randomized trial comparing weight loss treatment delivered in large versus small groups

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