S2068 The Role of Intermittent Fasting in Shaping Gut Microbiota and Improving Metabolic Health: A Systematic Review

Introduction: Intermittent fasting (IF), including time-restricted feeding (TRF) and religious fasting such as Ramadan, has gained popularity due to its metabolic benefits, including weight loss, improved insulin sensitivity, and reduced inflammation. The gut microbiome is emerging as a key mediator of these effects, playing a central role in energy balance, immune regulation, and metabolic homeostasis. This systematic review summarizes human studies assessing the impact of IF on gut microbiota composition and related metabolic parameters. Methods: The search was conducted on Ovid MEDLINE, Ovid Embase, and Cochrane Central databases to identify studies evaluating the effects of IF on gut microbiota. Eligible studies included randomized controlled trials (RCTs), prospective and retrospective observational cohorts, and interventional studies involving adult participants. Included studies assessed gut microbiota composition, diversity, or function using 16S rRNA gene sequencing or comparable microbial profiling methods, alongside metabolic or clinical parameters such as insulin sensitivity, body weight, liver function, or inflammatory markers. Results: IF consistently led to significant shifts in gut microbiota composition, with increases in microbial diversity and beneficial taxa such as Akkermansia, Faecalibacterium, Bacteroides, and Prevotella. TRF protocols, particularly early TRF (e.g., 8 AM–2 PM), improved insulin sensitivity, reduced adiposity, and enhanced microbial richness. Early TRF showed greater improvements in metabolic outcomes compared to mid-day TRF. Ramadan fasting also resulted in greater species diversity, decreased inflammatory markers, and improved gut hormone profiles (glucagon-like peptide-1, PYY, leptin, and CCK), with minimal impact on ghrelin. Several studies found IF altered the Firmicutes-to-Bacteroidetes ratio and correlated with reductions in body mass index, waist circumference, triglycerides, and systemic inflammation. Importantly, baseline microbiota composition influenced individual responsiveness to IF, with greater initial microbial diversity or abundance of certain taxa was linked to more favorable metabolic outcomes. Conclusion: Human studies demonstrate that intermittent fasting, both religious and structured, modulates gut microbiota composition and diversity, with beneficial effects on metabolic health, liver function, and hormone regulation. These results support IF as a promising lifestyle intervention targeting the gut microbiome in the prevention and management of metabolic diseases.