Indirect estimates of cellular hydration and relative water content and their associations with muscle strength and...

AIM: This study aimed to examine the associations between clinical characteristics, body composition parameters, and functional outcomes in older adults, with a specific focus on bioelectrical impedance analysis (BIA)-derived estimates related to hydration status—intracellular water (ICW) and the total body water-to-weight ratio (TBW/BW%). Outcomes included handgrip strength (HGS), performance in the Short Physical Performance Battery (SPPB), and calf circumference (CC), used here as an indirect indicator of muscle mass and lower-limb function. ICW and TBW/BW% were selected instead of extracellular water (ECW) or the ICW/ECW ratio to minimize collinearity and emphasize hydration-related estimates with physiological relevance to muscle function in older adults. METHODS: A cross-sectional study was conducted with 1,009 community-dwelling older adults (≥ 60 years) from Parnamirim, RN, Brazil. Clinical variables (sex, diabetes, hypertension, polypharmacy, and education ≥ 4 years), anthropometry (BMI), and hydration-related parameters estimated via single-frequency BIA (Biodynamics, model 450) were assessed. Structural equation modeling with path analysis estimated the standardized direct associations (β) of these variables with HGS, CC, and SPPB scores. RESULTS: For HGS, female sex, ICW, and TBW/BW% were significant correlates (β = –6.66, β = 0.79, and β = 0.03, respectively). In the CC model, ICW, BMI, and diabetes were significant predictors (β = 0.26, β = 0.37, and β = –0.52, respectively). For SPPB, higher ICW, TBW/BW%, and education (≥ 4 years) were positively associated (β = 0.14, β = 0.07, and β = 0.44, respectively), while hypertension (β = –0.29) and polypharmacy (β = –0.69) showed negative associations. Model explained variances (R2) were 51.0% (HGS), 42.1% (CC), and 14.2% (SPPB). CONCLUSION: ICW, as an indirect hydration-related estimate derived from BIA, was consistently associated with muscle strength and functional measures. TBW/BW% demonstrated smaller but significant associations with global physical performance. These findings indicate that hydration-related parameters obtained from BIA may contribute to identifying functional vulnerability in older adults, although they should not be interpreted as direct markers of cellular integrity or muscle tissue properties.