Planetary Health Diet and the Gut–Liver–Brain Axis: Implications for MASLD and Cognitive Aging

Background: Metabolic dysfunction-associated steatotic liver disease (MASLD) and cognitive aging are prevalent, interrelated conditions that share gut-derived pathomechanisms. The Planetary Health Diet (PHD), a sustainable plant-forward framework proposed by the EAT–Lancet Commission, may offer dual-target benefits for hepatic and neurological health through the gut–liver–brain axis. Methods: A structured narrative literature search of PubMed, Web of Science, and Scopus (2015–2026) was conducted to identify observational, mechanistic, and interventional studies addressing PHD adherence in relation to MASLD, cognitive aging, and gut–liver–brain axis pathways. Results: Epidemiological evidence from large cohorts links higher PHD adherence to lower MASLD prevalence and severity, slower cognitive decline, reduced dementia incidence, and more favorable Alzheimer’s disease neuropathology. Mechanistically, PHD components may modulate gut microbiota composition and diversity, enhance short-chain fatty acid production, regulate bile acid signaling through farnesoid X receptor (FXR) and Takeda G-protein-coupled receptor 5 (TGR5), reinforce intestinal barrier integrity, and attenuate metabolic endotoxemia and neuroinflammation. Conclusions: Although current evidence remains predominantly observational, the PHD is a promising dietary framework for simultaneously supporting liver and brain health. Future research should prioritize randomized controlled trials integrating liver imaging, cognitive assessment, microbiome profiling, and culturally adapted dietary interventions.