title: Friends and social contexts as unshared environments: a discordant sibling analysis of obesity- and health-related behaviors in young adolescents.
abstract: This study examines the contribution of best friends' weight and the peer social context (time spent alone versus with friends) as sources of unshared environment associated with variability in weight and health behaviors among weight-discordant siblings.
  
  
  
  Pairs of same-sex biologic siblings (N=40 pairs; ages 13-17) were originally recruited as part of a study evaluating putative factors contributing to differences in adiposity among weight-discordant siblings. Siblings were asked to bring their best friends to the laboratory and siblings and friends' height and weight were objectively measured. Siblings also completed multi-pass dietary recalls to assess energy intake and sugar sweetened beverage (SSB) consumption. Siblings' physical activity was measured using accelerometry. Experience sampling methodology was used to assess sedentary behaviors/screen time and the number of occasions siblings spent alone and in the presence of friends. Multilevel models were used to estimate the relationships between predictors (best friends' zBMI, time spent alone or with friends) and outcomes (siblings' zBMI and obesity-related health behaviors).
  
  
  
  Best friends' zBMI was the best predictor of participants' zBMI, even when controlling for child's birth weight. Best friends' weight (zBMI) further predicted participants' SSB intake and time engaged in sedentary behaviors. Being active with friends was positively associated with participants' overall physical activity, whereas spending time alone was negatively associated with accelerometer counts regardless of siblings' adiposity.
  
  
  
  A friends' weight and the social context are unshared environmental factors associated with variability in adiposity among biologically-related weight-discordant siblings.
authors.affiliation: Department of Preventive Medicine, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: S-J
authors.initials: SJ
authors.lastname: Salvy
authors.affiliation: Pediatrics, Behavioral Medicine, University at Buffalo, Buffalo, NY, USA.
authors.forename: D M
authors.initials: DM
authors.lastname: Feda
authors.affiliation: Pediatrics, Behavioral Medicine, University at Buffalo, Buffalo, NY, USA.
authors.forename: L H
authors.initials: LH
authors.lastname: Epstein
authors.affiliation: United States Department of Agriculture, Agricultural Research Service, Grand Forks, ND, USA.
authors.forename: J N
authors.initials: JN
authors.lastname: Roemmich
mesh_terms: D061725:Accelerometry; D050154:Adiposity / Q000502:physiology; D000293:Adolescent; D000294:Adolescent Behavior / Q000523:psychology*; D000704:Analysis of Variance; D004032:Diet / Q000009:adverse effects; D002149:Energy Intake / Q000502:physiology; D015444:Exercise / Q000523:psychology*; D005260:Female; D033062:Friends / Q000523:psychology*; D015438:Health Behavior; D006801:Humans; D007398:Interpersonal Relations; D008297:Male; D063766:Pediatric Obesity / Q000453:epidemiology / Q000517:prevention & control / Q000523:psychology*; D010379:Peer Group; D035781:Siblings / Q000523:psychology*; D014481:United States / Q000453:epidemiology