title: Effects of reallocating time in different activity intensities on health and fitness: a cross sectional study.
abstract: BACKGROUND
  The effects of replacing time in specific activity categories for other categories (e.g. replacing sedentary time with light activity) on health and fitness are not well known. This study used isotemporal substitution to investigate the effects of substituting activity categories in an equal time exchange fashion on health and fitness in young people.
  
  
  METHODS
  Participants were drawn from schools in Camden, London (n = 353, mean age 9.3 ± 2.3 years). Time sedentary, in light and in moderate-to-vigorous activity (MVPA) was measured via accelerometry. The effects of substituting time in activity categories (sedentary, light and MVPA) with equivalent time in another category on health and fitness were examined using isotemporal substitution.
  
  
  RESULTS
  In single and partition models, MVPA was favourably associated with body fat %, horizontal jump distance and flexibility. Time sedentary and in light activity were not associated with health and fitness outcomes in these models. In substitution models, replacing one hour of sedentary time with MVPA was favourably associated with body fat % (B = -4.187; 95% confidence interval (CI), -7.233, -1.142), horizontal jump distance (B = 16.093; 95% CI, 7.476, 24.710) and flexibility (B = 4.783; 95% CI, 1.910, 7.656). Replacing time in light activity with MVPA induced similar benefits but there were null effects for replacing sedentary with light intensity.
  
  
  CONCLUSIONS
  Substituting time sedentary and in light activity with MVPA was associated with favourable health and fitness. Time in sedentary behaviour may only be detrimental to health and fitness when it replaces time in MVPA in young people.
authors.affiliation: Health Behaviour Research Centre, University College London, 1-19 Torrington Place, WC1E 6BT, London, UK. d.aggio@ucl.ac.uk.|Department of Epidemiology and Public Health, Physical Activity Research Group, University College London, 1-19 Torrington Place, WC1E 6BT, London, UK. d.aggio@ucl.ac.uk.
authors.forename: Daniel
authors.initials: D
authors.lastname: Aggio
authors.affiliation: Health Behaviour Research Centre, University College London, 1-19 Torrington Place, WC1E 6BT, London, UK.|Department of Epidemiology and Public Health, Physical Activity Research Group, University College London, 1-19 Torrington Place, WC1E 6BT, London, UK.
authors.forename: Lee
authors.initials: L
authors.lastname: Smith
authors.affiliation: Department of Epidemiology and Public Health, Physical Activity Research Group, University College London, 1-19 Torrington Place, WC1E 6BT, London, UK.|National Centre Sport & Exercise Medicine, Loughborough University, Loughborough, UK.
authors.forename: Mark
authors.initials: M
authors.lastname: Hamer
mesh_terms: D000293:Adolescent; D002648:Child; D002652:Child Behavior / Q000523:psychology*; D003430:Cross-Sectional Studies; D015444:Exercise / Q000523:psychology; D005260:Female; D015438:Health Behavior; D006801:Humans; D008297:Male; D009043:Motor Activity; D010809:Physical Fitness / Q000523:psychology*; D057185:Sedentary Behavior; D013997:Time Factors