory the next morning to assess rates of protein synthesis.
To assess rates of protein synthesis, isotopically-labeled phenylalanine was infused in order to establish a labeled-amino acid:unlabeled amino acid equilibrium. (This isotope was 13C, a stable isotope of carbon. This isotope accounts for a small, but constant, percentage of all carbon found in nature. Alterations of the normal ratio of 13C:12C amino acids in muscle protein allows for determination of the incorporation of the administered labeled amino acids into the various muscle proteins). A muscle biopsy was then acquired. The participants were fed a beverage containing 15 g whey protein that was enriched with tracer. A second biopsy was acquired 3 hrs after the beverage. The same protocol was followed without exercise (rested fasted and protein-fed states) and 24 hrs after exercise (post-exercise fasted and protein-fed states). Protein synthesis resulting from protein ingestion in the resting state was referred to as FED and protein synthesis resulting from protein ingestion at 24 h after exercise was referred to as EX-FED. Blood draws were acquired every 1 hr in the fasting state and every 30 min in the fed state.

Summary of research findings:
  • The rate of myofibrillar muscle protein synthesis was greater 24 hrs after the 30WM exercise than no exercise. However, muscle protein synthesis was fastest after 90FAIL and 30FAIL (p<0.05).
  • The rate of sarcoplasmic protein synthesis was greater 24 hrs after 90FAIL and 30FAIL, but not enough to reach statistical significance.
  • There was no significant change in the rate of total mixed muscle protein synthesis among the treatments.
  • Akt phosphorylation was significantly greater in the EX-FED vs. FED condition only for the 90FAIL exercise treatment.
  • mTOR phosphorylation was significantly greater in the EX-FED vs. FED condition only for the 30FAIL exercise treatment.
  • Phosphorylation of 4E-BP1, which indicates lessened inhibition of translation, and p70S6K phosphorylation (which would drive translation forward) were not significantly different among treatments. However, there were some trends toward increased 4E-BP1 phosphorylation for EX-FED vs. FED in the 90FAIL and 30FAIL treatments.

Interpretation of findings/Key practice applications

Muscle protein synthesis is activated even 24 hrs after resistance exercise. This highlights the importance of consuming quality protein not just post-workout, but habitually in order to achieve maximal efficiency of muscle growth. It was observed that only resistance training that went to failure (90FAIL and 30 FAIL) sustained the increase in protein synthesis after 24 hrs, implying that fiber-fatiguing training may be required for maximal durations of increased protein synthesis.

One key limitation of such studies is that only muscle protein synthesis was determined. However, muscle protein status is affected by both muscle protein synthesis and muscle protein breakdown. Without information on muscle protein breakdown, it is difficult to determine if the increases in muscle protein synthesis actually increase overall muscle protein balance.

NOTE: Figure 1 reports that the sample sizes were 9 for 90FAIL and 30WM and 10 for 30FAIL. Given that each subject did only 2 of the 3 treatments and that it was impossible for subjects to do only 30WM and 30FAIL (because doing 30WM implies that subject had to have already done 90FAIL), it is not clear how these sample size numbers were obtained.