BACKGROUND: Short-term (<1 wk) muscle disuse lowers daily myofibrillar protein synthesis (MyoPS) rates resulting in muscle mass loss. The understanding of how daily dietary protein intake influences such muscle deconditioning requires further investigation.
OBJECTIVES: To assess the influence of graded dietary protein intakes on daily MyoPS rates and the loss of muscle mass during 3 d of disuse.
METHODS: Thirty-three healthy young men (aged 22 ± 1 y; BMI = 23 ± 1 kg/m2) initially consumed the same standardized diet for 5 d, providing 1.6 g protein/kg body mass/d. Thereafter, participants underwent a 3-d period of unilateral leg immobilization during which they were randomly assigned to 1 of 3 eucaloric diets containing relatively high, low, or no protein (HIGH: 1.6, LOW: 0.5, NO: 0.15 g protein/kg/d; n = 11 per group). One day prior to immobilization participants ingested 400 mL deuterated water (D2O) with 50-mL doses consumed daily thereafter. Prior to and immediately after immobilization upper leg bilateral MRI scans and vastus lateralis muscle biopsies were performed to measure quadriceps muscle volume and daily MyoPS rates, respectively.
RESULTS: Quadriceps muscle volume of the control legs remained unchanged throughout the experiment (P > 0.05). Immobilization led to 2.3 ± 0.4%, 2.7 ± 0.2%, and 2.0 ± 0.4% decreases in quadriceps muscle volume (P < 0.05) of the immobilized leg in the HIGH, LOW, and NO groups (P < 0.05), respectively, with no significant differences between groups (P > 0.05). D2O ingestion resulted in comparable plasma free [2H]-alanine enrichments during immobilization (∼2.5 mole percentage excess) across groups (P > 0.05). Daily MyoPS rates during immobilization were 30 ± 2% (HIGH), 26 ± 3% (LOW), and 27 ± 2% (NO) lower in the immobilized compared with the control leg, with no significant differences between groups (P > 0.05).
CONCLUSIONS: Three days of muscle disuse induces considerable declines in muscle mass and daily MyoPS rates. However, daily protein intake does not modulate any of these muscle deconditioning responses.Clinical trial registry number: NCT03797781.