Serum Uric Acid and Phase Angle in Middle-Aged and Older Korean Adults: Spline-Based Associations and Implications for Muscle Health in KNHANES 2022–2024

Background/Objectives: Serum uric acid, which has endogenous antioxidant properties, has shown inconsistent associations with muscle health. Bioelectrical impedance-derived phase angle reflects cell membrane integrity and muscle quality, but its relationship with uric acid remains insufficiently characterized in nationally representative populations. Methods: This cross-sectional study used Korea National Health and Nutrition Examination Survey (KNHANES) 2022–2024 data from Korean adults aged 55 years or older (N = 7201; 3139 men and 4062 women). Survey-weighted regression models with natural spline terms evaluated sex-stratified associations between serum uric acid and phase angle, adjusting for age, BMI, renal function, smoking, alcohol consumption, diabetes, and hypertension. Results: Significant spline-based associations between uric acid and phase angle were observed in both men (p = 0.0028) and women (p = 0.00002) in the fully adjusted model and remained robust across multiple sensitivity analyses. The uric acid × sex interaction was not significant (p = 0.499). Conclusions: Serum uric acid was significantly associated with phase angle in spline-based analyses, and the relationship may not be adequately summarized by a single linear coefficient. These findings suggest that both low and high uric acid levels should be interpreted cautiously and in conjunction with other clinical and metabolic indicators when interpreting phase angle as a BIA-derived physiological marker.