Participants
Members of a health maintenance organization (HMO) located in northern Florida were recruited for participation. HMO members were men and women who either self-referred or received physician referral to the program. HMO members were made aware of the study through the HMO’s website, member newsletters, and during physician office visits. Members attended an initial information session, and interested members completed a brief screening assessment by telephone to determine eligibility.
Participants were ≥21 years-old, had a body mass index (BMI; kg/m2) between 30-45, and had current insurance coverage through the HMO. Individuals were excluded if they had lost >4.5 kg in the last six months, had a medical condition likely to affect participation in physical activity, planned to relocate from the coverage area in the next 12 months, were unable or unwilling to attend weekly group sessions, or were unwilling to accept random assignment to treatment groups.
Among 103 individuals who attended the informational session, 99 completed a subsequent telephone screening and 70 were deemed eligible for participation. Of these, 68 individuals provided informed consent and completed a baseline assessment at the medical center; however, two individuals dropped out prior to treatment assignment. Thus, 66 participants were randomly assigned to the treatment conditions (Figure 1). Thirty-one individuals were assigned to one LG, and 35 individuals were assigned to one of three SGs (approximately 12 individuals/group).
Prior to recruitment, four meeting times for treatment groups were determined based on availability of clinic staff and space, and participants were asked to provide their availability for each of the four times during the screening process. After all participants provided availability, a computerized random number generator was used to determine which of the four meetings would be small or large groups. Participants were de-identified (represented by study ID and meeting time availability only) in order for staff to assign them to one of the four groups. Thus, the research team was masked to the identity of participants during treatment assignment (rather than being masked to condition itself) until all assignments had been made. Approval for this study was obtained from the IRBs of participating institutions.
Measures
Demographics and medical history
At baseline, participants reported their age, sex, race, education level, marital status, and current tobacco and alcohol use. Participants also self-reported a history (yes/no) of the following medical conditions: high blood pressure, heart attack, chest pain, type 2 diabetes, gestational diabetes, pre-diabetes, arthritis, sleep apnea, high cholesterol, dizziness/fainting, asthma or chronic lung disease, and orthopedic problems. The total number of medical conditions was computed for each participant.
Weight and height
Weight and height were measured at each assessment in the medical clinic and used to calculate BMI. Weight was measured to the nearest 0.1 kg using a calibrated digital scale. Height was measured to the nearest 0.1 cm using a wall-mounted stadiometer. For participants who failed to complete the Month 6 and/or Month 12 assessment, recent clinic visit weights were accessed via electronic health records (EHR). EHR weights were imputed if they were recorded within an eight-week period (+/- four weeks) of the study assessment. EHR weights were used for six participants at Months 6 and 12.
Treatment attendance
Group leaders recorded participant attendance at group sessions. If a participant missed a group session, an individual make-up session was not provided, although the participant received session materials for review.
Self-monitoring adherence
Participants were instructed to self-monitor food intake using forms provided. Instructions included daily self-monitoring during Months 0-6 (i.e., up to 168 days of completed logs) and 3 days/week during Months 7-12 (i.e., up to 72 days of completed logs), which resulted in a total possible 240 days of completed logs. A completed log was defined as having at least two designated meals or eating episodes recorded within the day.
Treatment climate
Participants completed the Group Climate Questionnaire-Short Form (GCQ-S), a widely used measure of group process [24], at Months 6 and 12. The GCQ-S consists of 12 items rated on a six-point scale. The GCQ-S provides scores on three domains: Engagement, Conflict, and Avoidance [24]. For each domain, higher scores indicate higher perceived levels of that group process. The Engagement scale measures the extent to which the working group atmosphere is positive and cohesive. The Conflict scale reflects the amount of interpersonal anger and friction in the group. The Avoidance scale measures the extent of avoidance of responsibility for change by group members. The GCQ-S has demonstrated satisfactory reliability (Cronbach’s α ranging from 0.72-0.95) [25]-[27].
Study design
Treatment description
The 12-month intervention was modeled after the lifestyle intervention of the Diabetes Prevention Program [28] and the treatment protocols developed and evaluated by Perri and colleagues [19]. Grounded in social-cognitive theory [12], these interventions seek to enhance individuals’ social support, personal motivators, and self-efficacy for behavior change [5]. Further, they have demonstrated efficacy for producing clinically meaningful weight reduction [19],[29]. The format and content of sessions were the same for both conditions. Participants received pedometers, food scales, and measuring cups/spoons to facilitate self-monitoring and behavior change, and group leaders distributed self-monitoring logs at each session.
During Months 0-6, participants attended 24 weekly 90-minute group sessions. Prior to each session, participants were weighed privately. Each session included presentation, discussion, and practice of skills related to nutrition, exercise, and other self-management strategies. Participants also received training in self-monitoring, problem-solving, stimulus control, cognitive restructuring, and relapse prevention. Consistent with NHLBI guidelines [30], participants were encouraged to work towards a 10% reduction in body weight during the initial six-month period. To achieve this goal, participants were instructed to reduce caloric intake to 1,200 kcal/day (for participants weighing <250 pounds) or 1,500 kcal/day (for participants weighing ≥250 pounds). Participants were also encouraged to increase levels of moderate-intensity physical activity to 180 minutes/week.
Participants attended six monthly extended care sessions between Months 6-12. The purpose of extended-care was to maintain adherence, bolster motivation, and reinforce information previously discussed with a focus on the maintenance of healthy lifestyle behaviors. All study-related visits were held at one of the HMO outpatient clinics.
Intervention staff
All interventions were facilitated by a licensed clinical psychologist with expertise in delivery of weight management programs and/or doctoral students in clinical and counseling psychology. Each group included two co-facilitators. The intervention team met weekly to discuss all aspects of treatment delivery.
Statistical analyses
Descriptive statistics, including sample mean and sample proportions, were used to summarize sample characteristics. T-tests were used to compare population means, and χ2 tests or Fisher’s exact tests were used to assess association between categorical variables. To analyze the main outcome of weight change over time while accounting for the dependence among measurements of the same participant, we applied a mixed effects model with unstructured covariance matrix and adjusted for baseline body weight. Without imputation, this method allowed for participants with missing measurements at either follow-up (n = 7 at Month 6; n = 4 at Month 12) to be retained in analyses. Comparison of attendance, adherence, and group climate scores (i.e., Engagement, Conflict, and Avoidance scales of the GCQ-S) for the SG and LG conditions were examined using t-tests at Months 6 and 12. Cumulative attendance was analyzed at Month 12.
Evaluating measures of group climate (i.e., Engagement, Conflict, and Avoidance), attendance, and adherence as potential mediators of the association between group assignment and weight change were proposed. To explore this, additional regression analyses were conducted based on the mediation approach initially described by Baron and Kenny [31] and recently updated by Cerin and MacKinnon [32], as well as the multiple mediator modelling method detailed by Preacher and Hayes [33]. In this approach, the change between overall treatment effect and the residual treatment effect of group size on weight change, after simultaneously accounting for the impact of the specified mediator variables, were examined using the product-of-coefficient estimate method. First, a test of the action theory was performed examining the association between treatment and the proposed mediators. For constructs demonstrating a significant association with group size, a test of the conceptual theory further assessed the relationship between the mediators and the outcome (i.e., weight change), while adjusting for the effect of treatment in the model. In these analyses, baseline weight was included as a confounder variable. For each mediator, the point estimator of mediation effect was calculated as the product of the action theory test and the conceptual theory test regression coefficients. Due to small sample size and subsequent skewed distributions of path effects, we obtained bias-corrected bootstrap confidence intervals based on 10,000 bootstrap samples [32],[34]-[36]. Also consistent with Cerin and MacKinnon [32], multiple-mediator models were employed, and mediation effect estimates were reported in the units of the outcome variable (kg). Statistical analyses were conducted using SAS/STAT® software, Version 9.3 of the SAS System for Windows, and R statistical software [37].