The Epidemiology of Nonalcoholic Fatty Liver Disease: A Global Perspective

Prevalence of NAFLD

Despite the limitations of the widely used diagnostic tools, it has become clear that NAFLD is an increasingly common problem in the United States and worldwide. Table 1 and Table 2 summarize the results of recent epidemiologic cross-sectional studies from the United States and other countries. Interpretation of the studies should take into account the source population and the operational definition of NAFLD.

As shown in the Table 1 ,^{[29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,54,55]} the majority of studies in the general population have used hepatic ultrasound as part of surveys or population studies. Results based on liver biopsy are mostly restricted to high-risk populations ( Table 2^{[61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85]}). The majority of studies excluded patients with a history of alcohol consumption and hepatitis on the basis of self-report data, but the criteria varied widely.

General Population

A study using proton-MRS to measure HT content determined that the prevalence of hepatic steatosis (5.5% or more fat) in the United States was 31%.^[46] In other countries, the prevalence of US-defined NAFLD in the general population ranges from 9.3 to 29% in Asia,^[39,56] 16% in Mexico,^[41] 23% in Italy^[57] and 30% in Israel.^[34] Fewer studies have used elevated liver enzymes to determine the prevalence of presumed NAFLD and the results ranged from 5.4% in the United States^[49] to 9.3% in Japan.^[48]

Estimates of steatosis using liver biopsies in the general population or low-risk groups have been reported in series of autopsies with non-liver-related deaths such as traffic and aviation accidents. The profile of people involved in these studies is only partially known, and information on alcohol consumption is largely hypothetical. Among these studies the prevalence of fatty liver ranged from 16% (in 1982) to 24% (in 1977).^[58,59,60] More recently, several biopsy studies among both living and cadaveric liver donors have been published. The prevalence of NAFLD among living liver donors ranges from 17.9% in Japan (in 2007)^[52] to 34% in the United States (in 2006).^[55] It is likely that these studies represent the best reflection of a low-risk population (apparently healthy, young, nonobese but may be overweight, with low or no alcohol consumption).

Bariatric Surgery Patients

At the other end of the distribution, data on the prevalence of NAFLD among high-risk individuals with severe obesity have become widely published ( Table 2 ).^{[61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85]} Patients are generally eligible for bariatric surgery if they have a body mass index (BMI) ≥ 40 kg/m² or ≥ 35 kg/m² with significant medical comorbidities such as type 2 diabetes. The patients in these series tend to be middle-aged and female. On average, these studies reported that 76% (range 33 to 99%) of the patients undergoing bariatric surgery have steatosis, 37% (range 9.8 to 72.5%) have NASH, 23% (range 7.3 to 49%) fibrosis, and 5.8% (range 1.6 to 10%) cirrhosis.^{[69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85]}

People with Type 2 Diabetes

Several studies have described a higher prevalence of NAFLD among people with type 2 diabetes compared with nondiabetics, with prevalence estimates ranging from 40% to 69.5%.^[61,86,87] Moreover, a recent case-control study using proton-MRS demonstrated that people with type 2 diabetes have on average 80% more liver fat than age-, weight-, and sex-matched controls. This difference was not explained by the type of medications used. Furthermore, aspartate aminotransferase (AST) and alanine aminotransferase(ALT) underestimated liver fat content among people with diabetes; for any given ALT or AST level, adults with type 2 diabetes had 40 to 200% more liver fat than nondiabetic adults.^[88] Patients with type 2 diabetes not only have a higher prevalence of NAFLD, but also appear to have more severe forms of the disease, including NASH and fibrosis.^{[82,89,90,91,92]}

Semin Liver Dis. 2008;28(4):339-350. © 2008  Thieme Medical Publishers