Organized sport and physical activity participation and body mass index in children and youth: A longitudinal study.

The relationship between sport participation and BMI in children and adolescents is unclear, with some studies showing no association at all and others suggesting that sport is linked to lower BMI. Another possibility, however, is that this relationship is bidirectional, with sport leading to lower BMI but BMI also influencing sport participation. Here, we examine the direction of this association by analyzing a longitudinal dataset. Data come from the Physical Health Activity Study Team (PHAST) study, a prospective open cohort study including 2278 children at baseline, followed from 2004 to 2010. We fit 3 lagged mixed effects models: One examining the simultaneous relationship, one regressing past BMI on present sport participation, and one regressing sport participation on present BMI. Our baseline sample included 1999 children, of whom 50% were female. Mean BMI increased over the study period from 19.0 (SD = 3.7) to 21.2 (SD = 4.1), while organized sport participation declined. Model results showed that BMI and sport are weakly associated, and that each of these variables predicts the other, which generally supports a bidirectional relationship. Consistent with some previous reports, however, the effect size in both directions is very small. At the levels of participation in our sample, activity and BMI are very weakly related. Findings should not obscure the other benefits of physical activity.