BACKGROUND & AIMS: Muscle mass loss is a major contributor to morbidity and mortality in Intensive Care Unit (ICU) patients, but the role of metabolic state - particularly energy expenditure - in this process remains unclear. This study investigates the association between metabolic status and muscle mass loss in critically ill adults using indirect calorimetry and CT imaging assessed muscle quantification.

METHODS: In this observational study, adult ICU patients with at least two indirect calorimetry measurements and matched abdominal CT scans were included. Resting energy expenditure (REE) was measured by indirect calorimetry, and muscle mass was quantified as the cross-sectional area (CSA) of the posterior muscle group at the L3 vertebral level. Statistical analyses included regression modeling and group comparisons.

RESULTS: The observational study included 88 patients (n = 88), all of whom underwent at least two calorimetric measurements with corresponding CT scans, and 43 patients (n = 43) had at least three assessments. Persistently elevated normalized energy expenditure per kilogram of body weight (nREE) was independently associated with greater muscle loss. Patients classified as hypermetabolic by nREE experienced significantly more muscle wasting than those with lower metabolic activity. Hypermetabolism was associated with increased inflammatory markers, while sedation or agitation (RAAS) and higher level of consciousness (GCS) were not related to metabolic state.

CONCLUSIONS: Persistent hypermetabolism in ICU patients is independently associated with accelerated muscle mass loss. Early identification of hypermetabolic patients using indirect calorimetry may enable targeted nutritional interventions to reduce muscle mass wasting and improve clinical outcomes.