A systematic review and meta-analysis of weight loss in control group participants of lifestyle randomized trials

Randomized clinical trials (RCTs) of lifestyle modification have reported beneficial effects of interventions, compared to control. Whether participation in the control group has benefits is unknown. To determine whether control group participants experience weight loss during the course of RCTs. After prospective registration (PROSPERO CRD42021233070), we conducted searches in Medline, Scopus, Web of Science, Cochrane library and Clinicaltrials.gov databases from inception to May 2021 without language restriction to capture RCTs on dietary advice or physical activity interventions in adults with overweight, obesity or metabolic syndrome. Data extraction and study quality assessment was performed by two independent reviewers. Weight loss in the control group, i.e., the difference between baseline and post-intervention, was pooled using random effects model generating mean difference and 95% confidence interval (CI). Heterogeneity was assessed using the I(2) statistical test. Subgroup meta-analysis was performed stratifying by follow-up period, type of control group protocols and high-quality studies. Among the 22 included studies (4032 participants), the risk of bias was low in 9 (40%) studies. Overall, the controls groups experienced weight loss of − 0.41 kg (95% CI − 0.53 to − 0.28; I(2) = 73.5% p < 0.001). To identify a result that is an outlier, we inspected the forest plot for spread of the point estimates and the confidence intervals. The magnitude of the benefit was related to the duration of follow-up (− 0.51 kg, 95% CI − 0.68, − 0.3, for 1–4 months follow-up; − 0.32 kg, 95% CI − 0.58, − 0.07, 5–12 months; − 0.20 kg, 95% CI − 0.49, 0.10, ≥ 12 months). In high-quality studies we found an overall weight loss mean difference of − 0.16 (95% CI − 0.39, 0.09) with a considerable heterogeneity (I(2) = 74%; p < 0.000). Among studies including control group in waiting lists and combining standard care, advice and material, no heterogeneity was found (I(2) = 0%, p = 0.589) and (I(2) = 0%, p = 0.438); and the mean difference was − 0.84 kg (95% CI − 2.47, 0.80) and − 0.65 kg (95% CI − 1.03, − 0.27) respectively. Participation in control groups of RCTs of lifestyle interventions had a benefit in terms of weight loss in meta-analysis with heterogeneity. These results should be used to interpret the benefits observed with respect to intervention effect in trials. That control groups accrue benefits should be included in patient information sheets to encourage participation in future trials among patients with overweight and obesity.