Background: It has not been established whether control conditions with large weight losses (WLs) diminish expected treatment effects in WL or prevention of weight gain (PWG)-randomized controlled trials (RCTs).
Subjects/methods: We performed a meta-analysis of 239 WL/PWG RCTs that include a control group and at least one treatment group. A maximum likelihood meta-analysis framework was used to model and understand the relationship between treatment effects and control group outcomes.
Results: Under the informed model, an increase in control group WL of 1 kg corresponds with an expected shrinkage of the treatment effect by 0.309 kg (95% confidence interval (-0.480, -0.138), P=0.00081); this result is robust against violations of the model assumptions.
Conclusions: We find that control conditions with large WLs diminish expected treatment effects. Our investigation may be helpful to clinicians as they design future WL/PWG studies.