The metabolic basis of nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is a major cause of chronic liver disease and is associated with significant morbidity and mortality worldwide, with a high incidence in Western countries and non-Western countries that have adopted a Western diet. NAFLD is commonly associated with components of the metabolic syndrome, type 2 diabetes mellitus and cardiovascular disease, suggesting a common mechanistic basis. An inability to metabolically handle free fatty acid overload-metabolic inflexibility-constitutes a core node for NAFLD pathogenesis, with resulting lipotoxicity, mitochondrial dysfunction and cellular stress leading to inflammation, apoptosis and fibrogenesis. These responses can lead to the histological phenotype of nonalcoholic steatohepatitis (NASH) with varying degrees of fibrosis, which can progress to cirrhosis. This perspective review describes the key cellular and molecular mechanisms of NAFLD and NASH, namely an excessive burden of carbohydrates and fatty acids that contribute to lipotoxicity resulting in hepatocellular injury and fibrogenesis. Understanding the extrahepatic dysmetabolic contributors to NASH is crucial for the development of safe, effective and durable treatment approaches for this increasingly common disease.