Nonalcoholic Fatty Liver Disease NAFLD

Epidemiology of NAFLD

INTRODUCTION

Over the past 5 years several comprehensive reviews studied the epidemiology and natural history of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH). Among them, Byrne et al. [1] when discussing the metabolic disturbances in NAFLD, emphasized that since obesity and diabetes are increasing in prevalence worldwide, there is a marked increase in NAFLD, which occurs in individuals of all ages and ethnic groups, and at the same time, it is an independent risk factor for cardiovascular disease (CVD). Cheung and Sanyal also underlined that NAFLD is the most common cause of chronic liver disease in North America and some Western countries associated with the metabolic syndrome, and its prevalence is estimated to be as high as 35% in some populations linked to the growing epidemic of obesity [2]. Ratziu et al., based on the EASL 2009 special conference, outlined the concepts on NAFLD, as an increasingly relevant public health issue. Since recently NAFLD is a frequent condition, it often co-exists with other chronic liver diseases, such as alcohol abuse, and HCV infection or hemochromatosis, EASL suggested a shift in diagnostic concept of NAFLD from a negative diagnosis of exclusion (to drop the negative definition of nonalcoholic) to a positive one, based on the presence of an underlying condition, instead of the absence of an unrelated disease. Thus, according to EASL, there would be a need for a change in nomenclature, discarding the negative definition, and using a name of "metabolic steatosis or metabolic steatohepatitis" [3]. In their overview, Lewis and Mohanty pointed out that NAFLD affects approximately 20-30% of population in industrialized countries, males and females are both equally affected; however, some studies revealed that it is more common in men than women in morbidly obese and Asian populations [4]. In 2011, Vernon et al. in a paper summarized the epidemiology and natural history of NAFLD based on clinical literature published over the past 30 years. They emphasized that understanding the epidemiology of NAFLD is essential for developing effective treatment and prevention strategies, which is of pivotal importance, because the prevalence of NAFLD and NASH is expected to increase in the next decade, leading to advanced fibrosis, cirrhosis and even HCC [5]. Bhatia et al. [6] provided a state-of-art article of the evidence linking NAFLD with CVD, the potential mechanisms underlying this association, its relation to insulin resistance and the metabolic syndrome, in the context of increased CV risk. The role of NASH as a potential independent CV risk factor has gained considerable importance such as an awareness of this disease is essential for practicing cardiologists, given that it affects 20-30% of general population. Weiβ et al. [7] when reviewing the publications on NAFLD that appeared between 1995 and 2013 stated that at present some 5% to 20% of patients with NAFLD develop NASH, and fewer than 5% of cases progress to cirrhosis. Approximately 0.05% to 0.3% may be the prevalence of cirrhosis in the general population and 2% of cirrhotic patients per year develop HCC. Recently Bedogni et al. provided a concise overview of the epidemiology of NAFLD published between January 2011 and October 2013; they noted, that NAFLD may be a separate entity, rather than an additional component of metabolic syndrome, but it remains to be tested, whether metabolic syndrome and NAFLD contribute to hard outcomes in the general population independently [8].

PREVALENCE OF NAFLD AND NASH

The prevalence of NAFLD and NASH in general population depends on the diagnostic methods, such as liver biopsy, radiological and ultrasonic techniques, elevated liver enzymes, and combination of clinical variables.

Liver Biopsy and Autopsy Studies

Liver biopsy (LB) has long been regarded as gold standard for the diagnosis and staging of NAFLD and NASH, though it is an invasive measure and cannot be used in population-based studies. Yet, valuable findings have been reported for liver transplant donors who are considered healthy people. A Korean study in which LB-s were performed on 589 consecutive potential liver transplant donors reported NAFLD prevalence of 51% [9].

In the USA 20% of donors were ineligible for donation based on the degree of steatosis (>30%) [10]. A high prevalence of histological NAFLD has been described in healthy living liver donors: 12-18% in Europe [11, 12], and 27-38% in the USA [13, 14]. In healthy living liver donors, the prevalence of NASH ranges from 3% to 16% in Europe [11, 12], and from 6% to 15% in the USA [13, 14].

The results of autopsy series showed mixed data regarding the prevalence of NAFLD. In an autopsy series of lean individuals from Canada, the prevalence of NASH and fibrosis was 3% and 7%, respectively [15], while a study from Greece showed evidence of steatosis in 31% and NASH in 40% of autopsied cases of ischemic heart disease or traffic accident death after exclusion of HBV positivity or known liver disease [16]. Furthermore, the studies using current histological definition, have shown a high prevalence of NASH among steatosis cases: 43-55% in patients with increased aminotransferases [17], and 49% in morbidly obese patients [18, 19]. On the basis of autopsy studies it has been suggested, that 3-5% of individuals in the general population might have NASH [20, 21].

Ultrasound (US)

US as a non-invasive method is the technique most commonly used to assess NAFLD in the general population. Although, it may have a sensitivity of 84.8% and specificity of 93.6% for the detection of moderated fatty liver [20, 22], US is unable to diagnose steatosis when it is less than 20-30% on liver biopsy, thus the prevalence of NAFLD may be underestimated by this means. Yet, most general population studies are based on US studies. The prevalence of NAFLD in the general population is assessed by US is 20-30% in Europe [23], as well as in the Middle East [24], and 15% in the Far East [25].

Armstrong et al. [26] studying NAFLD in a large, primary care practice in the UK, identified 1118 asymptomatic individuals with increased liver function tests, and established NAFLD in 25% of subjects. In a multicentric population study from Spain, the prevalence of NAFLD by US was 33% in men and 20% in women [27].

In a study performed in middle-aged USA Army personnel seeking medical care for unspecified medical condition, the prevalence of US-defined and histo- logically confirmed fatty liver was 48% [28].

Over 10 to 12 years, the prevalence of NAFLD identified by US in 35 519 Japanese individuals increased from 13% to 30% [29].

Magnetic Resonance Imaging (MRI)

MRI has also been used to perform population studies, but is less portable and much more expensive than US [20, 30]. MRI identified NAFLD in 31% of multi-ethnic, population based samples from the USA [28].

Non-invasive Biomarker for Fibrosis: Fibrotest

When screening the general population with a noninvasive biomarker Fibrotest, for fibrosis, Poynard et al. found a prevalence of advanced fibrosis of 3% [31]. It is estimated that 25% of the adult Western population have NAFLD and only half of them have elevated liver function tests, and between 1% and 2% of the general population might have advanced fibrosis due to NASH [32].

Elevation in Liver Enzymes and Combinations of Clinical Variables

Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) as non-invasive indicators, showed the prevalence of NAFLD ranging from 3% to 21% in population studies [33, 34]. Although elevated ALT is generally associated with histological NASH, several patients with normal ALT may also have NASH and even advanced fibrosis. Thus, ALT activity alone cannot be used to rule out significant liver disease in patients suspected of having NAFLD and NASH [35].

Two combinations of clinical variables for NAFLD epidemiology studies are Fatty liver Index, FLI (BMI, waist circumference, triglyceride and GGT) [36 - 39], and Lipid Accumulation Product, LAP (waist circumference and fasting triglyceride) [40]. These indices may contribute to epidemiological studies.

Taken together, now it is widely accepted that 20-30% of individuals in Western world have NAFLD and similar figures are being provided for Eastern countries as well. NAFLD has become the most commonly US-diagnosed cause of chronic liver disease, partially in connection with the epidemic of obesity as well as with the greater awareness of the disease. Worldwide, the assumed prevalence of NAFLD is between 6% and 33%, with a median of 20%, while the prevalence of NASH is 3% to 5% [3, 5, 41, 42].

RISK FACTORS

Vernon et al. studying the risk factors of NAFLD have pointed out that age, gender, race and ethnicity as well as the metabolic alterations, all play a significant role in the development of the disease [5].

Age

The prevalence of NAFLD increases with age [43 - 47], and it is highest in males between 40 and 65 years [14]. Frith et al. showed an association between the prevalence of NAFLD and fibrosis with age; they found that older patients had higher grades of fibrosis, and cirrhotic patients were older than non-cirrhotic patients [48]. Another study performed in a cohort of octogenarians reported a prevalence rate of 46% [43].

Cryptogenic cirrhosis, (which is considered burned-out NASH) was found to be more common in older diabetic patients with current or past obesity [49]. Older age increases the risk of developing fibrosis, cirrhosis and hepatocellular carcinoma (HCC) as well as type 2 diabetes mellitus [50]. It seems that the association between age and the high prevalence of NASH as well as the higher stage of fibrosis in NAFLD may be related to the duration of disease rather than to age itself [5].

Gender

NASH was initially thought to be more common in women, and a few studies suggested that female gender is associated with NAFLD and fibrosis [51, 52]. On the other hand, some studies revealed that NAFLD is more common in men than women in morbidly obese and Asian populations [53, 54]. It is possible that NAFLD behaves differently in men and women, but the reason is not known yet.

Race, Ethnicity and Gene Variants

Prevalence of NAFLD differs with race and ethnicity. While Hispanics have the highest frequency of NAFLD, followed by non-Hispanic Whites, with the lowest rate reported in African-Americans [28, 55 - 57]. The prevalence of NAFLD in American Indian and Alaska-Native populations ranges from 0.4% to 2% [58]. The rising incidence of NAFLD in several Asian countries including China, Korea, Japan and India was reported. Asian patients with NAFLD display higher levels of visceral adiposity at lower body mass index than the Caucasian population [59]. Central obesity is associated with insulin resistance and may be a more important risk factor in Asian population than in Caucasian population [60]. The explanation for ethnic differences in the prevalence is not clear; it is possible that NAFLD may be affected by yet unidentified genetic and environmental factors.

Earlier Browning et al. [55] reported that these differences may be attributable to biologic differences rather than lower rate of insulin resistance. Since African-American (AA) individuals were less likely to develop steatosis than White and Hispanic subjects, it seemed that inherent biological differences, e.g. lower rates of hypertriglyceridemia and low density lipoprotein cholesterol levels among AA subjects might have contributed to this lower incidence of steatosis in AA population.

Family members of subjects with NAFLS are also at increased risk. The observed familiar clustering on NADFLD suggests the role of genetic factors. Some studies of family-based cohorts indicate that the heritability of steatosis is approximately 27% [61, 62].

According to the genom-wide associate studies, the most important genetic factor to NAFLD seems to be the I148M allele of the PNPLA3 gene, which encodes adiponutrin [63 - 65]. This allele is prevalent in Hispanics (49%), the group most susceptible to NAFLD, while lower frequencies were found in Caucasians (23%) and African Americans (17%). NASH occurred with higher prevalence in I148M homozygotes (OR 3.488) [66].

Metabolic Disorders

It is well known, that NAFLD is more prevalent in patients with metabolic alterations such as type 2 diabetes mellitus, obesity, hypertriglyceridemia than the general population. A study of patients with type 2 diabetes mellitus reported a 69% prevalence of ultrasonographic NAFLD. Similarly, the prevalence of NASH increases in parallel with components of metabolic syndrome [67 - 69]. Longitudinal studies revealed a chronological association between the progression of the metabolic syndrome and the occurrence of NAFLD [70].

In the USA, the proportion of NAFLD among patients with chronic liver disease rose from 47% to 75%, over a twenty-year period, between 1988 and 2008, due to the increase in metabolic diseases and the aging population. At the same time, the prevalence of obesity rose from 21% to 33%, type 2 diabetes mellitus from 5.6% to 9.1%, and insulin resistance from 23% to 35% [71, 72].

The frequency of simple steatosis in obese individuals ranges from 30% to 37% and NAFLD ranges from 57% of overweight individuals to 98% of nondiabetic obese patients. The prevalence of NAFLD and cirrhosis in a cohort of obese patients undergoing intraoperative liver biopsy and gastric bypass was 63% and 2%, respectively [73]. Others showed that over 95% of bariatric surgery patients had fatty liver, 20-30% has NASH and 10 have advanced fibrosis. The median prevalence of NASH in the obese population is 33% ranging from 10 to 56% [74, 75].

Smits et al. suggested that NAFLD may be a separate entity, rather than an additional component of metabolic syndrome [76], but it remains to be tested, whether MS and NAFLD contribute to hard outcomes in the general population independently [8].

Polycystic Ovarian Syndrome (PCOS)

Interestingly, polycystic ovarian syndrome (PCOS) was proposed as the ovarian manifestation of metabolic syndrome as 41-55% of PCOS women had concomitant NAFLD, while in the non-PCOS control group the incidence of NAFLD was 19% [77, 78]. Thus, PCOS patients may be at risk for developing NAFLD and NASH [5]. Many potential links between PCOS and NAFLD have been proposed, most notably insulin resistance and hyperandrogenemia. Further studies are needed to clarify the association between PCOS and NAFLD.

Hepatitis C Virus (HCV)

HCV infection may also be a risk factor for NAFLD. It was shown that HCV exacerbates metabolic syndrome, increasing insulin resistance [79]. Steatosis is present in approximately 50% of patients with chronic hepatitis C, especially often in association with HCV genotype 3 infection [79, 80].

NATURAL HISTORY

Regarding the long-term outcomes and in terms of potential for progression, NAFLD patients fall into two categories: NASH and non-NASH. The non-NASH subtype includes all individuals with simple steatosis, which does not progress or progresses very slowly [81]; on the other hand, NASH patients at diagnosis showed inflammation and even bridging fibrosis in 25-33% including cirrhosis in 10-15% [82].

Harrison et al. reported that based on paired liver biopsies, 32% of NASH patients showed progressing fibrosis over a median follow-up of 5.7 years [83]. Similarly, a repeat biopsy study showed that 53% of NASH patients had fibrosis progression by at least one stage after a median time of 6.4 years [52].

Independent predictors of fibrosis are mainly age (>45 years), diabetes, BMI (>28 kg/m²), hypertension, and degree of IR [84]. The strongest predictor of progression is necro-inflammation on the initial biopsy. End-stage NASH is a cause of cryptogenic cirrhosis, mainly because steatosis and liver cell injury can disappear at this stage. Approximately 30-75% of cryptogenic cirrhosis can be attributed to burned-out NASH [49].

Liver failure is often the first presentation of NASH cirrhosis, and occurs after 7-10 years in 38-45% of cirrhotic patients [85]. Obese and diabetic patients have an increased risk of hepatocellular carcinoma (HCC) [86]. While simple steatosis does not increase overall or liver-related mortality, NASH increases overall mortality by 35-85% compared to general population and cardiovascular mortality is increased twofold in NASH patients [87]. Liver related mortality is increased 9-10 fold with NASH cirrhosis [88].

According to Adams et al. [89] in a community-based report of NAFLD patients from the U.S.A, mortality rate was 13% after a mean follow-up of 7.6 years. When Rafiq et al. [90] assessed the mortality in biopsy proven NAFLD patients after 8 years follow-up, liver related mortality developed in 11% of NASH patients compared to 2% of the non-NASH patients. Ekstedt et al. found increased mortality in NASH after 13.7 years of follow-up, but not in patients with simple steatosis [87]. Furthermore, it was also shown that the lifestyle modification and weight loss, as well as the bariatric intervention (gastric bypass) may lead to the resolution of steatosis. Already 4% body weight loss can result in significant decrease in steatosis in 56% of patients [91]. An earlier study of 90 biopsied bariatric patients showed that 18% had the same degree of steatosis, histologically improved 28%, and 54% had normal histology by the second biopsy [92].

Bedogni et al. [8] and Kim et al. [93] also emphasized that while studies have shown that NASH is a risk factor for liver fibrosis, cirrhosis and HCC; at the same time, even fibrosis can regress. Serial biopsy studies demonstrated that fibrosis in NAFLD patients might improve. Wong et al. [81] found that 27% NAFLD patients had fibrosis progression, 48% had static disease, and 25% had fibrosis regression after 3 years of the initial diagnosis.

Taking these studies into consideration, based on histology data suggest that only patients with NASH show the progressive form of the NAFLD; in addition, insulin resistance, type 2 diabetes mellitus or other components of metabolic syndrome are risk factors for developing advanced liver injury. As patients with cryptogenic cirrhosis are, on average, 10 years older than patients with NASH, it is conceivable, that in patients with cirrhosis the disease started with NASH and on over a decade it progressed to cirrhosis.

Although NAFLD is mainly associated with obesity (e.g. with prevalence of 28%), it can be found even in lean individuals (BMI <25 kg/m²): a prevalence of 7% NAFLD was observed in such cases, characterized by younger age, higher insulin sensitivity and lower frequency of metabolic syndrome [26].

Finally, the association between NASH and hepatocellular carcinoma (HCC) may provide another type of indirect evidence supporting of the progressive nature of NASH [94]. Hashimoto et al. [50] found that with a median follow-up of 40.3 months the 5-year cumulative incidence of HCC was 8% among patients with NASH and advanced fibrosis. Bugianesi et al. showed a 7% overall prevalence of HCC in cryptogenic cirrhosis [95]. A multivariate analysis revealed that hypertriglyceridemia and type 2 diabetes mellitus are associated with cryptogenic cirrhosis and HCC. In another study, 91% of NASH related HCC patients were obese or had type 2 diabetes mellitus or dyslipidemia, suggesting that these risk factors may be involved in the pathogenesis of NASH related HCC [96].

BRIEFLY ON NAFLD AND CARDIOVASCULAR DISEASE (CVD)

Targher et al. [97] emphasized that the role of NAFLD as a potential CV risk factor is of considerable importance, and an awareness of this disease is essential for practicing cardiologists, given that it affects 20-30% of general population. Up to now, several studies reported an increased incidence of adverse cardiovascular (CV) events in NAFLD patients compared with the general population [98 - 103]. A significant association between increased gamma-glutamyl transferase (GGT) values and CVD mortality has been shown over a median of 12-year follow-up [100]. A meta-analysis of 10 studies also confirmed the independent association between elevated GGT and CV events [101]. Yet, alanine aminotransferase (ALT) has been reported to be more closely related to liver fat content than GGT [14], and population-based examinations have shown an independent association between ALT and CVD mortality [102]. The correlation of elevated ALT and GGT with CV disease reflected their significant associations with insulin resistance, which is itself a risk factor for CVD. Using US as a diagnostic tool for NAFLD, large, community-based studies also documented an independent association with CV disease [103]. Hamaguchi et al. performing a prospective analysis of 1637 healthy subjects found 19% NAFLD by US. After a 5-year follow-up, 5.2% of the NAFLD patients suffered from CV event compared with 1.0% of the non-NAFLD group [98]. Smaller long-term prospective studies in biopsy-proven NAFLD showed higher mortality rates compared with a matched reference population. Only patients with NASH rather than simple steatosis had significantly reduced survival [17, 87].

SUMMARY

NAFLD is present in 20-30% of general populations of industrialized countries and is the most prevalent chronic liver disease, often associated with obesity and diabetes mellitus. Among NAFLD patients, NASH occurs in 10-20%, thus the prevalence of NASH in the population is approximately is 2-5%. NASH accounts for more than 50% cryptogenic cirrhosis which may