FROM LOW-FIBER TO NO-FIBER: A SYSTEMATIC REVIEW COMPARING THE GUT MICROBIOME AND INFLAMMATORY IMPACTS OF KETOGENIC VS. CARNIVORE DIETS

Background: The ketogenic diet (KD) and the emerging carnivore diet (CD) represent two extremes of carbohydrate restriction, with distinct implications for gut microbiome ecology and inflammation. Although both regimens minimize dietary fiber intake, their comparative effects on microbial composition and immune modulation remain unclear. Objectives: This systematic review aimed to synthesize current evidence on the impact of ketogenic and carnivore diets on gut microbiota structure, microbial metabolites, and inflammatory outcomes in human and animal studies. Methods: A comprehensive search of PubMed, Scopus, and Web of Science databases was conducted according to PRISMA guidelines, including studies published between 2014 and 2025. Eligible studies evaluated microbiome composition or inflammatory biomarkers following adherence to KD or CD. Data were extracted and compared narratively due to heterogeneity in study designs. Results: Across reviewed studies, ketogenic diets consistently reduced saccharolytic taxa (e.g., Bifidobacterium, Roseburia) and short-chain fatty acid production, yet in some cases improved systemic inflammation through β-hydroxybutyrate–mediated mechanisms. In contrast, carnivore diets led to further reductions in microbial diversity, enrichment of bile-tolerant taxa, and elevated proteolytic fermentation byproducts linked to intestinal barrier dysfunction. Evidence for long-term adaptation or recovery remains limited. Conclusions: Both KD and CD reshape the gut ecosystem toward reduced fermentation capacity and altered inflammatory signaling. While ketogenic interventions may confer transient metabolic benefits, the carnivore diet represents an extreme model of fiber deprivation with uncertain safety. Controlled longitudinal trials are urgently needed to define the long-term microbiome and immune consequences of these ultra-low-fiber regimens.