Epidemiology and risk factors of nonalcoholic fatty liver disease (NAFLD)

The nonalcoholic fatty liver disease (NAFLD) is defined as the presence of hepatic steatosis, determined by either imaging or histology, in the absence of secondary causes of hepatic fat accumulation. Nonalcoholic fatty liver is defined as the presence of hepatic steatosis with no evidence of hepatocellular injury in the form of ballooning of the hepatocytes or fibrosis. NASH is defined as the presence of hepatic steatosis and inflammation with hepatocyte injury (ballooning) with or without fibrosis. Although initial epidemiological studies have focused on its prevalence in the Western countries, it is becoming increasingly clear that NAFLD is highly prevalent in the Asia Pacific region, and there may be important distinctions in its phenotype between Asia Pacific and Western countries. Of particular interest are "lean NAFLD" and the "urban-rural divide," which will be discussed in this review article. Obesity, dyslipidemia, type 2 diabetes and metabolic syndrome are established risk factors for developing NAFLD. Many other risk factors (e.g., hypothyroidism, polycystic ovary syndrome, obstructive sleep apnea, hypopituitarism and hypogonadism) for NAFLD have been described in the Western countries, but these associations are yet to be investigated adequately in the Asia Pacific region.