Physical activity and sedentary behavior: a review of longitudinal studies of weight and adiposity in youth.

AIM: To review the published prospective observational studies of the relationship of physical activity and sedentary behavior with the development of overweight and adiposity, with an emphasis on methodologic issues.
METHODS: Sample size, population studied, length of follow-up, assessment of exposure (physical activity, inactivity, or sedentary behavior), assessment of outcome (relative weight, overweight, % body fatness, adiposity), statistical approach, and main findings were extracted, summarized, and key methodological issues highlighted.
RESULTS: In total, 17 studies of physical activity and 15 studies of inactivity/sedentary behavior were identified; as these were not mutually exclusive, 20 unique studies were reviewed. Results were mixed, with most studies showing an inverse association of physical activity with weight or fatness outcomes and/or a direct association of inactivity/sedentary behavior with weight or fatness outcomes. The effects identified were generally of small magnitude. Imprecise measurement of activity exposures likely weakens the observed relationships. Most studies used a pre-post design and had limited duration of follow-up (< or = 2y). Studies with longer and more frequent follow-up did not always use the most advantageous statistical approach.
CONCLUSIONS: On balance, the available evidence from prospective observational studies suggests that increased physical activity and decreased sedentary behavior are protective against relative weight and fatness gains over childhood and adolescence. In addition to improved measurement methods, longer and more frequent follow-up as well as truly longitudinal analysis methods would help establish these important prevention and intervention targets, and identify subgroups or development periods where interventions would likely be effective.