Can Supplemental Leucine Offset Disuse-induced Muscle Atrophy?

Ageing is associated with a gradual decline in muscle mass that is detrimental to both physical function and metabolic health, increasing the risk of morbidity and mortality. The loss of protein muscle mass with ageing is poorly understood, but it may partly relate to inactivity/disuse (i.e. during injury or hospitalization). Periods of inactivity/disuse blunt the ability of muscle to grow (termed anabolic blunting), leading to a loss of muscle mass and strength. An accumulation of these periods over a lifetime promotes the devastating loss of muscle protein mass and strength seen with ageing. Disuse-induced muscle loss is underpinned by a blunted muscle anabolic response to protein nutrition. Supplementing the diet with the amino acid leucine may offer a potential solution to alleviate muscle mass and strength loss during disuse. In fact, leucine is suggested to promote muscle protein growth and reduce muscle protein loss during disuse in rats, but this is yet to be shown in humans. Accordingly, the proposed study will investigate whether leucine supplementation can offset muscle and strength loss during short-term disuse. Twenty-four healthy (non-obese, non-diabetic, non-smokers) men aged 18-35 years will initially complete a lower-limb strength assessment and undergo a body composition scan three days later. The following morning, participants will be randomly assigned to ingest either 5g of leucine (n=12) or a caloric-matched placebo (n=12) with each meal over a 7 d period of a single-leg immobilisation. Immediately following immobilisation participants will undergo another body composition scan. Additionally, a stable isotope infusion will be combined with serial muscle biopsies from the thigh of each leg to determine the measure rates of muscle protein synthesis in the fasted state and in the 'early' and 'late' phase of feeding. A day later, the assessment of muscle strength will be repeated.