Introduction

Around age 40, muscle mass begins to decline, potentially leading to sarcopenia, a condition associated with frailty and increased fall risk (1). This muscle loss has been attributed to “anabolic resistance”, defined as a reduced capacity to stimulate muscle protein synthesis (MPS) in response to essential amino acids (EAAs) or resistance exercise (REx), potentially due to impaired mTORC1 signalling (2-3). However, it remains unclear whether anabolic resistance and age-related myocellular changes, such as denervation and satellite cell loss, reflect intrinsic aging or are confounded by lifestyle factors, such as inactivity, adiposity, or medications.

Methods and materials       
We addressed these questions by recruiting healthy, non-smoking young (n=10, 18-35 years) and older (n=11, 65-75 years) men who were lean, physically active, and free of medications. Participants underwent an acute trial consisting of high-volume unilateral REx (10 sets of 10 reps) followed by ingestion of EAAs (240 mg per kg bw). To investigate age-related muscle decline, we applied several complementary approaches, including stable isotope tracer infusions, immunoblotting, immunofluorescence, single-fiber experiments, and phosphoproteomics.

Results

Rates of MPS were comparable across age groups in response to EAA intake, both alone and after exercise. Similarly, mTORC1 signalling was maintained or more pronounced in older compared with younger men. Notably, older men displayed higher levels of amino acid transporters, nutrient sensors, and mTORC1 pathway activators (p<0.05). From the same cohorts, analyses of cross-sections revealed that older men had a lower proportion of type II fibers, with smaller and misshaped type II fibers, as well as fewer satellite cells and capillaries surrounding these fibers (p<0.05). In addition, older men showed more denervated and “grouped” muscle fibers compared with young men (p<0.05). These findings prompted further investigation of anabolic responses in different fiber types. To enable such analyses, we first developed a new method (THRIFTY) for fiber typing individual fibers, which was valid and shown to be more time-efficient than reference methods. We then applied THRIFTY and examined cell signalling responses to EAA intake alone and in combination with REx in pooled type I and type II fibers. The anabolic signalling response was similar or even more pronounced in older compared to younger individuals, with a more robust response observed in type I than in type II fibers (p<0.05). Finally, phosphoproteomic analysis of muscle biopsies from the acute intervention revealed that aging was associated with global suppression of the growth-induced phosphoproteome, despite intact mTORC1 activation in older muscle.

Conclusion

Healthy, lean, physically active older men did not display deficits in MPS or mTORC1 signalling in response to anabolic stimuli, indicating that anabolic resistance is not an inevitable consequence of healthy aging. Reduced activation of the mTORC1 pathway or an impaired capacity to stimulate MPS is therefore unlikely to be driving muscle loss in this population. Instead, our findings suggest that other mechanisms, including denervation, satellite cell loss, and altered activation of signalling pathways beyond mTORC1, may contribute to age-related muscle decline.