Purpose of review: The intended purpose of nutritional and exercise interventions during ICU stay is often to limit the muscle loss associated with critical illness. Unfortunately, direct measurements of muscle protein turnover or potential surrogates have often been neglected in clinical trials.
Recent findings: We discuss the potential advantages and drawbacks of common outcome measures for assessing changes in muscle structure and function over time, and how temporal changes in patient physiology require consideration. There is an increasing awareness of emphasizing functional outcomes in recent clinical trials. We here summarize the latest research on therapies attempting to limit muscle loss in ICU patients, with a focus on muscle protein metabolism. No recent or older studies show any effect of nutritional interventions on muscle protein gain, although some smaller studies show a promising positive effect on muscle thickness and function. Some studies show a positive effect of increased physical activity in the ICU on muscle mass and function but heterogeneity of the interventions and outcome measures make any general conclusions impossible.
Summary: Several knowledge gaps remain regarding the importance of muscle protein regulation as a driver of improved physical function following ICU discharge. In our opinion, physiological investigations are needed to guide the design and interpretation of future clinical trials.