Relationship between treatment preference and weight loss in the context of a randomized controlled trial

Abstract

Randomized controlled trials (RCTs) are considered the gold standard used to assess the efficacy of treatment. While a well implemented RCT can produce an unbiased estimate of the relative difference between treatment groups, the generalizability of these findings may be limited. Specific threats to the external validity include treatment preference. The purposes of this study were to: (i) assess whether receiving one's treatment preference was associated with weight loss and retention and (ii) whether receiving one's treatment preference modified the relationship between the treatments and weight loss. Treatment preference was assessed in 250 subjects prior to but independent of randomization into either low-carbohydrate or low-fat diets. Treatment preference was a predictor of weight loss (P = 0.002) but not retention (P = 0.90). Participants who received their preference lost less weight (-7.7 kg, 95% confidence interval (CI): -9.3 to -6.1) than participants who did not receive their preference (-9.7 kg, 95% CI: -11.4 to -8.1) and participants who did not report a strong preference at baseline (-11.2 kg, 95% CI: -12.6 to -9.7) (P = 0.04 and P = 0.0004, respectively). Treatment preference did not modify the effect of the treatment on weight loss. Contrary to conceptual predictions, this study failed to identify an interaction between treatment preference and weight loss in the setting of a randomized trial. Until treatment preference effects are definitively ruled out in this domain, future studies might consider stratifying their randomization procedure by treatment preference rather than excluding participants with strong treatment preferences.

Trial registration: ClinicalTrials.gov NCT00143936.

Figure 1

The trichotomy of revalenced diet preference scores.

Figure 2

Change in weight (kg) by treatment preference group (those who received their preference, preference congruent, those who did report a strong preference, and those who did not receive their preference, preference incongruent). Data are from a repeated measures mixed effects model that included fixed effects for time, treatment (i.e., low-carbohydrate and low-fat), a time × treatment interaction, baseline weight, preference category (i.e., received preference, did not report a strong preference, and did not receive preference), and research site. A random effect was included for participant cohort. Over the course of the 24 month study, the preference congruent group lost less weight than the preference incongruent group (P = 0.04), and the group that did not report a strong preference (P = 0.0004). There was no difference in mean weight loss between the group that did not report a strong preference and the preference incongruent group (P = 0.14). The effect of preference on weight loss persisted across the 24 month study (the preference × time interaction was not statistically significant).