Association of bioimpedance phase angle with aerobic work capacity independent of muscle mass and function in 4,540 adults aged 20 to 91 years

OBJECTIVES: Phase angle (PhA), derived from bioelectrical impedance analysis, has been associated with aerobic fitness, but whether it is independently associated with aerobic work capacity after accounting for body composition and muscle characteristics remains unclear. We examined this association across a broad adult age range and whether it differed by sex or age range. DESIGN: Cross-sectional study of 4,540 adults aged 20-91 years (2,052 men and 2,488 women) participating in a paid fitness assessment program. MEASUREMENTS: Aerobic work capacity was assessed using a submaximal incremental cycle ergometer test as physical work capacity at 75% of the age-predicted maximal heart rate (PWC75%HRmax), expressed as absolute and body weight-normalized values. Multivariable regression models included PhA, body composition, muscle mass, muscle function, and disease-related variables. PhA-by-sex and PhA-by-age-range interactions were additionally examined. RESULTS: PhA was positively associated with both aerobic work capacity outcomes in BMI- and body fat percentage-based models (standardized β=0.046-0.157; all p<0.001), and these associations persisted after adjustment for disease-related variables and in fully specified sensitivity analyses. For absolute aerobic work capacity, significant PhA-by-sex interactions were observed in both model specifications (both p for interaction < 0.001); PhA was positively associated with aerobic work capacity in men but not in women. No significant sex interaction was observed for body weight-normalized aerobic work capacity at the prespecified significance threshold. PhA-by-age-range interactions were not significant for either outcome. CONCLUSION: PhA was independently associated with aerobic work capacity after accounting for body composition, muscle mass, muscle function, and disease-related variables. The association with absolute aerobic work capacity differed by sex, whereas no clear effect modification by age range was observed.