Segmental validation of the 50-kHz phase angle as a valid, practical proxy that closely approximates the...

Although phase angle at 50 kHz (PhA50) is widely used, its validity relative to the theoretically correct characteristic-frequency PhA (PhAcf) remains unclear. This study evaluated the validity of the PhA50 by comparing it with the PhAcf and examining its associations with body composition, muscle strength, and physical function.

This cross-sectional study included 1266 community-dwelling older adults. Bioimpedance spectroscopy (BIS) was used to assess whole-body, thigh, and arm impedance parameters. PhAcf, PhA50, membrane capacitance (Cm), extracellular water (ECW), intracellular water (ICW), total body water (TBW), and ECW/ICW ratio were derived. Muscle strength (grip strength and knee extension) and physical function (Timed Up-and-Go test, 6-m walk, 5-times chair stand, and vertical jump) were assessed.

Characteristic frequency was positively associated with age, whereas PhA50 and PhAcf were negatively associated with age. PhA50 was strongly correlated with PhAcf and ECW/ICW (|r|>0.90), with Cm (|r|>0.70), and with ICW and TBW (|r|>0.40) across segments in both sexes. Both PhA50 and PhAcf were significantly associated with muscle strength and physical function, and no significant differences were observed in the correlation coefficients between the two PhA measures for muscle strength and physical function. Thigh PhA (both PhA50 and PhAcf) showed significantly stronger correlations with lower extremity muscle strength and performance than arm PhA.

The commonly used PhA50 closely approximates PhAcf and ECW/ICW and reflects Cm, ICW, and TBW. PhA50 could assess muscle strength and physical function with similarity to the theoretically optimal yet harder-to-measure PhAcf. These findings support PhA50 as a practical index for clinical nutrition research and potential healthcare screening.