title: Leisure-time physical inactivity and association with body mass index: a Finnish Twin Study with a 35-year follow-up.
abstract: We investigated the stability and change of leisure-time physical inactivity in adult men and women during a 35-year follow-up. We also analysed the impact of long-term physical inactivity on the development of body mass index (BMI).
  
  
  
  : In this population-based cohort study, 5254 Finnish twin individuals (59% women) participated in four surveys in 1975, 1981, 1990 and 2011. Mean age at baseline was 23.9 years. Individual long-term leisure-time physical activity (LTPA) was categorized into seven classes varying from 'persistently inactive' to 'persistently active'. We used the multivariate multilevel mixed-effects linear regression model and paired-sample t-test in the analyses. Co-twin control design was used for examining within-pair associations.
  
  
  
  : Of men 11%, and of women 8%, were persistently inactive. Among both sexes, the mean BMI slope trajectories were steeper among the persistently inactive and those who became inactive than among those who were persistently active. Overall, the inactive participants gained 1.4 kg/m 2 [95% confidence interval (CI) 1.2 to 1.7] more in weight than did the active participants from 1975 to 2011. Among twin pairs discordant for LTPA, the corresponding difference was 1.4 kg/m 2 (95% CI 0.83 to 2.0) in dizygotic pairs and 0.68 kg/m 2 (95% CI 0.05 to1.3) in monozygotic pairs.
  
  
  
  Over a 35-year time span from young adulthood, persistently inactive participants and those who had become inactive had greater weight increases than those who were persistently active. This association was also found in twin-pair analyses, although attenuated in monozygotic pairs. This may support the importance of LTPA in weight management, although further causal inference is required.
authors.affiliation: University of Helsinki, Department of Public Health, Helsinki, Finland.
authors.forename: Maarit
authors.initials: M
authors.lastname: Piirtola
authors.affiliation: University of Helsinki, Department of Public Health, Helsinki, Finland.
authors.forename: Jaakko
authors.initials: J
authors.lastname: Kaprio
authors.affiliation: University of Helsinki, Department of Public Health, Helsinki, Finland.
authors.forename: Katja
authors.initials: K
authors.lastname: Waller
authors.affiliation: University of Helsinki, Department of Public Health, Helsinki, Finland.
authors.forename: Kauko
authors.initials: K
authors.lastname: Heikkilä
authors.affiliation: University of Helsinki, Department of Public Health, Helsinki, Finland.
authors.forename: Markku
authors.initials: M
authors.lastname: Koskenvuo
authors.affiliation: University of Helsinki, Department of Public Health, Helsinki, Finland.
authors.forename: Pia
authors.initials: P
authors.lastname: Svedberg
authors.affiliation: University of Helsinki, Department of Public Health, Helsinki, Finland.
authors.forename: Karri
authors.initials: K
authors.lastname: Silventoinen
authors.affiliation: University of Helsinki, Department of Public Health, Helsinki, Finland.
authors.forename: Urho M
authors.initials: UM
authors.lastname: Kujala
authors.affiliation: University of Helsinki, Department of Public Health, Helsinki, Finland.
authors.forename: Annina
authors.initials: A
authors.lastname: Ropponen
mesh_terms: D000328:Adult; D000368:Aged; D015992:Body Mass Index; D015444:Exercise; D005260:Female; D005387:Finland; D005500:Follow-Up Studies; D015438:Health Behavior; D006801:Humans; D007899:Leisure Activities; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D057185:Sedentary Behavior; D011795:Surveys and Questionnaires; D015430:Weight Gain; D055815:Young Adult
keywords: Physical activity; behaviour; body mass index; cohort study; exercise; twin study; weight gain