Non-alcoholic fatty liver disease and the risk of dementia

Liver Diseases and Brain Disorders: Genetic Mechanisms and Biomarker Pathways in a Prospective Cohort Study From the UK Biobank

Population-based evidence directly linking liver diseases to brain disorders is limited, and its genetic and biochemical associations remain unclear. Our aim is to examine the links between liver diseases and brain disorders. This prospective cohort study utilized data from 492 059 participants in the UK Biobank. We identified 508 cases of alcoholic liver disease (ALD), 583 cases of non-alcoholic fatty liver disease (NAFLD), and 557 cases of viral hepatitis (VH) based on International Classification of Diseases (ICD) codes. Initially, we employed multiple linear and logistic regression to assess associations between liver diseases, polygenic risk score (PRS), inflammatory and metabolic biomarkers, and brain function. Cox proportional hazard models were then applied to determine the impact of liver diseases on the incidence of brain disorders. Ultimately, structural equation models were used to explore potential genetic and biomarker pathways. During a median follow-up of 14.46 years, participants with ALD, NAFLD, and VH demonstrated poorer cognition, mental health, and motor function compared to the healthy group, with p < 0.05 for false discovery rate (FDR-Q < 0.05). They exhibited increased risks for dementia (hazard ratios [HRs]: 2.28-4.10; FDR-Q < 0.001), major depressive disorder (HRs: 2.25-3.23; FDR-Q < 0.001), and generalized anxiety disorder (HRs: 1.70-2.66; FDR-Q < 0.01). Additionally, C-reactive protein, neutrophil-to-lymphocyte ratio, platelets, and low-density lipoprotein lipid components mediated the associations between PRS, liver diseases, and brain disorders. Our findings demonstrated that liver diseases were risk factors for brain disorders, with genetic and biochemical associations contributing to these risks.

Emerging Links between Nonalcoholic Fatty Liver Disease and Neurodegeneration

The association between liver and brain health has gained attention as biomarkers of liver function have been revealed to predict neurodegeneration. The liver is a central regulator in metabolic homeostasis. However, in nonalcoholic fatty liver disease (NAFLD), homeostasis is disrupted which can result in extrahepatic organ pathologies. Emerging literature provides insight into the mechanisms behind the liver-brain health axis. These include the increased production of liver-derived factors that promote insulin resistance and loss of neuroprotective factors under conditions of NAFLD that increase insulin resistance in the central nervous system. In addition, elevated proinflammatory cytokines linked to NAFLD negatively impact the blood-brain barrier and increase neuroinflammation. Furthermore, exacerbated dyslipidemia associated with NAFLD and hepatic dysfunction can promote altered brain bioenergetics and oxidative stress. In this review, we summarize the current knowledge of the crosstalk between liver and brain as it relates to the pathophysiology between NAFLD and neurodegeneration, with an emphasis on Alzheimer's disease. We also highlight knowledge gaps and future areas for investigation to strengthen the potential link between NAFLD and neurodegeneration.