Longitudinal changes in phase angle and its association with body composition in adolescent athletes.

BACKGROUND: Phase angle (PhA) has been recognized as a marker of cellular integrity and muscle quality. Although cross-sectional evidence suggests associations between PhA and body composition components, longitudinal data in adolescent athletes remain limited. OBJECTIVE: To analyze longitudinal changes in PhA over a 6-month period and to investigate its associations with lean soft tissue and body fat percentage in adolescent athletes. METHODS: This longitudinal study included 47 adolescent athletes (40.4 percent female; median age 15.2 years at baseline) followed over 6 months. Phase angle (PhA) was assessed via bioelectrical impedance analysis, while lean soft tissue and body fat percentage were measured using dual-energy X-ray absorptiometry. Generalized linear mixed-effects models (GLMMs) were used to examine longitudinal changes in PhA over time and subsequently to investigate the longitudinal association of PhA with lean soft tissue and body fat percentage. RESULTS: Adjusted models showed a 1.97% rise in PhA over six months (p = 0.48), but the effect lost significance upon adjusting for body composition (p > 0.05). Regarding longitudinal associations, GLMMs demonstrated that PhA was directly and positively associated with lean soft tissue (β = 1.638; 95% CI: 0.277 to 2.999; p = 0.021), whereas no significant association was observed with body fat percentage [exp(β) = 0.981; 95% CI: 0.947 to 1.016; p = 0.284]. Complementary analyses using fat mass instead of body fat percentage yielded similar non-significant results. CONCLUSIONS: In adolescent athletes, changes in PhA appear to be concurrently linked to changes in body composition. In addition, these findings indicate that PhA is a robust marker of lean soft tissue in athletes.