The long-term success of metabolic and bariatric surgery (MBS) depends largely on adherence to health-promoting behaviour following MBS. Especially, adherence to healthy dietary behaviour in line with guidelines appears to be of the utmost importance. The primary objective of the present study is to evaluate the effect of adherence to dietary behaviour recommendations and the percentage of total weight loss (%TWL) after MBS. Adherence is hypothesised to have a positive influence on the %TWL, 24 months after MBS. Furthermore, we investigate the association of various sociodemographic, organisational, psychological and behavioural factors prior to and after MBS and their effect on %TWL.

The present study is a single-centre observational, prospective, longitudinal cohort study conducted in Germany. Data are collected at nine measurement points (T0: 4 weeks prior to MBS; T1: 2 weeks after MBS, T2: 5 weeks after MBS; T3: 3 months after MBS; T4: 6 months after MBS; T5: 12 months after MBS; T6: 18 months after MBS; T7: 24 months after MBS; and T8: 36 months after MBS). Adherence to dietary behaviour recommendations is assessed using the Dietary Behavior Inventory-Surgery (DBI-S). N=325 patients applying for MBS will be included in the study. A regression analysis approach is chosen to answer the primary research question. The primary outcome %TWL is regressed at T7 (24 months after MBS) in a causal analysis on dietary adherence (DBI-S score) at T3–T7, with the covariates age, gender, marital status, educational attainment, employment status, Patient Health Questionnaire-4 score and body mass index at T0 and MBS method at T1. Stepwise hierarchical regression analyses are performed and analysed for significant model differences using ² difference tests. Effect sizes are estimated by R². Group differences are analysed using t-tests and Analyses of variance (ANOVAs). Bivariate correlations of continuous variables are examined using regression/correlation analyses.

The Ethics Committee of the Medical Faculty of the University of Essen-Duisburg has approved the conduct of the study (24-11969-BO). Results will be disseminated through manuscripts in clinical/academic peer-reviewed journals, presentations at academic conferences and communications with partners, participants and other stakeholders. Key findings will also be published in lay language on a publicly accessible website and disseminated via various (social) media channels.

The study has been prospectively registered on 8 October 2024 in the German Clinical Trials Register (DRKS00034888).