Muscle full effect after oral protein: time-dependent concordance and discordance between human muscle protein synthesis and mTORC1 signaling.
Level 4 - case-series / case-control
Single-arm uncontrolled physiological time-course study in humans.
PubMed 20844073 · doi:10.3945/ajcn.2010.29819
What was done
Eight healthy postabsorptive young men (~21 years of age) underwent an 8.5-hour primed continuous infusion of [1,2-¹³C₂]leucine with intermittent quadriceps biopsies to measure myofibrillar protein synthesis (MPS) and anabolic signaling. After a 2.5-hour baseline period, participants consumed an oral bolus of 48 g whey protein.
What was found
At 45–90 minutes post-ingestion, mean (± SEM) myofibrillar protein synthesis increased from 0.03 ± 0.003%/h to 0.10 ± 0.01%/h, but returned to baseline thereafter despite plasma essential amino acids remaining elevated (+130% at 120 min, +80% at 180 min). PKB activity and eIF4G phosphorylation preceded the rise in MPS, while S6K1 and 4EBP1 phosphorylation matched MPS up to 90 minutes. Beyond 90 minutes, MPS declined while mTORC1 signaling and plasma EAAs remained elevated. Phosphorylation of eIF2α increased only at 180 minutes.
Why it matters
This study demonstrates the "muscle full" phenomenon with whole-protein ingestion, showing that muscle protein synthesis stimulation is refractory and disconnects from sustained systemic amino acid availability and mTORC1 signaling after 90 minutes.
Limits
The study had a very small sample size (n = 8) consisting exclusively of young healthy men, used an uncontrolled single-arm design without a non-protein comparator, tested only a single large dose of whey protein, and was restricted to an acute 8.5-hour laboratory window.
Cited by
- contradicts Ingesting 20 grams of protein induces an increase in muscle protein synthesis that peaks around 3 hours post-meal and wears off in approximately 5 to 6 hours.