Time-restricted eating and gut microbiota alpha diversity in adults with overweight or obesity: an updated rapid systematic review.

BACKGROUND: Time-restricted eating (TRE) is widely used for weight management, but its effect on gut microbiota alpha diversity in adults with overweight or obesity remains uncertain. METHODS: We updated a rapid systematic review of intervention studies in adults with body mass index (BMI) of at least 25 kg/m2. The original searches covered PubMed, Web of Science Core Collection, Embase, and CENTRAL from inception to 10 February 2026. For the update period from 11 February through 4 August 2026, PubMed was rerun and Europe PMC, OpenAlex, and ClinicalTrials.gov were searched as supplementary discovery sources, with backward and forward citation checking. Eligible studies applied a daily eating window of 12 h or less for at least 2 weeks and reported an alpha-diversity metric. Risk of bias was assessed with RoB 2, ROBINS-I, or the JBI quasi-experimental checklist, as appropriate. Certainty was assessed with GRADE. Because study designs, comparators, metrics, and reporting were not sufficiently compatible, results were synthesized narratively and no meta-analysis was performed. RESULTS: Nine reports included 517 participants with microbiome data (14-138 per study), including five randomized comparative reports (n = 377), two non-randomized controlled studies, and two uncontrolled longitudinal analyses. Three randomized reports found no clear between-group change, one reported timing-dependent divergence between early and late TRE, and one reported increased richness measures after 6-h TRE. Non-randomized and uncontrolled reports generally found no clear change, but numerical estimates were frequently unavailable. The certainty of the overall evidence was very low because of risk of bias, inconsistency, imprecision, and incomplete outcome reporting. CONCLUSION: Available evidence does not establish either a beneficial effect or an absence of effect of TRE on gut microbiota alpha diversity. Timing-specific differences are plausible but remain hypothesis-generating. Adequately powered trials should prespecify microbiome outcomes, retain early and late TRE as separate interventions, and report paired and between-group estimates with uncertainty.