Elevated extracellular water to total body water ratio and low phase angle in relation to muscle function in middle-aged and older adults

Background: Decline in muscle function is a major health concern in aging populations, increasing the risk for disability and frailty. Bioelectrical impedance analysis (BIA) offers a practical method to assess physiological status in community settings. BIA-derived parameters include the phase angle (PhA), an indicator of cellular health and integrity, and the extracellular water to total body water (ECW/TBW) ratio, which reflects fluid balance and systemic inflammation. Objective: While these markers have been independently linked to adverse health outcomes, their combined utility for stratifying the risk of functional decline remains underexplored. This study aimed to investigate the joint association of PhA and ECW/TBW status with poor muscle function in middle-aged and older adults. Methods: This cross-sectional study included 695 community-dwelling adults aged ≥ 50 years (mean age 75.7 ± 8.9 years; 21.4% male; mean BMI 24.6 ± 3.6 kg/m²). PhA and ECW/TBW were measured using a BWA 2.0 Body Water Analyzer (InBody BWA, Inc. Audubon, PA, USA) in a seated position. Participants were categorized into three groups based on their PhA and ECW/TBW status: Normal PhA/Normal ECW/TBW (n = 258), Normal PhA/Elevated ECW/TBW (n = 323), and Lower PhA/Elevated ECW/TBW (n = 114). Logistic regression was used to assess the association with low physical function (Short Physical Performance Battery score ≤ 9) and low grip strength. Results: After adjustment for demographic and clinical confounders, the Lower PhA/Elevated ECW/TBW group showed significant associations with both low physical function (OR = 3.07, 95% CI = 1.63–5.81) and low grip strength (OR = 2.41, 95% CI = 1.20–4.85), as well as their co-occurrence (OR = 3.10, 95% CI = 1.53–6.27). No significant associations were found for the Normal PhA/Elevated ECW/TBW group after adjustment. Conclusion: The combination of a lower PhA and an elevated ECW/TBW ratio is significantly associated with poor muscle function. This combined BIA profile may serve as a useful, noninvasive screening tool for identifying individuals at high risk of functional decline in community settings.