Domain-Specific Associations of Physical Activity With Phase Angle and Extracellular Water–to–Total Body Water Ratio in Middle-Aged or Older Adults: Cross-Sectional Study

Background: The phase angle (PhA) and extracellular water (ECW)-to-total body water (TBW) ratio serve as indicators of cell membrane integrity and body fluid balance in older adults. Objective: This study examined the domain-specific associations of physical activity (PA) with PhA and the ECW-to-TBW ratio. Methods: A nationally representative sample of 5211 adults aged 50 years and older was analyzed. Weekly amounts of moderate-to-vigorous PA (MVPA) in leisure-time, occupational, and transport-related domains were assessed using the Global Physical Activity Questionnaire. A bioelectrical impedance analyzer was used to measure PhA (°) and the ECW-to-TBW ratio (%). Linear regression analyses were conducted. Results: For leisure-time PA, compared with 0 min/week, participating in 150 to 299 min/week and ≥300 min/week of MVPA was associated with 0.08 (95% CI 0.02-0.14) and 0.13 (95% CI 0.06-0.20) degrees higher PhA, respectively. Additionally, compared with 0 min/week, engaging in 150 to 299 min/week and ≥300 min/week of MVPA was associated with -0.11 (95% CI -0.19 to -0.03) and -0.15 (95% CI -0.22 to -0.07) percentage points lower ECW-to-TBW ratio, respectively. For transport-related PA, compared with 0 min/week, engaging in ≥300 min/week of MVPA was negatively associated with the ECW-to-TBW ratio (β=-0.07, 95% CI -0.14 to -0.01). Occupational PA showed no clear associations with PhA or the ECW-to-TBW ratio. Conclusions: While the observed associations were modest, the associations of PA with both PhA and the ECW-to-TBW ratio differed across PA domains. Specifically, leisure-time PA was positively associated with PhA and inversely associated with the ECW-to-TBW ratio. These findings highlight the potential role of leisure-time PA in improving bioelectrical markers of cellular health and fluid distribution.