Macro partitioning
In uncomplicated human disuse states (bed rest, immobilization), the dominant mechanism driving muscle atrophy is a significant reduction in muscle protein synthesis (MPS), not an increase in muscle protein breakdown (MPB).
If you are immobilized or inactive, your muscles shrink primarily because protein synthesis drops by about 50%, not because breakdown increases. To prevent this, you must actively stimulate muscle protein synthesis through low-volume resistance exercise or amino acid/protein intake, as the body's natural response to inactivity is to shut down muscle building.
Thus, in humans disuse induces not only a reduction in resting MPS of ~50% but also a reduction in the meal-induced rise in MPS of ~50%... Thus, if breakdown were a predominant, or even substantial, contributor to muscle atrophy during disuse then the loss of muscle mass would be far greater than what is observed during the period of greatest muscle loss. Thus, contrary to our opponent’s thesis, it appears when measured in humans with simple disuse there is minimal elevation in muscle protein breakdown.
Why this rating
Based on dynamic in vivo measurements in humans across multiple studies cited (Gibson, Ferrando, Glover, Wall), excluding disease states.
Source
CrossTalk proposal: The dominant mechanism causing disuse muscle atrophy is decreased protein synthesis
Stuart M. Phillips et al. · The Journal of Physiology · 2014
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