Dietary Periodicity Disrupts the Gut Microbiota–Enterolactone Axis to Exacerbate MASLD in a Translational Guinea Pig Model

Background/Objectives: Metabolic dysfunction-associated steatotic liver disease (MASLD) is closely linked to Western dietary patterns. Yet, preclinical studies rely on continuous high-fat feeding, overlooking the intermittent nature of human eating. Whether dietary periodicity itself influences the gut–liver axis and MASLD pathogenesis remains unknown. We compared continuous and intermittent high-fat, high-cholesterol (HFHC) diets in guinea pigs, a model that mirrors human lipoprotein metabolism, hepatic cholesterol handling, and hindgut fermentation. Methods: Integrated multi-omics analyses (serum metabolomics, fecal 16S rRNA sequencing, and liver transcriptomics) were employed in a guinea pig model, stratified into normal diet (ND), intermittent HFHC diet (IHD), and IHD with flaxseed lignan supplementation, to compare the effects of dietary regimens and the therapeutic efficacy of lignan on hepatic pathology. Results: Both diets induced hallmark hepatic features of MASLD; however, the intermittent regimen provoked significantly more severe hepatic steatosis, inflammation, oxidative stress, and fibrosis. Hepatic transcriptomic analysis identified the PI3K-Akt signaling pathway as the most significantly enriched pathway in the IHD group. Mechanistically, this aggravated liver injury was linked to gut microbiota dysbiosis and a marked depletion of the microbial metabolite enterolactone. Fecal microbiota transplantation confirmed that the dysbiotic microbiota directly transmits the aggravated liver injury phenotype. Supplementation with flaxseed lignan, the dietary precursor of enterolactone, restored enterolactone production, corrected the dysregulated gut microbiota–enterolactone axis, and largely normalized the expression of PI3K-Akt downstream targets, thereby alleviating hepatic pathology. Conclusions: These findings suggest that alterations in the gut microbiota–enterolactone axis may contribute to diet-periodicity-driven liver injury and highlight its potential involvement in disease progression. Modulating this axis through dietary interventions, such as flaxseed lignan supplementation, may represent a promising nutritional strategy for mitigating MASLD associated with cyclical dietary exposure.