In the current study, we observed that while all groups lost weight, the treatment congruent group lost less weight than the treatment incongruent group or the group who did not report a strong preference at baseline. This effect was observed throughout the duration of the study (a preference × time interaction was not statistically significant). There were no differences between preference groups with respect to attrition. Previous studies on treatment preference in the context of weight loss studies were mixed (4
). Burke et al.
), observed greater weight loss in participants who did not receive their preferred treatment. Other studies that also dichotomized preference found no effect of preference on weight loss (4
An explanation for these discrepant findings (4
) may be the oversimplification of preference (i.e., received preference vs. did not receive preference). Subjects who have strong preferences may behave differently than subjects who expressed no strong preferences at baseline. Treating preference as a trichotomy may clarify this important qualitative distinction between preference and no preference and was a statistically significant predictor of weight loss.
In the current study, that the treatment congruent group loss less weight than the treatment incongruent group or the group who did not report a strong preference at baseline may seem counter intuitive. According to behavioral choice theory, participants who received their preference would have better outcomes than those who received their nonpreferred treatment (2
). This theory; however, assumes that the behavior being modified is a deficit behavior. In the case of a weight loss study, where the desired outcomes are weight loss and retention, the behavior that participants desire to be changed is a dependent or preferred behavior (i.e., a preferred way of eating). In the context of a weight loss study, dietary modifications, such as low-carbohydrate or low-fat diets are offered to participants as alternative reinforcers. Whether participants in the current study made their preference selection based on their preferred way of eating or as an alternative to their preferred way of eating is unknown.
In the current study, participants were asked to qualitatively explain their preference selection; however, they were not asked whether their selection was based on their preferred diet or a preferred alternative to their dependent behavior (i.e., what they already enjoyed eating). It is, therefore, unknown whether participants who received their preference lost less weight because they selected a diet that was consistent with what they enjoyed eating. The qualitative data suggest that participants selected their preferred or desired diet, which may explain why participants in this group lost the least amount of weight. For example, the participants that provided the following qualitative explanations for their preference, “Because I am a carbohydrate addict,” “Because I am the BBQ King,” preferred the low-fat and low-carbohydrate diets, respectively. Future studies of preference may want to confirm this by explicitly asking subjects whether their preference was based on a dependent behavior or a preferred alternative.
Alternative explanations for the observed relationship between preference and weight loss may also be considered. For example, participants who received their preference may have greater expectations for it to produce weight loss (6
). Alternatively, participants who did not express a strong preference at baseline or those who do not receive their preference may internalize the responsibility of weight loss. This internalization was refected in the qualitative data collected when the respondents submitted their diet preference (i.e., “No preference because I was willing to give either diet 100%.”).
In the current study, receiving one's treatment preference was associated with differential weight loss; however, it did not modify the effect of the treatments (low-carbohydrate and low-calorie diet) on weight loss, which was consistent with previous studies (4
). While more participants who received the low-carbohydrate diet received their preference (38.0 vs. 20.7%), the groups were fairly comparable with respect to preference. A greater threat to validity may exist if participants widely preferred one treatment over another treatment, which would result in an imbalance in receiving a preferred treatment across groups.
This study had several strengths including the duration (24 months), multi-center design (three sites), sample size (N
= 250), and an a priori
analytic plan to assess preference effects. In addition, the two treatments (low-carbohydrate and low-fat diets) produced equal weight loss between groups. This allowed a more sensitive assessment of the relationship between preference and weight loss without treatment as a confounder. Participants were also approximately evenly split into preference congruent (29.6%), did not report a strong preference (39.2%), and preference incongruent (31.2%) groups. Finally, our assessment of treatment preference using a 6-item Likert scale allowed us to build on the work of previous studies in the area of weight loss and treatment preference (4
). We chose to trichotomize treatment preference, which refected a compromise between utilizing all available information (i.e., maintaining all 6 categories) and simplicity (i.e., dichotomization).
A limitation, which is consistent with most studies of weight loss, is the under representation of male participants. Attrition may be another limitation. Of the 250 participants assessed at baseline, 216 (86.4%), 185 (74.0%), and 157 (62.8%) participants remained in the study at 6 months, 1 year and 2 years, respectively.
In summary, in this large, 2-year study of weight loss, receiving one's treatment preference was a predictor of successful weight loss. While all groups lost weight, participants who received their preference lost less weight than participants who either did not receive their preference or who did not report a strong preference at baseline. This effect persisted across the duration of the study. Receiving one's treatment preference did not modify the effect of the treatment on weight loss. Future studies should assess treatment preference at baseline and explore whether preference modifies the effect of the treatment on the main outcomes. In addition, to help ensure that treatment preference is equally distributed between groups, when the external validity of results is a goal, studies should stratify the randomization procedure by preference as an alternative to excluding participants with strong treatment preferences.