Epidemiological aspects of obesity and NASH/NAFLD in Japan.

Although the characteristics of non-alcoholic steatohepatitis (NASH) and non-alcoholic fatty liver disease (NAFLD) have become well recognized, mainly in the clinical and pathological fields, epidemiological findings, including magnitude of the disease in a population (prevalence, outcomes, and mortality) and risk factors (relative risk and population attributable risk) are scarce in the Asia-Oceania region. Obesity is known to be one of the most important risk factors for NASH/NAFLD, and observed increases in NASH/NAFLD in selected areas or settings may be linked to the increase in overweight and obesity in Japanese adults in the last few decades. Obesity could be a major key to explaining NASH/NAFLD, because of the complex causal web that includes obesity, insulin resistance, dyslipidemia, high blood pressure, etc. In this paper epidemiological data on overweight and obesity were summarized as underlying factors for NAFLD with special reference to yearly trends and regional differences in the prevalence of obesity and overweight in Japanese adults using large-scale representative samples from the National Nutrition Surveys. Unfortunately, as very few published reports on population-based epidemiological data for NAFLD are currently available, we referred to selected data obtained from field settings to estimate the prevalence of NAFLD in the general population and its risk factors or associated factors. It is important to understand NASH/NAFLD as a common "burden of disease" in the populations if effective strategies to control these disorders are to be devised as part of public health initiatives.