Changes in physical activity explain paradoxical relationship between baseline physical activity and adiposity changes in adolescent girls: the FLVS II study.

OBJECTIVE: To investigate in adolescents relationships between habitual physical activity (PA) and changes in several indicators of adiposity.
DESIGN: Longitudinal population-based study. The Fleurbaix-Laventie Ville Sante II (FLVS II) study in Northern France.
SUBJECTS: A total of 222 boys and 214 girls aged 8-18 y, not obese at inclusion. MEASUREMENTS: PA (Modifiable Activity Questionnaire) and adiposity (body mass index (kg/m2), percent body fat by bioimpedance analysis, sum of four skinfolds, waist circumference) were assessed in 1999 and 2001. Adiposity indicators adjusted for age and pubertal stage were compared between groups of baseline PA and between groups of PA changes, separately by sex.
RESULTS: At baseline, PA was not associated with adiposity indicators in both sexes. In girls only, being in the group with the highest age-relative level of moderate PA at baseline predicted the highest adiposity gain from baseline to follow-up, for all indicators. After adjustment for baseline values, all adiposity indicators were higher at follow-up in girls who had decreased their relative level of moderate PA during follow-up. In boys, changes in adiposity during follow-up were not significantly different across groups of PA changes. However, the sum of skinfolds at follow-up, adjusted for baseline value, tended to be higher in those who decreased their vigorous PA level and lower in those who increased it.
CONCLUSION: In adolescent girls, a decrease in PA rather than inactivity per se may result in increased adiposity over time.