Summary
Weight gain prevention is a global public health priority to address escalating adiposity in adults. This review evaluates the efficacy of weight gain prevention trials targeting adults aged 18–50 years and adheres to the Preferred Reporting Items for Systematic Reviews and Meta‐Analyses (PRISMA) protocol. Trials of any duration from inception to May 2020 that evaluated a weight gain prevention intervention (using either prescriptive diet, prescriptive physical activity, prescriptive diet, and/or physical activity or non‐prescriptive lifestyle) and included weight or body mass index (weight [kg]/height [m2]) were eligible. Twenty‐nine trials across 34 publications (participants n = 37,407) were included. Intervention resulted in less weight gain compared with controls (−1.15 kg [95% CI −1.50, −0.80 kg] p < 0.001). Subgroup analysis demonstrated greater effectiveness with prescriptive (−1.60 kg [95% CI −2.00, −1.19] p < 0.001) compared with non‐prescriptive (−0.81 kg [95% CI 1.10, −0.53] p < 0.001) intervention types. Interventions had greatest impact in healthy weight (18.5–24.9 kg/m2) (−0.82 kg [95% CI −1.5, −0.50] p < 0.001) or overweight (25.0–29.9 kg/m2) (−1.48 kg [95% CI −1.85, −1.12] p < 0.001) compared with obese populations (≥30.0 kg/m2) (−0.56 kg [95% CI −1.40, 0.27] p = 0.19). These findings demonstrate that lifestyle intervention prevents cumulative weight gain in non‐obese adults, with future research required to inform cost‐effectiveness and implementation feasibility.