Behavioral weight management interventions often yield small, short-term effects. Acceptance and Commitment Therapy (ACT)-based interventions show promise for long-term weight outcomes, but are typically delivered in-person. The COVID-19 pandemic highlighted the urgent need for inexpensive, scalable weight management interventions, particularly given the increased risk of weight gain and mental health challenges during lockdowns. The SWIM-C intervention, a three-month, remotely delivered ACT-based guided self-help program, was developed to address this need. A previous four-month evaluation showed improvements in psychological determinants of weight management but inconclusive results on weight change. This study aimed to evaluate the twelve-month effects of SWIM-C on weight change and other relevant outcomes.

A systematic review and network meta-analysis found that interventions incorporating ACT strategies may have better long-term weight outcomes. ACT focuses on increasing psychological flexibility and reducing experiential avoidance to facilitate improved behavioral responses. However, evidence on remotely delivered ACT-based weight management interventions was scarce at the time of the study's inception, with only three studies identified in a recent review, and none designed to detect effects on weight. The need for inexpensive, scalable interventions for behavioral weight management became particularly urgent during the COVID-19 pandemic due to social distancing measures and increased vulnerability of adults with overweight and obesity to weight gain and changes in eating behavior.

This randomized, parallel, two-group trial (ISRCTN12107048) included 388 participants (≥18 years, BMI ≥25 kg/m²) randomized to either the SWIM-C intervention or a standard advice group. Participants completed online questionnaires at baseline, four months, and twelve months. The primary outcome was change in self-reported weight from baseline to twelve months. Secondary outcomes included eating behavior (using the TFEQ-R21), experiential avoidance (AAQW-R), depression (PHQ-8), anxiety (GAD-7), stress (PSS-4), wellbeing (ICECAP-A), and physical activity (IPAQ). SWIM-C involved a web platform with 12 weekly modules, email, and telephone contact with a coach. Standard advice consisted of a leaflet on managing weight and mood. Randomization was 1:1, stratified by BMI and sex. Statistical analyses included random intercepts linear regression models, with sensitivity analyses addressing missing data.

At twelve months, the baseline-adjusted difference in weight change between SWIM-C (n=119) and standard advice (n=147) was -0.81 kg (95% CI: -2.24 to 0.61 kg), not statistically significant. However, SWIM-C participants showed a greater reduction in experiential avoidance (-2.45, 95% CI: -4.75 to -0.15), uncontrolled eating (-5.52, 95% CI: -9.67 to -1.37), and emotional eating (-4.49, 95% CI: -7.57 to -1.42), and a greater increase in physical activity (8.96 MET-min/week, 95% CI: 0.29 to 17.62) compared to the standard advice group. Per-protocol analyses, excluding standard advice participants who engaged with the SWIM-C platform, yielded similar results. Sensitivity analyses assuming data were missing not at random did not alter the conclusions. From four months to twelve months, SWIM-C participants showed a small increase in anxiety and a decrease in cognitive restraint, while other outcomes were generally maintained.

While SWIM-C did not produce significant weight loss, it improved several key psychological and behavioral factors associated with successful weight management. The sustained effects on uncontrolled eating and experiential avoidance suggest lasting positive changes. The increase in physical activity is also a significant finding, offering potential health benefits independent of weight loss. The lack of significant weight loss might be due to the relatively short duration of the intervention and the inherent challenges of long-term weight management. The delayed effects on some outcomes might suggest a need for longer intervention durations for ACT-based approaches to fully manifest their impact. The changes observed in eating behavior and experiential avoidance, while statistically significant, require further investigation to determine their clinical significance.

SWIM-C demonstrated improvements in several key determinants of weight management, including eating behaviors, experiential avoidance, and physical activity. While the effect on weight change was not statistically significant, the sustained positive impacts on psychological and behavioral factors suggest the potential of remotely delivered ACT-based interventions. Further research is needed to refine the intervention, potentially through increased coach support or longer intervention duration, to optimize its effectiveness in achieving clinically meaningful weight loss.

The study's limitations include the use of self-reported weight, potential limitations in the recruitment strategy (social media recruitment may limit generalizability), and a high rate of missing data at follow-up, particularly in the intervention group. The relatively high proportion of female, White, and university-educated participants may limit generalizability. The study did not account for participants potentially engaging in other weight management programs or treatments.