Novel Hypothesis on Phase Angle as a Candidate Marker Associated with Physiological Reserve in Geriatric Patients in Long-Term Care

Background: Physiological reserve reflects the capacity of the organism to maintain homeostasis under biological stress. We suggest the hypothesis that phase angle derived from bioelectrical impedance analysis could be a potential objective marker of Physiological reserve. Methods: An analytical cross-sectional study was conducted in 45 older adults residing in long-term care units at Hospital Español de México (2025). Frailty was assessed with the FRAIL index, sarcopenia using EWGSOP2 criteria, functionality with the Barthel Index, and quality of life with WHOQOL-OLD. Body composition and phase angle were measured by multifrequency bioelectrical impedance (InBody BWA 2.0®). Associations were analyzed using ROC curves and regression models. Results: Phase angle (PA) was closely associated with key geriatric domains. Frail participants showed significantly lower PA values (3.1 ± 0.7 vs. 3.8 ± 0.9; p = 0.009), while sarcopenic patients presented lower PA (3.1 ± 0.7 vs. 4.1 ± 0.9; p < 0.001). PA demonstrated a strong positive correlation with functional status (r = 0.668; p < 0.001). A moderate positive correlation was observed with quality of life (r = 0.551; p = 0.001). In an exploratory multivariable analysis, PA was independently associated with age (B = −0.063; p < 0.001), functionality (B = 0.013; p < 0.001), and frailty (B = −0.137; p = 0.044). Conclusions: The phase angle is associated with key geriatric conditions that are related to reduced physiological reserves in older adults in long-term care, addressing the hypothesis that phase angle reflects physiological reserve at the cellular level by providing preliminary cross-sectional evidence.