Abstract
Skeletal muscles comprise a substantial portion of whole body mass and are integral for locomotion and metabolic health. Increasing age is associated with declines in both muscle mass and function (e.g. strength‐related performance, power) with declines in muscle function quantitatively outweighing those in muscle volume. The mechanisms behind these declines are multi‐faceted involving both intrinsic age‐related metabolic dysregulation and environmental influences such as nutritional and physical activity. Ageing is associated with a degree of ‘anabolic resistance’ to these key environmental inputs, which likely accelerates the intrinsic processes driving ageing. On this basis, strategies to sensitize and/or promote anabolic responses to nutrition and physical activity are likely to be imperative in alleviating the progression and trajectory of sarcopenia. Both resistance‐ and aerobic‐type exercises are likely to confer functional and health benefits in older age, and a clutch of research suggests that enhancement of anabolic responsiveness to exercise and/or nutrition may be achieved by optimizing modifications of muscle‐loading paradigms (workload, volume, blood flow restriction) or nutritional support (e.g. essential amino acid/leucine) patterns. Nonetheless, more work is needed in which a more holistic view in ageing studies is taken into account. This should include improved characterization of older study recruits, that is physical activity/nutritional behaviours, to limit confounding variables influencing whether findings are attributable to age, or other environmental influences. Nonetheless, on balance, ageing is associated with declines in muscle mass and function and a partially related decline in aerobic capacity. There is also good evidence that metabolic flexibility is impaired in older age.
Highlights
Stable isotope tracers to quantify muscle protein turnoverMuscle mass is regulated via the maintenance of a dynamic equilibrium between MPS and MPB
In constituting ~40% of body weight, skeletal muscle is the largest organ in the body, one of the fundamental roles of skeletal muscle is to maintain skeletal structure and locomotion enabling completion of essential daily activities (Reid & Fielding 2012)
6.7 g of essential amino acids (EAAs) enriched with leucine enhanced MPS in a group of older adults compared with there being no increase in the absence of supplemental leucine (Katsanos et al 2006); there was a ~50% increase in MPS in older men who consumed a meal enriched with leucine (Rieu et al 2006)
Summary
Muscle mass is regulated via the maintenance of a dynamic equilibrium between MPS and MPB. Measurements of FSR are tissue specific and unaffected by blood flow perturbations (unlike A-V balance techniques), by stopping a steady-state tracer infusion, the measurement of the decay of the tracer enrichment from the arterial and intracellular pool over time can give a measurement of fractional breakdown rate (FBR; Zhang et al 1996, 2002) In another approach, using arterial–venous (A-V) balance kinetics, rates of tissue synthesis and breakdown can be determined by monitoring the rate of disappearance of the tracer from the arterial pool (as a proxy of synthesis), or the rate of appearance of the tracer into the venous pool (as a proxy of breakdown), assuming that the AA being studied is not subject to secondary metabolism within the tissue. For more information on this important technique and its applications to metabolic research, we direct the reader to the following seminal articles
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