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This study investigates the effects of two weeks of reduced daily ambulation and a subsequent return to normal physical activity on metabolic health of older adults.
The over-65 population is not only increasing at an alarming rate, but because six out of 10 will be managing more than one chronic condition by 2030, they will make up a much greater proportion of hospitalizations than ever before. Hospitalizations for disease, injury, and/or surgery in this group are likely to impair physical mobility and, therefore, the older adults capacity to be physically active both during hospitalization and beyond. The resulting sedentary lifestyle is likely to be accepted as the "new normal", ultimately increasing the risk of skeletal muscle and metabolic dysfunction (e.g. impaired glucose disposal, insulin resistance). These devastating outcomes are neither inevitable nor necessary if prevented with an appropriate mechanism-based intervention. A novel mechanism that may contribute to physical inactivity-induced insulin resistance is accumulation of inflammation and ceramide within skeletal muscle. Of interest, increased skeletal muscle inflammation and ceramide has been tied to various metabolic disturbances such as diabetes and insulin resistance. However, it is currently unknown if skeletal muscle inflammation and ceramide are a key mechanism associated with insulin resistance due to physical inactivity in older adults.
As the loss of muscle mass with aging is associated with a dysregulation of muscle protein synthesis (MPS) that is generally characterized by an 'anabolic resistance' to nutrients, the primary outcome of the trial is to investigate the effect of a mixed macronutrient beverage on post-prandial myofibrillar MPS. However, the 'anabolic resistance' of aging may be predominantly influenced by the contractile activity of skeletal muscle; for example, the investigators have demonstrated that 2 weeks of reduced habitual activity (i.e. daily step count) induces anabolic resistance in otherwise healthy older adults. In contrast, resistance exercise is well-known to enhance muscle mass and can increase the sensitivity of skeletal muscle to dietary amino acids. Therefore, all participants will undergo a 2-week reduced step count intervention combined with a unilateral leg resistance exercise protocol to induce an 'unhealthy' or 'anabolically resistant' leg and a relatively 'healthy' or 'anabolically sensitive' leg, respectively; this within subject model will be utilized to investigate the effect of the mixed macronutrient beverage in both relatively 'healthy' and 'unhealthy' older muscle. The investigators will also measure changes in muscle mass, muscle function, and cardiovascular health of both legs before and after the 2-week intervention to determine the effect of reduced activity and the protective effect of minimal, low-intensity exercise on these outcomes.