Purpose: To assess whether forming general behavioral intentions and implementing intentions through action plans promotes weight loss and is moderated by weight loss goals and self-efficacy.
Design: A quasi-experimental study comparing change in body mass index (BMI) for 15 months between a behavioral intentions condition (BIC), an implementation intentions condition (IIC), and a comparison condition (CC).
Setting: Ten-week weight loss program delivered in weekly group meetings at community medical centers.
Subjects: Six hundred thirty-two attendants at the weight loss program (80% of program attendants participated in the study).
Intervention: Weight loss program focused on lifestyle changes, augmented by two experimental conditions (vs. a comparison condition): BIC, intended use of weight loss techniques; IIC, intended use and detailed plans for two techniques. Phone follow-up was conducted 3 and 12 months later.
Main outcome: BMI during the program (computed on the basis of weight and height measured on a mechanical medical scale). Experimental manipulations included exposure to list of techniques (BIC/IIC) and structured planning form (IIC); independent variables were assessed with questionnaires (eating self-efficacy, weight loss goal, demographics).
Analysis: Linear mixed models estimating changes in BMI and their interactions with the planning interventions, goals, and self-efficacy.
Results: Participants in the BIC and IIC lost 40% more weight during the 10-week program than those in the CC (1.10 and 1.11 BMI points compared with .79; ts < -2.76, ps < .01). Weight loss goals interacted with implementation intentions (t = 2.98, p < .01). Self-efficacy was unrelated to weight loss. No differences were found between conditions at 3 and 12 months after the program.
Conclusion: Findings revealed that forming implementation intentions promotes weight loss within a weekly program at a field setting and that its effectiveness depends on initial high goals.