Non-alcoholic fatty liver disease (
NAFLD) is histologically characterized by the aberrant accumulation of lipid droplets in the liver, which is positively correlated with
insulin resistance. Within the spectrum of this disease, patients can develop
hepatitis and
cirrhosis; i.e., non-
alcoholic steatohepatitis (NASH). The mechanisms responsible for the progression of
NAFLD are not fully understood.
Triacylglycerol (TAG), which is mainly found in lipid droplets, is currently considered to act as a
buffer against the accumulation of non-TAG toxic
lipid species. In line with this, recent studies have revealed that
insulin resistance is driven by the accumulation of
phosphatidic acid and
diacylglycerol in hepatocytes and that
cholesterol-overloaded stellate cells are associated with
fibrosis in the liver. Therefore, it is important to identify the toxic
lipid species that contribute to
NAFLD progression in order to clarify the pathogenesis of NASH and find novel targets for its treatment. In this review, we divided
lipids into five classes; i.e., into fatty acyls,
glycerophospholipids, glycerolipids,
sphingolipids, and
sterol lipids, and described their molecular structures, distributions, and metabolism under physiological conditions, as well as the contributions they make to the progression of
NAFLD.