Behavioral interventions for obesity typically result in body weight reductions of 8-10% during the initial six months of treatment [1]-[3]. While treatment may be offered in group or individual settings, it is commonly delivered in groups of 8-15 participants [1],[4],[5]. Studies suggest that participants receiving group-based treatment lose significantly more weight than those receiving one-on-one treatment [6]-[8]. Group-based treatment has resulted in greater weight loss even among participants who expressed initial preference for individual treatment [7] and when treatment was delivered by telephone (i.e., group vs. individual calls) [6].

Potential reasons for the superior efficacy of group-based obesity treatment include the provision of additional social support, empathy, role-modeling of healthy choices, and a healthy dose of competition [4],[9]-[11]. Rooted in social-cognitive theory [12], group-based lifestyle interventions utilize interaction among members to promote positive changes in self-efficacy for increased weight loss [13]. Through vicarious learning, verbal persuasion, and emotional and physiological activation, group members can impact each other and encourage behavior change [13].

While clinical experience and/or practical considerations (e.g., space constraints) have driven the traditional delivery of obesity treatment to groups of 8-15 individuals, research is lacking on whether this is more efficacious than alternative group sizes. A limited number of studies in the psychotherapy literature compared groups of differing size for the treatment of conditions such as anxiety, depression, and stress, but results have been mixed. While some studies found few differences between large and small groups [14], at least one demonstrated sustained improvements in anxiety symptoms and better session attendance with groups of 10-12 members versus larger groups of 20 members [15]. For interventions focused on changing health behaviors, one smoking cessation study found no association between group size and outcomes [16].

Participants within smaller groups may experience greater benefit regarding social support [17] and group cohesion [18]. Group-based obesity treatments capitalize upon positive group interactions to promote social learning, reinforcement, and support that ultimately lead to behavior change [4],[9]-[13]. Given there are more opportunities for meaningful social interactions, smaller groups may promote higher quality, cohesive relationships among participants [17]. With the greater opportunity for social interaction and feedback, smaller groups could also promote increased self-efficacy and adherence (e.g., session attendance, completion of self-monitoring records) to treatment recommendations [13], which are strongly associated with weight loss success [19]-[23]. To our knowledge, however, no previous study has empirically tested the effects of group size on weight loss and related outcomes.

Examining the association between group size and treatment outcomes has important clinical and practical implications for weight loss treatment. Given the limited resources often available in applied clinical settings, treatment offered in larger groups may maximize available staff time and other organizational resources while promoting clinically significant weight loss. However, the delivery of group-based interventions to larger numbers of individuals has not been empirically evaluated. Therefore, it is important to compare the outcomes achieved with traditional, smaller groups and larger groups, as this decision about group sizes has direct implications for the implementation, dissemination, and sustainability of evidence-based weight loss treatments into “real world” settings.

This study examined the effect of group size on weight loss within a 12-month randomized behavioral weight loss intervention delivered in a managed care setting. Participants were randomized to receive treatment in either a “small” group (i.e., approximately 12 members per group, SG) typical of most group-based interventions [1],[4],[5],[15] or a “large” group (i.e., approximately 30 members per group, LG). Given the theoretical rationale for delivering obesity treatment in a group format [12],[13] along with previous results indicating the potential benefits of smaller groups [15],[17],[18], it was hypothesized that SG participants would demonstrate greater weight loss, better treatment attendance, better dietary self-monitoring adherence, and a more positive attitude toward the group than LG participants. Additionally, this study explored potential social (e.g., group climate) and behavioral (e.g., self-monitoring adherence) mediators of the association between group size and weight change.

The present study examined whether delivery of a 12-month behavioral weight loss intervention in either “small” or “large” group formats within a managed care setting impacted weight loss outcomes. The primary finding demonstrates that participants assigned to the SG condition lost significantly more weight at both 6 and 12 months of the intervention when compared to participants within the LG. Participants in both groups achieved significant weight losses at Month 6 (-6.4 kg for SG versus -3.2 kg for LG). Participants in the LG, however, experienced weight regain from Months 6 12, resulting in a final weight that did not significantly differ from baseline. Conversely, participants in the SG demonstrated slight additional weight loss from Months 6-12, such that by Month 12 they achieved a significant average loss of -7.03 kg, which is comparable to weight losses seen in other trials [29],[38]. In addition, 51% of SG participants attained a 5% weight loss compared to only 26% in the LG at Month 12. A 5% body weight loss has clinical significance, resulting in reduced risk factors for diabetes and heart disease [29],[38]. While group-based interventions have demonstrated superiority when compared to individually-tailored delivery [6]-[8], this study further suggests that delivery to smaller groups of approximately 12 participants, versus larger groups of approximately 30 participants, results in more successful weight loss.

Both session attendance and self-monitoring adherence are strongly associated with overall weight loss success in lifestyle interventions for obesity [19]-[23]. SG participants attended nearly four more sessions on average than LG members, although this was a non-significant difference. Additionally, adherence to self-monitoring differed significantly between conditions, with higher rates of adherence observed among SG participants. This difference in self-monitoring adherence mediated the association between group size and weight loss, such that the residual direct effects of group size were no longer significant after accounting for adherence. Smaller groups, as compared to larger groups, may provide more opportunities for interventionists and participants to reinforce the accuracy and completion of self-monitoring records, problem-solve barriers to maintaining records, and set individualized goals for improving adherence. Therefore, increased accountability in smaller groups may promote greater adherence and contribute to weight loss success.

Within the psychotherapy literature, smaller group size is associated with increased social support [17] and group cohesion [18]. Larger groups may limit potential resources available and may cause individual members difficulties in making connections, leading to feelings of isolation or alienation [17]. In the present study, SG participants endorsed a more positive and cohesive group atmosphere than LG members, suggesting that smaller groups allow for greater allocation of resources and improved opportunities for supportive interactions that foster an environment encouraging of weight loss. Treatment engagement did not, however, mediate the association between group size and weight loss after accounting for self-monitoring adherence. While it is possible that the included measure did not fully capture the most salient features of engagement, the more robust influence of self-monitoring adherence may have also contributed to these findings. According to social-cognitive theory [12], the cohesive bonds and meaningful interactions experienced among SG participants may have promoted increases in self-efficacy for weight loss behaviors, such as self-monitoring [13]. Indeed, members who feel connected within their group have demonstrated greater responses to group treatment, such as an increased willingness to disclose during discussion [39], higher goal attainment [40], and better attendance rates [41],[42].

This study did have limitations. First, group members were recruited solely within HMOs, with the sample being comprised mainly of females (86%). While the sample was racially and educationally diverse, generalizability remains limited to other clinical settings. Second, the sample size was modest and may have been inadequately powered to detect some group differences. Third, the study timeline lasted one year. While participants in the SG continued to lose a small amount of weight from Months 6-12, it remains unclear whether this trend would continue in the long-term. Fourth, the LG condition included only one treatment group, while the SG condition included three groups that were combined for analyses. Thus, there was a potential cluster effect not fully accounted for by the analyses, as the inclusion of a single LG prohibited estimating random effects attributable to cluster (i.e., random cluster effect could not be separated from the treatment effect for the LG). Fifth, this trial only compared groups of approximately 12 versus 30 members, so the relative effects of moderate-sized groups (e.g., 15-30 participants) is unknown. Finally, while self-monitoring adherence fully mediated the association between group size and weight loss, there may be other process variables not assessed that additionally account for the weight change differences observed between the LG and SG conditions.

While this study provides novel information on the treatment effects of LG and SG conditions, cost-effectiveness of the two approaches was not evaluated. Even though SG participants achieved greater weight loss, it is important to consider whether LG treatment delivery remains a viable option from a cost-efficiency perspective. By formally measuring and calculating program operation costs (e.g., interventionist and administrator time, meeting space fees, etc.), future research could determine costs per kg of weight lost, or a similar metric of cost-effectiveness, to select the appropriate group size that maximizes weight loss at the lowest costs.

The present study also had a number of strengths. The trial utilized a randomized design, and retention was excellent with approximately 94% of participants contributing weight data at Month 12. In addition, all participants received a structured lifestyle intervention modeled after evidence-based protocols [19],[28], which incorporated state-of-the-art behavioral principles to promote weight loss and maintenance. Given the nature of the HMO setting, access to electronic health records provided objective weight data for participants who did not attend follow-up study assessment visits.

In addition to these methodological strengths, this study offers important clinical implications for the delivery of behavioral weight loss treatment. There may be a perceived benefit to delivering evidence-based weight loss treatment to larger numbers of participants in clinical settings to efficiently use the limited resources available. However, this study suggests that clinicians and administrators may need to balance this potential advantage with the improved weight loss outcomes achieved by delivering treatment in smaller groups. Recently, researchers have investigated alternate, more cost-effective methods of weight loss intervention delivery, including group phone calls [6] and group internet-based programs [43]. Utilizing these alternative methods may retain group cohesiveness while incorporating more cost-effective modes of delivery and reducing burden on staff and organizational resources. Future studies should continue to assess whether alternative methods of delivery prove superior for weight loss, cost-effectiveness, and dissemination when delivered to traditional groups of 8-15 participants.

To our knowledge, the present study represents the first empirical test of the effects of group size on weight loss outcomes. The findings suggest that providing lifestyle intervention for obesity in traditional groups of approximately 12 participants is superior to delivery in larger groups of 30 or more participants. Participants in the smaller groups achieved larger weight losses and were more likely to attain a clinically significant weight loss of ≥ 5%. A smaller group size also resulted in higher perceived group engagement, suggesting that smaller group settings provide more positive, cohesive, and collaborative atmospheres. Small groups also facilitated greater adherence to behavioral weight management strategies (i.e., self-monitoring), and this behavior mediated the relationship between group size and treatment outcomes. Future studies should continue to investigate the utilization of smaller group sizes with alternative modes of delivery (e.g., telephone, internet) to improve cost effectiveness within managed care and other applied clinical settings.

GD, LN, PD, SA, and MP contributed to study design; GD and NR collected data; GD, NR, LN, PD, SA, and MP assisted with intervention planning and delivery; FT performed data analyses. All authors were involved in writing the paper and had final approval of the manuscript.