title: Activity, inactivity, and screen time in relation to weight and fatness over adolescence in girls.
abstract: OBJECTIVE
  The impact of activity and inactivity on relative weight and fatness change are best evaluated longitudinally. We examined the longitudinal relationship of physical activity, inactivity, and screen time with relative weight status and percentage body fat (%BF) and explored how it differed by parental overweight status.
  
  
  METHODS
  Non-obese pre-menarcheal girls (173), 8 to 12 years old, were followed until 4 years post-menarche. %BF, BMI z-score, and time spent sleeping, sitting, standing, walking, and in vigorous activity were assessed annually. We developed a physical activity index to reflect time and intensity of activity. Inactivity was defined as the sum of time spent sleeping, sitting, and standing. Screen time was defined as time spent viewing television, videotapes, or playing video games. Parental overweight was defined as at least one parent with BMI>25.
  
  
  RESULTS
  In separate linear mixed effects models, activity, inactivity, and screen time were unrelated to BMI z-score longitudinally, with and without accounting for parental overweight. After controlling for parental overweight, activity was inversely related (p<0.001), and inactivity was directly related (p<0.035) to increased %BF longitudinally. Screen time was unrelated to %BF change. With stratification for parental overweight, effects of activity and inactivity on %BF were observed only among girls with at least one overweight parent.
  
  
  CONCLUSIONS
  In this cohort of initially non-overweight girls, activity and inactivity were related to accrual of BF over adolescence, particularly among children with at least one overweight parent. These results suggest that girls with a family history of overweight represent a target population of high priority for interventions around physical activity and inactivity.
authors.affiliation: Department of Public Health and Family Medicine, Tufts University School of Medicine, 136 Harrison Avenue, Boston, Massachusetts 02111, USA. aviva.must@tufts.edu
authors.forename: Aviva
authors.initials: A
authors.lastname: Must
authors.forename: Linda G
authors.initials: LG
authors.lastname: Bandini
authors.forename: David J
authors.initials: DJ
authors.lastname: Tybor
authors.forename: Sarah M
authors.initials: SM
authors.lastname: Phillips
authors.forename: Elena N
authors.initials: EN
authors.lastname: Naumova
authors.forename: William H
authors.initials: WH
authors.lastname: Dietz
mesh_terms: D000293:Adolescent; D000294:Adolescent Behavior; D015992:Body Mass Index; D001835:Body Weight; D002648:Child; D004032:Diet; D015444:Exercise; D005260:Female; D005500:Follow-Up Studies; D015438:Health Behavior; D006801:Humans; D008137:Longitudinal Studies; D008572:Menarche; D009765:Obesity / Q000453:epidemiology*; D012890:Sleep; D016138:Walking