Muscle Protein Synthesis

Sarcopenia describes the progressive decline of muscle mass and strength with advancing age and is associated with increased frailty and morbidity, however we do not currently have an effective treatment. Protein feeding and exercise is known to increase muscle mass, but aged muscle shows a lower response to these stimuli leading to muscle loss over time. We do know that ingesting leucine, a building block of protein, can overcome this reduced response to protein feeding and exercise leading to increased muscle mass in older people. However, we do not understand when the optimum time to ingest leucine is to maximise muscle mass after exercise in older people. In this study we will examine the effect of feeding leucine after exercise either with a meal or between meals.

In the present semi-crossover randomized design, 15 recreationally active young adults (19-40 y) will receive stable isotope tracer infusions and perform a single bout of resistance exercise. Immediately after exercise, participants will ingest either 1 medium russet potato with skins and 2 ounce-equivalent lean ground beef combination (274 kcals, 19.3 g protein, 5 g fat, 37 g carbohydrates, and 4 g fiber), 2 slices of white bread and beef combination (262 kcals, 19.8 g protein, 6.8 g fat, 28 g carbohydrates, and 1.8 g fiber) or 2 ounce-equivalent beef (93% lean meat/7% fat; 110 kcals, 14.8 g protein, and 5 g fat). Repeated blood and muscle biopsies will be collected to determine whole body leucine kinetics, amino acid concentrations, anabolic signaling and myofibrillar protein synthesis rates during the trials.

The goal of this intervention trial is to characterize skeletal muscle atrophy in healthy, young adults during short term bedrest. The main questions it aims to answer are: How much do skeletal muscle volume, strength, and fatigue resistance decline during bedrest? How much does whole-body insulin sensitivity change during bedrest? How do mitochondrial function and protein synthesis change during bedrest? Participants will undergo the following tests before and after a free-living control period and before and after a 5 day period of strict horizontal bedrest: * Magnetic resonance imaging of the thigh muscles * Strength testing of the thigh muscles * Insulin sensitivity testing in response to a mixed meal * Exogenous glucose oxidation in response to a mixed meal * Muscle biopsies from the thigh muscles * Blood samples

Muscle tissue consists of proteins. These proteins are built up of a collection of smaller building blocks: amino acids. When protein is consumed, it gets digested and absorbed into the blood. The body can use these amino acids, by taking them up from thecirculation. By consuming sufficient protein through our diet, we ensure that the body is provided with enough amino acids to enable muscle protein building. Sufficient muscle protein synthesis is important for maintaining muscle function and strength. Previous research has shown that when 20 to 25g of protein is eaten, muscle protein synthesis is maximized. It is therefore recommended to eat 20g of protein per meal. However, it is currently unclear what happens to muscle protein synthesis rates if multiple meals are eaten. When multiple meals are consumed, amino acids appear in the circulation for prolonged period of time. Theoretically, when there are a high amino acid concentrations in the blood, muscle protein synthesis rates will increase. Contrary to this theory, a study more than 20 years ago showed otherwise. It was observed that muscle protein synthesis rates are only elevated for2 hours afterwhich they decrease again. This phenomenon was referred to as the "muscle-full" effect. Because this phenomenon is in contrast with more previous studies, the objective is to replicate that study. This is important so that nutritional advice for healthy, but also clinical populations in the future can be improved.