Enhanced amino acid sensitivity of myofibrillar protein synthesis persists for up to 24 h after resistance exercise in young men.
Level 2 - randomized trial
Controlled within-subject physiological intervention trial in humans
PubMed 21289204 · doi:10.3945/jn.110.135038
What was done
Fifteen young men (age 21 ± 1 years, BMI 24.1 ± 0.8 kg/m²) underwent primed, constant infusions of L-[ring-¹³C₆]phenylalanine to measure muscle protein synthesis after consuming 15 g of whey protein at rest (FED) and 24 h after unilateral leg resistance exercise (EX-FED). Participants completed three unilateral leg exercise conditions: 4 sets at 90% of maximal strength to failure (90FAIL), 30% of maximal strength work-matched to 90FAIL (30WM), or 30% of maximal strength to failure (30FAIL). Mixed muscle, sarcoplasmic, and myofibrillar protein fractional synthetic rates as well as Akt and mTOR phosphorylation were evaluated.
What was found
Mixed muscle and sarcoplasmic protein synthesis rates were similarly stimulated in both FED and EX-FED states across all conditions. Whey protein ingestion increased myofibrillar protein synthesis above fasting by 0.016 ± 0.002%/h at rest. At 24 h post-exercise, this feeding response was significantly enhanced only in failure conditions, increasing by 0.038 ± 0.012%/h in 90FAIL and by 0.041 ± 0.010%/h in 30FAIL (not in 30WM). Akt Ser473 phosphorylation was higher at EX-FED than FED only in 90FAIL, whereas mTOR Ser2448 phosphorylation was significantly elevated at EX-FED above FED only in 30FAIL.
Why it matters
This study shows that resistance exercise extends the muscle-sensitizing effect to dietary amino acids for at least 24 hours, but this sustained enhancement requires lifting sets to failure rather than matching total external work volume.
Limits
The study was conducted in a small cohort (n = 15) of young, healthy males, limiting generalizability to females, older adults, or clinical populations. Only acute isotopic tracer endpoints and intracellular signaling events were measured rather than long-term muscle hypertrophy or functional strength changes.
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