The spectrum of nonalcoholic fatty liver disease (NAFLD), including nonalcoholic steatohepatitis (NASH), is rapidly becoming the major cause of chronic liver disease in the United States.34,40 Additionally, NAFLD is increasingly being reported in Europe, the Middle East, and Asia.3,5,9,22,23,36
The clinical manifestation of NAFLD is the presence of hepatic steatosis in individuals without other causes of chronic liver disease, including excessive alcohol consumption. Although most patients with NAFLD do not seem to have a progressive course, the data from natural history studies suggest that patients with NASH—reportedly 10%–40% of all individuals diagnosed with NAFLD—are at risk for an advanced liver disease, including hepatic fibrosis and cirrhosis and hepatocellular carcinoma. Established risk factors for advanced liver disease in patients with NAFLD include older age, obesity, and type 2 diabetes.1,8,10,16–20,24–27,33,35,39 Increased liver-related mortality has also been reported for individuals with NAFLD, and especially its progressive forms.15,26,29,34
It is important to emphasize, however, that the data for most of the previously reported epidemiologic studies for NAFLD and NASH originated from tertiary care medical centers with inherent biases that could lead to an overestimation of the prevalence of NAFLD. On the other hand, the previously reported population-based studies relied on the liver enzyme elevation to determine the prevalence rates for NAFLD.15,26,34 Given that a large number of patients with NAFLD could have normal or intermittently normal aminotransferases, these prevalence rates probably underestimate the true prevalence of NAFLD. Thus, the true prevalence of NAFLD and NASH in the general population remains largely unknown, being estimated somewhere between 5% and 50% depending on the method used to establish the diagnosis of NAFLD and the population sampled.36
In the absence of a validated noninvasive biomarker for NAFLD and NASH, liver biopsy remains the “suboptimal” gold standard for diagnosing NAFLD and its progressive stages.4 However, given the invasive nature of liver biopsy, it cannot be used in large scale population-based studies of NAFLD.2,30 Radiologic modalities such as ultrasound are excellent for detecting significant amounts of hepatic steatosis.32 Although it does not help to stage the progress of liver damage such as hepatic fibrosis in patients with NAFLD, ultrasound can be used as a good screening tool for detecting hepatic steatosis.12 Specifically, as reported in a 2011 meta-analysis of 46 studies,6 the accuracy of ultrasound for the evaluation of hepatic steatosis included the sensitivity of 73.3%–90.5% and specificity of 69.6%–85.2% when compared to histopathology as the reference standard. That study points to the limitation of radiologic modalities such ultrasound for evaluating hepatic steatosis. Nevertheless, the overall accuracy of ultrasound, computed tomography, and magnetic resonance imaging (MRI) was found to be similarly high (p > 0.05) for patients with a significant amount of hepatic steatosis (25% or more). Despite the limitation of ultrasound for detecting lower amounts of hepatic steatosis, its relatively low cost and noninvasive nature make it an attractive choice for population-based studies of NAFLD.
The diagnosis of a progressive form of NAFLD, namely NASH, is also typically based on liver biopsy, with its inherent problems.2 Several serum biomarkers have been developed to diagnose NASH but none has been fully validated.4 Studies of patients with NAFLD, however, have determined that the presence of type 2 diabetes (diabetes mellitus, or DM) or insulin resistance (IR) significantly increases the likelihood of having NASH.38,40,41 Furthermore, in patients with NAFLD, elevated liver enzymes have also been associated with the presence of NASH.1,25 Therefore, patients with the diagnosis of NAFLD who also have DM or IR as well as elevated aminotransferases are likely to have histologic NASH. Short of a liver biopsy, this definition of NASH should be able to capture the vast majority of patients with NASH.38
The role of obesity in the pathogenesis of NAFLD and NASH has been firmly established.36 Although most individuals with NAFLD and NASH are overweight or obese,40 some NAFLD patients present without being such. The prevalence and independent predictors of NAFLD in lean individuals are unknown, as are the clinical characteristics and mechanisms of the pathogenesis of NAFLD in lean individuals compared to those in overweight or obese individuals. We conducted the current study to determine the prevalence and independent predictors of radiologic NAFLD and radiologic-clinical NASH in the United States population, with a particular focus on lean NAFLD.
PATIENTS AND METHODS
The study is based on data obtained from the National Health and Nutrition Examination Survey III (NHANES III) conducted by the United States National Center for Health Statistics of the Centers for Disease Control and Prevention between 1988 and 1994.11
The inclusion criteria used for the study were age 20 years or older; availability of hepatic ultrasound data; and availability of relevant demographic, clinical, and examination data. Body mass index (BMI), waist circumference, and blood pressure were measured for all NHANES participants at the time of examination. We used previously reported definitions for DM, IR, visceral obesity, hypertension, and hypercholesterolemia, as well as elevated transferrin saturation.34 For the purposes of the current study, IR was defined using a homeostasis assessment model (HOMA); a HOMA score of 3.0 or higher was used as diagnostic of IR. We defined excessive alcohol consumption as >20 g per day for men and >10 g per day for women.
A lean cohort was defined as individuals with BMI <25.0. Individuals with BMI of 25.0 or higher were considered overweight or obese.
The study cohort was assessed according to 4 major racial or ethnic groups: non-Hispanic whites, non-Hispanic blacks, Hispanics, and “other,” (Aleut, Eskimo, American Indian, Asian, or Pacific Islander).
We used the dietary intake data collected as a part of the Dietary Recall Interview to estimate intakes of energy, nutrients, and other food components from the foods and beverages consumed by the participants during a 24-hour period before the interview. Daily aggregates of food amount (in g), energy, and 96 nutrients/food components from all foods included in NHANES were calculated using the food database by the Nutrition Coordination Center [ftp://ftp.cdc.gov/pub/Health_Statistics/NCHS/nhanes/nhanes3/2A/EXAMDR-acc.pdf].
The Diagnosis of NAFLD
Clinical, laboratory, and ultrasound data were used to define NAFLD. For NHANES III participants, the archived hepatobiliary ultrasound video images have recently been re-reviewed by the National Center for Health Statistics, and the presence of fat within the hepatic parenchyma was graded as normal-mild or moderate-severe [http://www.cdc.gov/nchs/nhanes/nhanes3/HGUHS.htm]. Quality control and quality assurance procedures were used to standardize the readings of 3 ultrasound readers who had no access to any other participant data.
Subjects used for this study were presumed to have NAFLD if moderate to severe hepatic steatosis was found by ultrasound in the absence of any other possible cause of chronic liver disease such as excessive alcohol use or elevated transferrin saturation (both defined above), or a positive hepatitis B surface antigen (HBsAg) or hepatitis C antibody (anti-HCV) test (anti-HCV by enzyme-linked immunosorbent assay [ELISA] and HCV RNA by polymerase chain reaction [PCR]).
NAFLD patients (using criteria noted above) with elevated aminotransferases in the presence of either DM or IR (as defined above) were considered to have NASH. In this definition, elevated aminotransferases were determined as follows: alanine aminotransferase (ALT) of 40 U/L or higher or aspartate aminotransferase (AST) of 37 U/L or higher in men, and ALT or AST of 31 U/L or higher in women.
Individuals without steatosis on the ultrasound and other causes of chronic liver disease were presumed to have no chronic liver disease and were used as the control cohort.
Sample weights were used to account for nonresponse and unequal selection probabilities for certain categories of the population. After weighting on the basis of age, sex, level of education, and race or ethnic group, the distribution of participants was representative of that of the United States population.11 Additionally, stratum and sampling units accounted for the survey design effects using Taylor series linearization. Continuous variables (such as liver enzymes measured in standard units) were compared using a t-test for a contrasted mean. The prevalence of various parameters, including demographic parameters and metabolic syndrome components, was compared between subjects with NAFLD and controls in lean and overweight or obese groups separately, as well as between lean and overweight or obese NAFLD subjects, by the stratum-specific chi-square test for independence. Continuous variables such as nutrients measured in grams or milligrams were also compared in the same cohorts using a t-test for a contrasted mean. Individuals with NASH were further compared to the non-NASH NAFLD cohort. Finally, logistic regression was used to identify independent predictors of lean NAFLD, as well as predictors of NAFLD in lean and overweight or obese cohorts separately, using all clinical and demographic parameters. Predictors of NASH were studied in the entire cohort of individuals with NAFLD. P values of 0.05 or less (of 0.07 or less for multivariate analysis) were considered potentially significant. All analyses were run with SAS 9.1 and SUDAAN 10.0 (SAS Institute Inc., Cary, NC). The study was approved by the Inova institutional review board.
Of the initial study population (20,050 adult participants from NHANES III), 11,613 were considered eligible for the study by the inclusion criteria. Of those, 2492 individuals (18.77% ± 0.76%) fulfilled the definition of NAFLD, 307 (2.21% ± 0.20% of the entire study population, or 11.78% ± 1.03% of the NAFLD cohort) fulfilled our definition for NASH and 9121 were used as controls.
Comparing Lean NAFLD and Non-Lean NAFLD
Eligible participants were subdivided into lean (BMI <25) and overweight-obese (BMI >25). The most relevant clinico-demographic differences between the lean and non-lean individuals with NAFLD are noted in Table 1. As expected, NAFLD was significantly less common in lean individuals than in the overweight or obese individuals (7.39% ± 0.65% vs. 27.75% ± 1.00%, respectively; p < 0.0001).
Comparing Lean and Non-Lean Patients With NAFLD to Their Own Controls
Compared to controls without liver disease in the same weight cohort, both lean and overweight-obese patients with NAFLD were older, more commonly of Hispanic ethnicity, and had higher prevalence of components of metabolic syndrome (visceral obesity, IR, DM, hypercholesterolemia, and hypertension). Although their mean aminotransferase levels were within the normal range, both AST and ALT were higher in NAFLD patients in all weight groups, and so was the prevalence of individuals with abnormal aminotransferases (p < 0.05) (see Table 1). In additional to in-depth clinical and laboratory data, we used nutrition data to assess potential associations with lean NAFLD. The results indicated that none of the important nutrition components was associated with lean NAFLD (Table 2).
Multivariate analysis comparing lean NAFLD patients with lean control individuals without liver disease showed that lean NAFLD patients were more commonly Hispanic with DM and hypertension (Table 3).
A separate multivariate analysis comparing overweight-obese NAFLD patients to the overweight-obese controls without liver disease showed that overweight-obese NAFLD was independently associated with being non-African-American, and, specifically, with being Hispanic; male; of older age; and with IR, hypercholesterolemia, and hypertension (see Table 3).
Comparing Lean NAFLD Patients with Non-Lean NAFLD Patients
We performed an analysis to compare lean NAFLD patients to overweight-obese NAFLD patients. This analysis showed that in comparison to the overweight-obese NAFLD group, the lean NAFLD cohort was younger (p < 0.0001), more commonly female (p = 0.189), with significantly lower prevalence of IR (p < 0.0001), DM (p < 0.0001), hypercholesteremia (p < 0.0001), and hypertension (p < 0.0001). Furthermore, in comparison to overweight-obese NAFLD patients, lean NAFLD patients had lower AST (p = 0.0002) and lower ALT (p < 0.0001) levels, lower HOMA score (p < 0.0001), and lower platelet count (p = 0.0075) (see Table 1).
Multivariate analysis comparing lean NAFLD to overweight-obese NAFLD showed that lean NAFLD was independently associated with younger age, female sex, and a decreased likelihood of having IR and hypercholesterolemia (see Table 3).
Assessment of Predictors of NASH in the US Population
As noted previously, 307 individuals (2.21% ± 0.20% of the entire study cohort) fulfilled our definition for NASH. This NASH cohort represents 11.78% ± 1.03% of the NAFLD cohort. In fact, the prevalence of NASH was 0.10% ± 0.05% in the lean cohort (1.38% ± 0.62% of lean individuals with NAFLD) and 3.86% ± 0.35% in the overweight-obese cohort (13.92% ± 1.25% of overweight-obese individuals with NAFLD) (p < 0.0001).
Individuals with NASH were less commonly African-American than NAFLD patients without NASH (5.51% ± 1.19% vs. 8.87% ± 1.39%, respectively; p = 0.0054) and more commonly Hispanic (13.07% ± 2.33% vs. 6.88% ± 1.28%; p < 0.0001). They were more likely to be aged less than 45 years (56.24% ± 5.02% vs. 43.90% ± 2.25%; p = 0.0340) and less likely to be aged more than 65 years (9.38% ± 2.74% vs. 16.35% ± 1.32%; p = 0.0296) compared to their non-NASH NAFLD counterparts (Table 4).
Additionally, NAFLD patients with NASH were more likely than NAFLD patients without NASH to have visceral obesity (88.67% ± 3.41% vs. 63.81% ± 1.75%, respectively; p < 0.0001), hypercholesterolemia (90.16% ± 3.31% vs. 81.96% ± 1.51%; p = 0.0293), and hypertension (45.58% ± 5.29% vs. 34.05% ± 1.76%; p = 0.0360) (see Table 4).
Multivariate analysis comparing the NASH cohort to the non-NASH NAFLD cohort showed that NASH was independently associated with being Hispanic (odds ratio [OR], 1.72; 95% confidence interval [CI], 1.28–2.33) and less associated with being African-American (OR, 0.52; 95% CI, 0.34–0.78). Patients in the NASH cohort were more likely to be younger (age OR, 0.96; 95% CI, 0.94–0.98) and more likely to have components of metabolic syndrome (other than DM and IR, which were part of the definition), such as hypertension (OR, 2.44; 95% CI, 1.44–4.13) and BMI >25 (OR, 13.41; 95% CI, 4.80–37.51) (Table 5).
In the current study, using a nationwide representative sample of the United States population, we found the prevalence of radiologically demonstrated NAFLD to be 18% and that of NASH to be 2.6%. These rates are similar to those previously reported for NAFLD and NASH (as reviewed in Introduction). Furthermore, as expected, we found that the prevalence of NAFLD and NASH are much higher in overweight or obese individuals than in the lean population. In fact, the prevalence of NAFLD in the lean population was only 7% (compared to 28% in the overweight-obese population). A similar prevalence of lean NAFLD, as low as 9%, was previously reported from the Dallas Heart Study,7,14 although to our knowledge the current study is the first to evaluate the prevalence of lean NAFLD using a nationwide sample.
Although the prevalence of NAFLD in lean individuals is lower than that in overweight-obese individuals, a substantial number of lean individuals in the United States have NAFLD. A comparison of lean and overweight-obese patients with NAFLD to their respective controls without liver disease shows that lean NAFLD patients are more likely to have components of metabolic syndrome. Additionally, they are most likely to be Hispanic. In fact, comparing both types of NAFLD patients (lean and overweight-obese) to their own controls without liver disease shows that predictors of NAFLD in both lean and non-lean patients are similar to those found in previous reports.7,13,21,28,31 These data validate the independent association of NAFLD with components of metabolic syndrome and Hispanic ethnicity in a large population-based study.
It is noteworthy that a different picture arises from a comparison of lean NAFLD patients to overweight-obese NAFLD patients. In fact, lean NAFLD patients are younger and are less likely to have components of metabolic syndrome. Notably, similar factors associated with lean NAFLD, including younger age, lower prevalence of metabolic syndrome components, and better insulin sensitivity, have recently been reported from a tertiary care setting.37 These and our findings together with the absence of difference in dietary intake between lean individuals with and without NAFLD indicate that lean NAFLD patients may have other metabolic abnormalities that produce NAFLD not only at a younger age but also in the setting of less severe metabolic conditions. Other postulated causes of their NAFLD could include genetic predisposition, intestinal dysmotility, and other metabolic abnormalities not associated with weight gain.14
Finally, to our knowledge the current study is the first population-based study to report the prevalence and predictors of NASH using a strict radiologic, clinical, and laboratory definition. Prevalence rates for NASH reported here are comparable to those reported from other tertiary care centers, 3%–5%.24,25,33,36 We also show that, at the population level, NASH is more common in Hispanic patients and in those who have components of metabolic syndrome. This is important because NASH is the only subtype of NAFLD42 that has been convincingly shown to progress. If these estimates are correct, about 2.6% of the US population has NASH. According to the US Census Bureau, the US population was estimated to be about 307 million people in July of 2009 [http://2010.census.gov], so about 9 million Americans may have NASH. Because natural history studies have estimated that about 10%–15% of NASH patients can progress to advanced liver disease with higher liver-related mortality, the future burden of this liver disease could be enormous. This is especially important in light of recent reports showing that all other causes of chronic liver disease in the US are stable. On the other hand, the prevalence of NAFLD is steadily increasing.40 Furthermore, the US Census Bureau reports that between 2000 and 2010, the Hispanic population in the US grew by 43%, or 4 times the nation’s 9.7% growth rate [http://2010.census.gov]. As we report here, being Hispanic is independently associated with NASH and the progressive form of NAFLD. Given these data, we believe that NASH will soon become the most common and important cause of chronic liver disease in the US.
A major limitation of the current study is the limited accuracy of the ultrasound used to establish the diagnosis of NAFLD. This is especially true for patients who have less than moderate-severe (<25%) hepatic steatosis (as determined by ultrasound). In these cases, other modalities, such as MRI or proton magnetic resonance spectroscopy (1H-MRS), may be more accurate to detect smaller amounts of hepatic steatosis.6 It should be noted that in the current study, we included only individuals with moderate or severe steatosis. Since the performance of ultrasound improves when hepatic steatosis exceeds 25%, our approach may have minimized this potential limitation. Nevertheless, this approach has certainly led to an underestimation of the true prevalence of NAFLD (it does not account for mild [5%–25%] hepatic steatosis). In fact, the group of patients with mild steatosis may be an important proportion of patients with lean NAFLD. Additionally, the current study might have benefited from follow-up laboratory tests, which might help to evaluate the natural history of NAFLD at the population level.
In summary, our data provide the first estimate of the population-based prevalence rate for lean NAFLD. We report that lean individuals with NAFLD have a different clinical profile than overweight-obese patients with NAFLD. Patients with NASH are commonly Hispanic and have components of metabolic syndrome. Given these data, we believe that NASH will soon become the most common and important cause of chronic liver disease in the United States.
ALT: alanine aminotransferase
anti-HCV: hepatitisC antibody
AST: aspartate aminotransferase
BMI: body mass index
CI: confidence interval
DM: diabetes mellitus
HBsAg: hepatitis B surface antigen
HOMA: homeostasis assessment model
IR: insulin resistance
MRI: magnetic resonance imaging
NAFLD: nonalcoholic fatty liver disease
NASH: nonalcoholic steatohepatitis
NHANES: National Health and Nutrition Examination Survey
OR: odds ratio
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