# Indirect estimates of cellular hydration and relative water content and their associations with muscle strength and physical function in older adults: a path analysis from the Pro-Eva study

## TL;DR

This study explores how hydration levels in older adults relate to muscle strength and physical function using bioelectrical impedance analysis.

## Contribution

The study introduces hydration-related BIA estimates (ICW and TBW/BW%) as indirect indicators of muscle function in older adults.

## Key findings

- Higher intracellular water (ICW) was positively associated with handgrip strength and calf circumference.
- TBW/BW% showed smaller but significant associations with physical performance scores.
- Education and hypertension were also significant predictors of physical function outcomes.

## Abstract

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.

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.

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).

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.