Extracellular Water and Phase Angle, Markers of Heightened Inflammatory State, and Their Prognostic Potential for Body Composition Outcomes in Adults

Background/Aim: Extracellular-to-total body water ratio (ECW/TBW) and phase angle (PhA, PA) reflect hydration and cellular health, but their relationship with bone, muscle, and fat, as primary components of body composition, is not fully elucidated. This study aimed to evaluate sex-specific differences in body composition and assess the diagnostic potential of ECW/TBW and PhA for low bone/muscle mass and increased fat mass in generally healthy adults. Methods: This post hoc analysis utilized data from a multicenter, cross-sectional, Italian study (2010–2014) that included 18–90 year adults (n=9717). Body composition was measured by bioelectrical impedance (BIA-ACC, Bio-Tekna®), assessing bone, muscle mass, fat mass, ECW, TBW, and PhA. Low bone/muscle mass and adiposity were defined using standard cutoffs. Associations were examined using nonparametric tests and multiple regression analyses. Results: The mean age of men and women was similar (mean ~48 years). Men had significantly higher body mass index (BMI), intramuscular adipose tissue (IMAT%), T-score (bone), S-score (muscle), and PhA, while women had significantly higher fat mass (FM%) and ECW/TBW. ECW/TBW showed excellent discrimination for low muscle mass (AUC 0.845–0.922) and low bone mass (AUC 0.696–0.885), outperforming PhA. Neither ECW/TBW nor PhA reliably predicted increased fat mass. Regression models indicated ECW/TBW was strongly associated with age, sex, BMI, fat mass, and bone/muscle scores (R² = 0.943), whereas PhA’s association was moderate (R² = 0.368). Conclusions: ECW/TBW and PhA reflected sex-specific differences for body composition and effectively identified low muscle and bone mass (with better predictability of the former). Both showed a limited predictive ability for fat mass. Overall, both parameters provide complementary insights into sarcopenia and osteopenia.