title: Quasi-causal associations of physical activity and neighborhood walkability with body mass index: a twin study.
abstract: OBJECTIVE
  Physical activity, neighborhood walkability, and body mass index (BMI, kg/m(2)) associations were tested using quasi-experimental twin methods. We hypothesized that physical activity and walkability were independently associated with BMI within twin pairs, controlling for genetic and environmental background shared between them.
  
  
  METHODS
  Data were from 6376 (64% female; 58% identical) same-sex pairs, University of Washington Twin Registry, 2008-2013. Neighborhood walking, moderate-to-vigorous physical activity (MVPA), and BMI were self-reported. Residential address was used to calculate walkability. Phenotypic (non-genetically informed) and biometric (genetically informed) regression was employed, controlling for age, sex, and race.
  
  
  RESULTS
  Walking and MVPA were associated with BMI in phenotypic analyses; associations were attenuated but significant in biometric analyses (Ps<0.05). Walkability was not associated with BMI, however, was associated with walking (but not MVPA) in both phenotypic and biometric analyses (Ps<0.05), with no attenuation accounting for shared genetic and environmental background.
  
  
  CONCLUSIONS
  The association between activity and BMI is largely due to shared genetic and environmental factors, but a significant causal relationship remains accounting for shared background. Although walkability is not associated with BMI, it is associated with neighborhood walking (but not MVPA) accounting for shared background, suggesting a causal relationship between them.
authors.affiliation: Department of Epidemiology, School of Public Health, University of Washington, Seattle, WA, United States; Nutritional Sciences Program, School of Public Health, University of Washington, Seattle, WA, United States. Electronic address: duncag@u.washington.edu.
authors.forename: Glen E
authors.initials: GE
authors.lastname: Duncan
authors.affiliation: Department of Epidemiology, School of Public Health, University of Washington, Seattle, WA, United States; Public Health Sciences Division, Fred Hutchinson Cancer Research Center, Seattle, WA, United States.
authors.forename: Stephanie Whisnant
authors.initials: SW
authors.lastname: Cash
authors.affiliation: Department of Psychology, University of Virginia, Charlottesville, VA, United States.
authors.forename: Erin E
authors.initials: EE
authors.lastname: Horn
authors.affiliation: Department of Psychology, University of Virginia, Charlottesville, VA, United States.
authors.forename: Eric
authors.initials: E
authors.lastname: Turkheimer
mesh_terms: D000328:Adult; D015992:Body Mass Index; D004779:Environment Design; D005260:Female; D040362:Geographic Information Systems; D006801:Humans; D008297:Male; D008991:Monitoring, Physiologic / Q000295:instrumentation / Q000379:methods; D009043:Motor Activity; D015999:Multivariate Analysis; D010641:Phenotype; D012111:Residence Characteristics; D014429:Twins, Dizygotic / Q000235:genetics / Q000706:statistics & numerical data*; D014430:Twins, Monozygotic / Q000235:genetics / Q000706:statistics & numerical data*; D016138:Walking / Q000502:physiology* / Q000706:statistics & numerical data; D014861:Washington
keywords: Body mass index; Environment and public health; Heredity; Motor activity; Twins