Non‐steady‐state modelling reveals reduced skeletal muscle protein synthesis as the primary driver of disuse‐atrophy in young, healthy females

Skeletal muscle mass reflects the balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). In healthy humans, a decline in MPS is recognized as the primary driver of muscle disuse‐atrophy, with limited contribution from increased MPB. However, disuse‐induced declines in protein pool size directly influence the calculation of muscle protein turnover. We examined how accounting for changes in protein pool size impacted MPS and MPB rates assessed using deuterium oxide in healthy humans. Fourteen females (21 ± 2 years of age; 24.5 ± 3.0 kg/m 2 ) had one leg subjected to 10 days of unloading, whilst the contralateral leg underwent loading in the form of resistance exercise. Steady‐state and non‐steady‐state modelling approaches were used to calculate MPS and MPB rates. In response to unloading, there was a significant decline in MRI‐derived vastus lateralis volume, and thus non‐steady‐state equations were applied exclusively to this limb. Regardless of whether steady‐state or non‐steady‐state approaches were applied, mixed MPS rates were significantly reduced in response to unloading, whilst mixed MPB rates remained unchanged. Notably, unloading elicited protein fraction‐specific responses, whereby myofibrillar MPS rates declined significantly whilst sarcoplasmic MPS rates were unaltered. We also found that in response to loading, mixed, myofibrillar and sarcoplasmic MPS rates remained unchanged compared with basal rates. Our findings show that unilateral unloading in females induces muscle disuse‐atrophy driven by reductions in MPS rates, rather than by elevations in MPB rates, a response that was protein fraction specific and preserved irrespective of whether steady‐state or non‐steady‐state equations were used. image Key points In humans, short‐term muscle disuse‐atrophy is generally attributed to reduced muscle protein synthesis (MPS) rates, but integrated tracer approaches to measure protein turnover can be influenced by changes in protein pool size. We used deuterated water ( 2 H 2 O) to measure rates of MPS and muscle protein breakdown (MPB) during 10 days of unilateral unloading in young, healthy females. Steady‐state and non‐steady‐state equations were applied to account for changes in the protein pool size that occur with disuse‐atrophy. Unloading reduced muscle size and MPS rates, whereas MPB rates were unchanged. Accounting for the change in the protein pool size using non‐steady‐state equations did not alter this conclusion. These findings refine our interpretation of the mechanisms driving muscle disuse‐atrophy in healthy humans that might differ from more severe clinical or preclinical scenarios of muscle wasting.