Muscle Strength and Functional Performance as Predictors of Metabolic and Body Composition Improvement in Anorexia Nervosa

<b>Background/Objectives</b>: Anorexia nervosa (AN) is a severe psychiatric disorder characterized by energy restriction and associated with profound metabolic and body composition alterations. The loss of Body Cell Mass (BCM) leads to impaired muscle strength and functional capacity. Traditional monitoring based on body mass index (BMI, kg/m²) and weight primarily captures quantitative recovery, failing to reflect early qualitative metabolic and functional restoration. This study evaluated the longitudinal associations between improvements in physical performance and metabolic and structural recovery during intensive rehabilitation. <b>Methods:</b> A prospective longitudinal study was conducted on 21 AN patients undergoing a four-month intensive nutritional and functional rehabilitation program at Villa Miralago. Anthropometry, BIVA-derived parameters, indirect calorimetry and physical performance tests were assessed at baseline (T0), 2 months (T1) and 4 months (T2). Longitudinal associations were explored using Generalized Estimating Equations (GEE). <b>Results:</b> Improvements in functional performance (KPI-SUSS) were significantly associated with metabolic recovery, with a positive association with resting metabolic rate (β = +0.3635; <i>p</i> = 0.0088), indicating early metabolic reactivation before full structural reconstruction. Grip strength (KPI-grip) was significantly associated with cellular integrity (Xc β = +0.4129; <i>p</i> = 0.0088) and with favorable fluid redistribution (↑ intracellular water percentage (ICWp), ↓ extracellular water percentage (ECWp)). Structural recovery markers (Fat-Free Mass (FFM, kg), Fat-Free Mass Index (FFMI, kg/m²), BCM) increased significantly over time, confirming a time-dependent restoration of metabolically active mass. Fat mass (FM, kg) and fat mass percentage (FMp) were positively associated with functional improvement, although this effect attenuated longitudinally. These findings support strength performance as a sensitive functional indicator longitudinally associated with qualitative metabolic and cellular recovery. <b>Conclusions:</b> Muscle strength and functional performance tests are simple, non-invasive and cost-effective tools longitudinally associated with metabolic and structural normalization in AN. Their integration into clinical practice may enable the monitoring of meaningful recovery, personalized rehabilitation strategies and improving long-term outcomes.