Change in Domain-specific but not General Psychological Flexibility Relates to Greater Weight Loss in Acceptance-Based Behavioral Treatment for Obesity.

Acceptance-based approaches have demonstrated promise for improving outcomes in behavioral treatments for obesity, but few studies have examined processes of change in these treatments. It is critical to identify mechanisms of action in treatment to further optimize this approach, refine theory, and inform future research. This study examined change in several domain-specific and general measures of psychological flexibility in a randomized controlled trial of an acceptance-based behavioral weight loss treatment. The relationships between change in these measures and weight loss outcomes were also examined. Adults ( = 283) were randomized to receive 12 months of acceptance-based (ABT) or non-acceptance-based (non-ABT) behavioral treatment and completed measures of general and weight control-specific psychological flexibility at months 0, 6, and 12. Participants in ABT demonstrated greater increases in psychological flexibility related to eating and physical activity experiences during treatment compared to participants in non-ABT, and changes in these processes were correlated with better weight loss. Parallel mediation analyses further revealed that psychological flexibility related to eating and physical activity experiences partially mediated the relationship between treatment condition and 12-month weight loss. Participants across conditions also experienced small increases in general psychological flexibility, but general psychological flexibility was not meaningfully related to weight loss outcomes. These findings indicate that domain-specific (versus general) psychological flexibility may be most impacted by ABT and most relevant to weight loss outcomes. Results also provide partial support for the theoretical model of ABT for weight management.