Differences In Body Composition Changes Based On Physical Activity Level In Adolescents: 20-week Longitudinal Study

INTRODUCTION: Nearly half of Mexican adolescents do not meet WHO recommendations for physical activity (PA), a trend that has remained steady for six years. This lack of adequate PA is associated with adverse body composition changes. However, there is limited longitudinal data on how PA influences body composition changes in adolescents over time within the Mexican population. PURPOSE: To compare body composition changes, specifically fat and lean mass, between physically active and inactive Mexican adolescents, assessing the impact of PA longitudinally. METHODS: Fifty-three adolescents (32 girls, 21 boys) were classified as active (n = 34) or inactive (n = 19) based on the International Physical Activity Questionnaire (IPAQ). Body composition, including fat and lean mass, was measured using the InBody 120 bioimpedance scale at the beginning and end of a 20-week period. Paired t-tests and Wilcoxon tests were used to assess changes over time in JAMOVI 2.4.8, with significance set at p < .05. RESULTS: Among active girls, lean mass decreased significantly (M = 23.9 kg, SD = 5.06 to M = 23.6 kg, SD = 4.98), t(44) = 1.9, p = .042, d = −.39d = −.39, 95% CI [-.84, .07], with no significant change in fat mass (p = .269). Active boys gained fat mass (M = 16.6 kg, SD = 7.69 to M = 18.7 kg, SD = 8.46), t(13) = 3.04, p = .009, d = .82, 95% CI [0.19, 1.41]. Inactive girls showed significant fat mass increases (M = 16.6 kg, SD = 5.79 to M = 18.9 kg, SD = 5.30), t(11) = 4.21, p = .001, d = 1.22, 95% CI [0.45, 1.96], with no change in lean mass (p = .611). Inactive boys experienced no significant changes in fat or lean mass (p = .793; p = .904). CONCLUSION: Findings suggest that PA level impacts body composition in Mexican adolescents. Active boys showed increases in fat mass, while inactive girls gained fat with no change in lean mass. These results highlight the potential benefit of accessible PA programs to help adolescents maintain lean mass and limit fat gain.