title: A randomized trial comparing weight loss treatment delivered in large versus small groups.
abstract: BACKGROUND
  Behavioral interventions for obesity are commonly delivered in groups, although the effect of group size on weight loss has not been empirically evaluated. This behavioral weight loss trial compared the 6- and 12-month weight changes associated with interventions delivered in a large group (LG) or small groups (SG).
  
  
  METHODS
  Obese adults (N = 66; mean age = 50 years; mean BMI = 36.5 kg/m2; 47% African American; 86% women) recruited from a health maintenance organization were randomly assigned to: (1) LG treatment (30 members/group), or (2) SG treatment (12 members/group). Conditions were comparable in frequency and duration of treatment, which included 24 weekly group sessions (months 1-6) followed by six monthly extended care contacts (months 7-12). A mixed effects model with unstructured covariance matrix was applied to analyze the primary outcome of weight change while accounting for baseline weight and dependence among participants' measurements over time.
  
  
  RESULTS
  SG participants lost significantly more weight than LG participants at Month 6 (-6.5 vs. -3.2 kg; p = 0.03) and Month 12 (-7.0 vs. -1.7 kg; p < 0.002). SG participants reported better treatment engagement and self-monitoring adherence at Months 6 and 12, ps < 0.04, with adherence fully mediating the relationship between group size and weight loss.
  
  
  CONCLUSIONS
  Receiving obesity treatment in smaller groups may promote greater weight loss and weight loss maintenance. This effect may be due to improved adherence facilitated by SG interactions. These novel findings suggest that the perceived efficiency of delivering behavioral weight loss treatment to LGs should be balanced against the potentially better outcomes achieved by a SG approach.
authors.forename: Gareth R
authors.initials: GR
authors.lastname: Dutton
authors.forename: Lisa M
authors.initials: LM
authors.lastname: Nackers
authors.forename: Pamela J
authors.initials: PJ
authors.lastname: Dubyak
authors.forename: Nicole C
authors.initials: NC
authors.lastname: Rushing
authors.forename: Tuong-Vi T
authors.initials: TV
authors.lastname: Huynh
authors.forename: Fei
authors.initials: F
authors.lastname: Tan
authors.forename: Stephen D
authors.initials: SD
authors.lastname: Anton
authors.forename: Michael G
authors.initials: MG
authors.lastname: Perri
mesh_terms: D000328:Adult; D001741:Black or African American; D015992:Body Mass Index; D005247:Feeding Behavior; D005260:Female; D005431:Florida; D005500:Follow-Up Studies; D015438:Health Behavior; D006279:Health Maintenance Organizations; D006801:Humans; D008019:Life Style; D008297:Male; D008875:Middle Aged; D009043:Motor Activity; D009765:Obesity / Q000628:therapy*; D012959:Socioeconomic Factors; D016896:Treatment Outcome; D015431:Weight Loss; D061217:Weight Reduction Programs / Q000379:methods*; D055815:Young Adult