Abstract
Sarcopenia is of important clinical relevance for loss of independence in older adults. The prevalence of obesity in combination with sarcopenia (“sarcopenic-obesity”) is increasing at a rapid rate. However, whilst the development of sarcopenia is understood to be multi-factorial and harmful to health, the role of obesity from a protective and damaging perspective on skeletal muscle in aging, is poorly understood. Specifically, the presence of obesity in older age may be accompanied by a greater volume of skeletal muscle mass in weight-bearing muscles compared with lean older individuals, despite impaired physical function and resistance to anabolic stimuli. Collectively, these findings support a potential paradox in which obesity may protect skeletal muscle mass in older age. One explanation for these paradoxical findings may be that the anabolic response to weight-bearing activity could be greater in obese vs. lean older individuals due to a larger mechanical stimulus, compensating for the heightened muscle anabolic resistance. However, it is likely that there is a complex interplay between muscle, adipose, and external influences in the aging process that are ultimately harmful to health in the long-term. This narrative briefly explores some of the potential mechanisms regulating changes in skeletal muscle mass and function in aging combined with obesity and the interplay with sarcopenia, with a particular focus on muscle morphology and the regulation of muscle proteostasis. In addition, whilst highly complex, we attempt to provide an updated summary for the role of obesity from a protective and damaging perspective on muscle mass and function in older age. We conclude with a brief discussion on treatment of sarcopenia and obesity and a summary of future directions for this research field.
Highlights
The degenerative, generalized, and precipitous loss of skeletal muscle mass, quality and strength associated with aging is termed “sarcopenia” and is characterized by muscle loss of ∼0.5–1.0% per year [1]
Sarcopenia is associated with decreased mobility and impaired whole-body metabolic health, as Skeletal Muscle Function in Older Obese well as impaired function during locomotion, reduced resting energy expenditure, a reduction in non-structured free-living physical activity and increased fat mass [8, 9], factors that have been found to be present with obesity
We showed that the muscle protein synthesis (MPS) response to protein ingestion correlated with physical activity [85]
Summary
The degenerative, generalized, and precipitous loss of skeletal muscle mass, quality and strength associated with aging is termed “sarcopenia” and is characterized by muscle loss of ∼0.5–1.0% per year [1]. Whilst there is evidence to suggest that a relative reduction in type I fibers is a result of excess adiposity, it is possible that this may reflect an intrinsic defect predisposing individuals toward obesity [91] This is pertinent to note as, aside from any impact on functional outcomes, a relative reduction in type I muscle fibers is related to impaired metabolic health, increased LDL content, decreased insulin sensitivity and decreased arterial elasticity [90]. In further support of this notion, a recent study demonstrated an additive effect of obesity, induced by a high-fat diet on the reduction in contractile function with aged rodents, demonstrating that the increase in intramyocellular lipid levels were associated with the degree of impaired muscle contractile force [120, 121]
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