title: Fatness predicts decreased physical activity and increased sedentary time, but not vice versa: support from a longitudinal study in 8- to 11-year-old children.
abstract: OBJECTIVE
  To examine independent and combined cross-sectional associations between movement behaviors (physical activity (PA), sedentary time, sleep duration, screen time and sleep disturbance) and fat mass index (FMI), as well as to examine longitudinal associations between movement behaviors and FMI.
  
  
  METHODS
  Cross-sectional and longitudinal analyses were done using data from the OPUS school meal study on 785 children (52% boys, 13.4% overweight, ages 8-11 years). Total PA, moderate-to-vigorous PA (MVPA), sedentary time and sleep duration (7 days and 8 nights) were assessed by an accelerometer and FMI was determined by dual-energy X-ray absorptiometry (DXA) on three occasions over 200 days. Demographic characteristics, screen time and sleep disturbance (Children's Sleep Habits Questionnaire) were also obtained.
  
  
  RESULTS
  Total PA, MVPA and sleep duration were negatively associated with FMI, while sedentary time and sleep disturbances were positively associated with FMI (P⩽0.01). However, only total PA, MVPA and sleep duration were independently associated with FMI after adjustment for multiple covariates (P<0.001). Nevertheless, combined associations revealed synergistic effects among the different movement behaviors. Changes over time in MVPA were negatively associated with changes in FMI (P<0.001). However, none of the movement behaviors at baseline predicted changes in FMI (P>0.05), but higher FMI at baseline predicted a decrease in total PA and MVPA, and an increase in sedentary time (P⩽0.001), even in normal-weight children (P⩽0.03).
  
  
  CONCLUSIONS
  Total PA, MVPA and sleep duration were independently associated with FMI, and combined associations of movement behaviors showed a synergistic effect with FMI. In the longitudinal study design, a high FMI at baseline was associated with lower PA and higher sedentary time after 200 days but not vice versa, even in normal-weight children. Our results suggest that adiposity is a better predictor of PA and sedentary behavior changes than the other way around.
authors.affiliation: Department of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: M F
authors.initials: MF
authors.lastname: Hjorth
authors.affiliation: Healthy Active Living and Obesity Research Group, Children's Hospital of Eastern Ontario Research Institute, Ottawa, Ontario, Canada.
authors.forename: J-P
authors.initials: JP
authors.lastname: Chaput
authors.affiliation: Department of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: C
authors.initials: C
authors.lastname: Ritz
authors.affiliation: Department of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: S-M
authors.initials: SM
authors.lastname: Dalskov
authors.affiliation: Division of Nutrition, National Food Institute, Technical University of Denmark, Søborg, Denmark.
authors.forename: R
authors.initials: R
authors.lastname: Andersen
authors.affiliation: Department of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: A
authors.initials: A
authors.lastname: Astrup
authors.affiliation: Division of Nutrition, National Food Institute, Technical University of Denmark, Søborg, Denmark.
authors.forename: I
authors.initials: I
authors.lastname: Tetens
authors.affiliation: Department of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: K F
authors.initials: KF
authors.lastname: Michaelsen
authors.affiliation: Department of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: A
authors.initials: A
authors.lastname: Sjödin
mesh_terms: D015502:Absorptiometry, Photon; D050154:Adiposity; D015992:Body Mass Index; D002648:Child; D003430:Cross-Sectional Studies; D003718:Denmark / Q000453:epidemiology; D004032:Diet; D015444:Exercise; D005260:Female; D006801:Humans; D007899:Leisure Activities; D008137:Longitudinal Studies; D008297:Male; D063766:Pediatric Obesity / Q000453:epidemiology / Q000517:prevention & control*; D010809:Physical Fitness; D040242:Risk Reduction Behavior; D012890:Sleep; D011795:Surveys and Questionnaires; D013690:Television