title: Time spent being sedentary and weight gain in healthy adults: reverse or bidirectional causality?
abstract: BACKGROUND
  Whether obesity is a cause or a consequence of a sedentary lifestyle has not yet been fully elucidated, which leaves uncertainty about the direction of causality.
  
  
  OBJECTIVE
  We aimed to assess the longitudinal associations between objectively measured time spent being sedentary (sedentary time) and obesity indicators.
  
  
  METHODS
  The study was a prospective, population-based cohort study in 393 middle-aged healthy whites (n = 176 M, 217 F). Sedentary time (% of daytime hours) was measured by individually calibrated monitoring of the heart rate. Body weight (BW), body mass index (BMI), and waist circumference (WC) were assessed by standard clinical procedures. Fat mass (FM) was assessed with bioimpedance. All measurements were collected at baseline and at 5.6-y follow-up.
  
  
  RESULTS
  At baseline, sedentary time was significantly correlated with FM (partial r = 0.10, P = 0.043) and WC (partial r = 0.11, P = 0.027) after adjustment for sex and age. At follow-up, sedentary time was significantly correlated with BW (partial r = 0.19, P < 0.0001), BMI (partial r = 0.20, P < 0.0001), WC (partial r = 0.15, P = 0.003), and FM (partial r = 0.19, P < 0.0001). Sedentary time did not predict any of the obesity indicators at follow-up. In contrast, BW (beta = 0.33; 95% CI: 0.15, 0.50), BMI (1.10; 0.58, 1.63), FM (0.59; 0.11, 0.40), and WC (0.44; 0.23, 0.66) predicted sedentary time at follow-up after adjustment for sex, baseline age, baseline sedentary time, baseline physical activity energy expenditure, and follow-up time.
  
  
  CONCLUSIONS
  BMI, FM, and WC may predict sedentary time, but our results do not suggest that sedentary time predicts future obesity.
authors.affiliation: Medical Research Council Epidemiology Unit, Institute of Metabolic Science, Cambridge, United Kingdom. ulf.ekelund@mrc-epid.cam.ac.uk
authors.forename: Ulf
authors.initials: U
authors.lastname: Ekelund
authors.forename: Søren
authors.initials: S
authors.lastname: Brage
authors.forename: Herve
authors.initials: H
authors.lastname: Besson
authors.forename: Stephen
authors.initials: S
authors.lastname: Sharp
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Wareham
mesh_terms: D000328:Adult; D001823:Body Composition; D001835:Body Weight; D003924:Diabetes Mellitus, Type 2 / Q000453:epidemiology / Q000517:prevention & control; D004734:Energy Metabolism / Q000502:physiology*; D005260:Female; D005500:Follow-Up Studies; D006304:Health Status; D006801:Humans; D008019:Life Style; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009765:Obesity / Q000453:epidemiology* / Q000209:etiology / Q000517:prevention & control; D012016:Reference Values; D012959:Socioeconomic Factors; D013997:Time Factors; D015430:Weight Gain / Q000502:physiology*