Nonalcoholic fatty liver disease (NAFLD) is defined by steatosis in more than 5% of liver cells, in the absence of a secondary cause such as drugs, alcohol, or other causes. The incidence of NAFLD is increasing every day; almost a quarter of the world's adult population is affected by this disease. The burden of NAFLD is affected by the epidemics of obesity and type 2 diabetes (T2DM), and therefore, we do not expect the prevalence of this disease to decrease in the future. The world is now in the process of passing on health to non-chronic diseases, like NAFLD. The most common cause of chronic liver disease worldwide is non-alcoholic fatty liver disease. About 25 percent of the world's population is affected by the disease, and it ranges from simple steatosis to cirrhosis. 1 in 4 individuals with NAFLD is a person with non-alcoholic steatohepatitis, which is associated with complications and significant mortality and morbidity due to complications such as liver cirrhosis and hepatocellular carcinoma. Non-alcoholic fatty liver disease is closely related to metabolic syndrome, and it can be said that the liver is an integral part of obesity. Diagnostic methods for this disease include laboratory tests, imaging studies and liver biopsy. Although NAFLD is observed predominantly in obese persons or type 2 diabetes, an estimated 7% to 20% of people with NAFLD have lean body habitus. Recent studies have shown that fatty liver can occur in lean individuals, even without abdominal and visceral fat. Fatty liver in lean people (Lean NAFLD) is a relatively new concept that has attracted many people to find the differences between lean and obese people. The pathophysiological mechanisms of lean NAFLD are still poorly understood. Studies have shown that NAFLD without obesity is more closely related to factors such as environmental, genetic susceptibility, and epigenetic regulation. In addition to lifestyle modifications such as weight loss, diet and physical activity, only a few NAFLD-specific drug treatment options such as vitamin E and pioglitazone are considered. This article discusses the pathogenesis of fatty liver in lean individuals, its treatment, prognosis, and its relationship with metabolic syndrome.
