Clinical differences between alcoholic liver disease and nonalcoholic fatty liver disease

Abstract

Alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD) are serious health problems worldwide. These two diseases have similar pathological spectra, ranging from simple hepatic steatosis to steatohepatitis, liver cirrhosis, and hepatocellular carcinoma. Although most subjects with excessive alcohol or food intake experience simple hepatic steatosis, a small percentage of individuals will develop progressive liver disease. Notably, both ALD and NAFLD are frequently accompanied by extrahepatic complications, including cardiovascular disease and malignancy. The survival of patients with ALD and NAFLD depends on various disease-associated conditions. This review delineates the clinical characteristics and outcomes of patients with ALD and NAFLD by comparing their epidemiology, the factors associated with disease susceptibility and progression, and the predictors and characteristics of outcomes. A comprehensive understanding of the characteristics and outcomes of ALD and NAFLD is imperative in the management of these chronic liver diseases.

Key Words: Alcoholic liver disease; Nonalcoholic fatty liver disease; Clinical characteristics; Outcomes; Chronic liver disease

Core tip: Although alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD) have similar pathological spectra, from simple hepatic steatosis to steatohepatitis and liver cirrhosis, the epidemiological and clinical characteristics of these two diseases differ. Comparative analyses of the factors associated with disease susceptibility and progression and the predictors and characteristics of outcomes would be helpful in the management of these diseases. Notably, both ALD and NAFLD are frequently accompanied by extrahepatic complications, including cardiovascular disease and malignancy, which can influence patient survival.

INTRODUCTION

Excessive alcohol intake is associated with various diseases, including alcoholic liver disease (ALD), cardiovascular disease, and cancer[1]. The prevalence of alcohol use disorders increased dramatically in the sixteenth century when alcohol distillation became more widespread, and alcohol-associated diseases have continued to afflict humanity[2]. A recent review reported that alcohol substantially contributes to the global burden of disease and is responsible for 4.6% of disability-adjusted life-years and 3.8% of all deaths[1]. Moreover, alcohol has a negative effect on socioeconomic activities; the cost of excessive social drinking is 1% or more of the gross domestic product in high-income countries[3].

Excessive food intake can lead to overweight and obesity. An estimated 1.46 billion adults worldwide had a body mass index (BMI) of 25 kg/m² or higher in 2008, which reflects modern overnutrition[4]. Metabolic syndrome often develops in obese individuals, and nonalcoholic fatty liver disease (NAFLD) has been recognized as the liver manifestation of metabolic syndrome[5]. A recent study from the United States found that the annual healthcare costs were 1.6-fold higher for subjects with metabolic syndrome than for those without metabolic syndrome[6].

ALD and NAFLD are both serious health and socioeconomic problems worldwide. Although these diseases have similar pathological spectra, ranging from simple hepatic steatosis to steatohepatitis and liver cirrhosis[7], ALD and NAFLD differ from each other in many characteristics, ranging from differences in clinical features to patient outcomes. A comparison of these diseases may result in a better understanding and management of both ALD and NAFLD. Therefore, in this review, we comprehensively characterized ALD by comparing its clinical features and outcomes with those of NAFLD.

EPIDEMIOLOGY

Table 1 displays the epidemiology of ALD and NAFLD. Comprehensive data regarding the prevalence of ALD are scarce. Geographic differences in alcohol intake have been reported. For example, Eastern Europe has the highest annual per capita intake (15.7 L per person), while North Africa and the Middle East have the lowest (1.0 L per person)[8]. Although patterns of alcohol intake may affect the prevalence of ALD, studies from Europe, the United States, and Asia, in which hepatic steatosis was assessed using ultrasonography or magnetic resonance imaging, reported that the prevalence of NAFLD ranged from 12.9% to 46.0%[9-12]. A study from Japan demonstrated that the prevalence of NAFLD had increased over time from 12.6% in 1989 to 28.4% in 2000[13]. Moreover, a recent study from China, where the per capita annual alcohol intake is 4-6 L per person[8], found that the prevalence rates of ALD and NAFLD were 4.5% and 15.0%, respectively, and both rates are expected to increase in the future[14].

Table 1 Epidemiology of alcoholic liver disease and nonalcoholic fatty liver disease.

	ALD	NAFLD	Ref.
Prevalence rate (overall)	Unclear	12.9%-46.0%	[8-12]
	(rates may reflect geographical differences in the amount of alcohol intake)
Prevalence rate (China)	4.50%	15.0%	[14]
Trend (Japan)		Increasing (12.6% in 1989; 28.4% in 2000)	[8]
Peak age	45-69 yr (United States)	40-49 yr in males; 60-69 yr in females (Japan)	[12,15]
		60-69 yr (China)	[16]
Gender	Male dominant	Male dominant	[11,12,18-20]
Ethnicity	South American males > Afro-Caribbean males	Hispanics > Whites > Blacks	[21,22]

ALD: Alcoholic liver disease; NAFLD: Nonalcoholic fatty liver disease.

Age

A study from the United States revealed that the peak prevalence of hospitalization for alcohol-related conditions was between 45 and 69 years of age[15]. In contrast, the prevalence of NAFLD in a large Japanese study peaked at 40-49 years of age in males and at age 60-69 years in females[12]; however, a large study in China found that the prevalence of NAFLD in both males and females peaked at 60-69 years of age[16]. In addition, NAFLD is commonly observed in children; a recent autopsy study from the United States revealed that approximately 10% of children had NAFLD[17].

Gender

ALD is predominantly observed in males. For example, a cross-sectional study revealed that the male to female ratio of patients with alcoholic liver cirrhosis was 9:1[18]. A nationwide study in the United States that assessed patients discharged from the hospital following a diagnosis of ALD found that 4.5 per 100000 persons had acute alcoholic hepatitis, with a male to female ratio of 1.83:1, and that 13.7 per 100000 persons had chronic alcoholic hepatitis with cirrhosis, with a male to female ratio of 2.64:1[19]. Studies have also revealed a male predominance in the prevalence of NAFLD. For example, a study from the United States found that 58.9% of patients with NAFLD were male[11], and a large study from Japan reported that the prevalence rates of NAFLD in males and females were 41.0% and 17.7%, respectively. The prevalence rates of NAFLD were higher at all ages in males than in females, but the rate gradually increased in females with age, from 3.3% in the second decade of life to 31.3% beyond the sixth decade[12]. Similar trends were observed in the Chinese population[20].

Ethnicity

Studies have reported ethnic differences in the prevalence rates of ALD and NAFLD. For example, the prevalence rate of alcoholic liver cirrhosis was higher among South Asian males (32.8%) than among Afro-Caribbean males (1.1%) in the United Kingdom[21]. A recent large study of patients with NAFLD indicated that the prevalence of this disease was higher in Hispanics (24.2%) than in non-Hispanic Whites (17.8%) and non-Hispanic Blacks (13.5%)[22].

DIAGNOSTIC PROCEDURES

Differentiation

As mentioned above, ALD and NAFLD have similar pathological spectra, from simple steatosis to liver cirrhosis, which makes confident differentiation of the two diseases difficult. However, the following histological findings are helpful in the differential diagnosis[7]. The fatty degeneration of liver cells occurs to a greater degree in NAFLD than in ALD. In contrast, inflammatory cell infiltration is more pronounced in ALD than in NAFLD. Furthermore, venous or perivenular fibrosis, phlebosclerosis, and (less commonly) lymphocytic phlebitis are more common in ALD than in NAFLD.

Clinically, the differentiation between ALD and NAFLD is usually performed by taking a history of a patient’s alcohol intake combined with laboratory and imaging examinations; however, the reliability of these methods may not be high[23]. Therefore, discriminant indices consisting of clinical parameters have been developed[24-26]. For example, the ALD/NAFLD index (ANI) is calculated from a patient’s gender, BMI, aspartate aminotransferase (AST)/alanine aminotransferase (ALT) ratio, and mean corpuscular volume (MCV)[25]. This index has exhibited a high diagnostic accuracy, with an area under the receiver operating characteristic curve (AUROC) of 0.983 (cut-off value, 0; sensitivity, 93.5%; specificity, 92.0%) in the derivation set and AUROCs of 0.974, 0.989, and 0.767 in the three validation sets. A recent validation study confirmed the high accuracy of the index[27]. However, the ANI may be less reliable in patients with end-stage liver disease because these patients frequently have an elevated MCV and an increased AST/ALT ratio[25]. Because the ANI was formulated based on data sets mainly obtained from Caucasians, this index may not be applicable to other ethnic groups, suggesting the need for indices specifically for these groups.

Assessment of disease stage

Following a differential diagnosis, the assessment of disease severity and stage is vital for optimal patient management. Although a liver biopsy is the gold standard, this procedure is invasive and costly and is accompanied by patient discomfort, which affects its feasibility. Non-invasive methods for assessing the stage of fibrosis have been recently developed, including fibrosis indices and transient elastography[28]. Many fibrosis indices have been devised based on clinical parameters, and the usefulness of these indices has been tested in patients with chronic liver diseases. The AST/platelet ratio index (APRI)[29], the FIB-4 index[30], and the FibroTest[31] have been validated for use in assessing the fibrosis stage in patients with both ALD and NAFLD. The AUROC of the FibroTest for the diagnosis of liver cirrhosis in patients with ALD was 0.95 (cut-off value, 0.7; sensitivity, 91%; specificity, 87%)[32], while a more recent study found that the AUROCs of the APRI, the FIB-4 index, and the FibroTest for liver cirrhosis in patients with ALD were 0.67, 0.80, and 0.94 (cut-off value, 0.7; sensitivity, 86.6%; specificity, 86.0%), respectively[33]. In patients with NAFLD, the AUROCs of the APRI and the FIB-4 index for the diagnosis of advanced fibrosis were 0.73 and 0.80 (cut-off value, 2.67; sensitivity, 33%; specificity, 98%), respectively[34], and in another study, the AUROCs for the APRI and the FIB-4 index were 0.82 (cut-off value, 1; sensitivity, 67%; specificity, 81%) and 0.87 (cut-off value, 3.25; sensitivity, 48%; specificity, 95%), respectively[35]. Another study in patients with NAFLD indicated that the AUROCs of the FibroTest for the diagnosis of advanced fibrosis in two independent cohorts were 0.92 (cut-off value, 0.7; sensitivity, 25%; specificity, 97%) and 0.81(cut-off value, 0.7; sensitivity, 25%; specificity, 99%), respectively[36].

Transient elastography has also been shown to be useful in assessing the disease stage. For example, the accuracy of this method in assessing the fibrosis stage was validated for ALD, with AUROCs of 0.94 (cut-off value, 11.60 kPa; sensitivity, 87%; specificity, 89%) for advanced fibrosis and 0.87 (cut-off value, 22.70 kPa; sensitivity, 84%; specificity, 83%) for liver cirrhosis[37]. The AUROCs of transient elastography, the APRI, and the FIB-4 index for the diagnosis of advanced fibrosis in patients with NAFLD were 0.93 (cut-off value, 9.6 kPa; sensitivity, 63.6%; specificity, 83.7%), 0.74 (cut-off value, 0.5; sensitivity, 65.1%; specificity, 72.3%), and 0.80 (cut-off value, 2.67; sensitivity, 20.6%; specificity, 95.5%), respectively[38]. Generally, transient elastography cannot differentiate simple hepatic steatosis from mild hepatic fibrosis but can assess the degree of hepatic fibrosis.

Recently, a novel measurement method for assessing the hepatic steatosis grade was developed, which has been designated as the controlled attenuation parameter (CAP)[39]. This method is based on transient elastography and permits a simultaneous evaluation of the fibrosis stage and the steatosis grade. Studies have demonstrated the usefulness of CAP in quantitatively assessing the steatosis grade with a high accuracy[40-42]. A large cohort study found that the AUROCs of the CAP for the diagnosis of steatosis > 10%, steatosis > 33%, and steatosis > 66% were 0.79, 0.84, and 0.84, respectively, in 440 patients who underwent a liver biopsy[42]. This study also found that elevated CAP values were significantly associated with excessive alcohol intake, indicating a possible clinical application of CAP in the diagnosis and management of ALD.

Further validation studies are required to establish the usefulness of these non-invasive methods in patients with ALD and NAFLD.

Scoring systems for ALD and NAFLD

Scoring systems specific for ALD or NAFLD have been developed to assess disease severity and stage and/or to predict patient outcomes. Scoring systems for ALD[43,44] include Maddrey’s discriminant function (mDF), the glasgow alcoholic hepatitis score (GAHS), the Age-Bilirubin-international normalized ratio (INR)-Creatinine (ABIC) score, and the Lille score. Studies have demonstrated that these systems are useful in predicting the short-term survival of patients with alcoholic hepatitis[44,45]. The Brunt score[46] and the NAFLD activity score (NAS)[47] are histological scoring systems for NAFLD. Studies have suggested that the NAS has an excellent ability to differentiate simple hepatic steatosis from nonalcoholic steatohepatitis (NASH)[48]. Other scoring systems for the assessment of NAFLD stage, such as the NAFLD fibrosis score and the BARD score, have shown promising results[49].

FACTORS ASSOCIATED WITH DISEASE SUSCEPTIBILITY AND PROGRESSION

Simple hepatic steatosis occurs in most subjects who ingest excessive amounts of alcohol[2] or food[50]. However, only a small percentage of these individuals will develop advanced liver fibrosis or liver cirrhosis. Repeat liver biopsies at 4-year intervals in patients with ALD indicated that liver cirrhosis developed in 11% of the patients with simple hepatic steatosis and 39% of those with alcoholic hepatitis[51]. A recent cohort study of patients with ALD estimated that the 5-year risk of cirrhosis was 6.9% for patients with simple hepatic steatosis and 16.0% for patients with steatohepatitis[52]. Studies examining the histological course of NAFLD over a mean follow-up period of 3.2-13.8 years found that liver cirrhosis developed in 0%-8% of the patients without hepatic fibrosis and 11.3%-17.6% of those with hepatic fibrosis[53,54]. A long-term (approximately 20 years) follow-up study on simple hepatic steatosis observed that 22% of patients with ALD and 1.2% of those with NAFLD developed liver cirrhosis[55]. A recent autopsy study found that the ratios of simple hepatic steatosis to steatohepatitis or liver cirrhosis were 2.33:1 in ALD patients and 3.60:1 in NAFLD patients[56]. The differences in disease progression may be caused by environment-host interactions. Table 2 contains the environmental and host factors that have been suggested by clinical and/or basic research. Recent advances in genetics have provided further insight into the host factors that can affect disease progression.

Table 2 Rates of alcoholic liver disease and nonalcoholic fatty liver disease progression and the factors associated with susceptibility and progression.

	ALD	NAFLD	Ref.
Rate of progression from simple hepatic steatosis to liver cirrhosis	1.0%-3.1% per year	0%-2.5% per year	[51-55]
Rate of progression from steatohepatitis to liver cirrhosis	3.2%-12.2% per year	1.3%-3.5% per year	[51-54]
Environmental factors associated with disease susceptibility	Increased alcohol intake	Increased calorie intake	[18,50,57]
	Daily heavy drinking, not episodic or binge drinking		[18,60]
		Fructose	[62]
Environmental factors associated with disease progression	Increased amount of alcohol intake	Higher intake of soft drinks and meats	[58,61]
	Spirits rather than beer or wine	Fructose	[59,62]
Host factors associated with disease susceptibility
Age	Older age		[65,66]
Gender	Female		[57]
Body mass index	Obesity	Obesity	[22,58,67]
Metabolic syndrome	Presence	Presence	[22,71,72]
Type 2 diabetes	Presence	Presence	[71,72]
Ethnicity	Hispanic, Black	Hispanic	[11,73,74]
Genetic variant	PNPLA3 rs738409 G	PNPLA3 rs738409 G	[80-84]
Host factors associated with disease progression
Age	Older age	Older age	[58,67,68]
Gender	Female	No difference	[52,55,58,67]
Body mass index	Obesity		[58,67]
Metabolic syndrome
Type 2 diabetes
Ethnicity
Genetic variant	PNPLA3 rs738409 G	PNPLA3 rs738409 G	[80-84]

ALD: Alcoholic liver disease; NAFLD: Nonalcoholic fatty liver disease.

Environmental factors

The risks for the development and progression of ALD are increased as the intake of alcohol increases[18,57,58]. The type of beverage may also modify ALD progression. In a pooled cross-sectional time-series analysis, the consumption of spirits, rather than beer or wine, was associated with mortality from cirrhosis in primarily beer-drinking countries[59]. Drinking patterns are also factors associated with ALD. Studies have demonstrated that daily or near-daily heavy drinking, not episodic or binge drinking, is closely associated with ALD development[18,60]. Moreover, it was demonstrated that alcohol intake outside of mealtimes and the intake of multiple, different beverages increase the risk of developing ALD[18].

Similarly, a high total energy intake is positively associated with the development of NAFLD[50], and specific dietary components affect the pathogenesis of this disease. A cross-sectional study found that a greater intake of soft drinks and meats was associated with an increased risk of NAFLD[61]. Fructose may contribute to disease progression[62] and disease development[63], whereas the ingestion of n-3 polyunsaturated fatty acids may decrease intrahepatic fat deposition[64].

Host factors

Age: The ability to metabolize alcohol decreases with age because aging causes reductions in liver size and blood flow to the liver and decreases the activity of enzymes related to alcohol metabolism, such as alcohol dehydrogenase, acetaldehyde dehydrogenase, and cytochrome P-4502E1[65,66]. Thus, the livers of older subjects become more vulnerable to alcohol toxicity. Indeed, studies in patients with ALD revealed positive correlations between age and advanced fibrosis or liver cirrhosis[58,67]. Similarly, a systematic review of NASH patients revealed that aging was significantly associated with increased fibrosis[68]. The aging of the human population worldwide will likely have an impact on the progression of ALD and NAFLD.

Gender: Females have been found to be more susceptible to alcohol-induced liver damage than males. For example, a large prospective study found that the amount of alcohol intake at which the relative risk of ALD is greater than 1 was lower in females (7-13 beverages per week) than in males (14-27 beverages per week)[57], and multivariate analyses indicated that female gender was significantly associated with increased fibrosis in patients with ALD[58,67]. Studies of patients with simple alcoholic steatosis found that females were at a higher risk for cirrhosis than males[52,55]. In contrast, although a gender difference was observed in the prevalence of NAFLD, no significant gender difference was observed in the risk of increased fibrosis among patients with NASH[68].

Obesity, metabolic syndrome, and type 2 diabetes: Obesity predisposes the individual to the development of both ALD[58] and NAFLD[22]. Although a higher BMI was significantly associated with increased fibrosis in ALD patients[58,67], obesity was not significantly associated with increased fibrosis in patients with NAFLD[68]. Insulin resistance (IR) is a key factor in the development of metabolic syndrome[69] and is largely responsible for the development of type 2 diabetes. Recent studies have demonstrated the close relationship between ALD and IR[70] and have suggested that metabolic syndrome and type 2 diabetes are associated with the development of ALD[71]. In contrast, IR or metabolic syndrome and type 2 diabetes were found to be highly associated with NAFLD development[22,71,72] but not NAFLD progression[68].

Ethnicity: Ethnicity may influence the pathogenesis of ALD and NAFLD. For example, a large cross-sectional study revealed that, among current drinkers, the activities of the liver enzymes AST and γ-glutamyl transpeptidase (GGT) were likely to be 2-fold higher in black non-Hispanic and Mexican Americans than in white non-Hispanic Americans[73]. Another study suggested that African-Americans were more susceptible to alcohol-induced hepatotoxicity than whites[74]. Although the homeostasis model assessment of insulin resistance (HOMA-IR) was not a significant risk factor for NASH among Latinos (OR = 0.93; 95%CI: 0.85-1.02), the HOMA-IR was significant among non-Latino whites (OR = 1.06; 95%CI: 1.01-1.11), which suggests that ethnicity may modulate the effects of IR on the risk of NASH[75].

Genetic factors: Considerable effort has been expended to identify genetic factors that contribute to the pathogenesis of ALD and NAFLD[76,77]. Close links have been found between single nucleotide polymorphisms (SNPs) in DNA and the susceptibility to and severity of many diseases. The patatin-like phospholipase domain-containing 3 gene, PNPLA3, encodes adiponutrin[78], a protein thought to play an important role in lipid metabolism, such as the hydrolysis of triacylglycerols[79]. A recent genome-wide association study (GWAS) of NAFLD suggested that a PNPLA3 SNP, I148M (rs738409 C/G), is associated with increased hepatic fat content and hepatic inflammation[80]. Moreover, a recent meta-analysis[81] confirmed that this SNP strongly correlates with both hepatic fat content (patients with the GG genotype had 73% more hepatic fat than those with the CC genotype) and disease progression (the pooled ORs of the GG vs the CC genotype were 3.25 for having higher necroinflammatory scores and 3.26 for developing fibrosis). Notably, these findings were consistent across ethnic groups. Similar results were obtained in patients with ALD[82-84]. For example, a multivariate analysis revealed that the PNPLA3 SNP was more frequently found in ALD patients than in controls (OR = 1.54) and that this SNP was the strongest independent predictor of the progression of alcoholic liver cirrhosis (OR = 2.08)[84]. These findings regarding the PNPLA3 SNP may explain, at least in part, the ethnic differences in the susceptibility to and prevalence rates of ALD and NAFLD. For example, Hispanics have been reported to be more susceptible to ALD[73] and have the highest prevalence rate of NAFLD[11]; this is consistent with the frequency of the PNPLA3 SNP, which is highest among Hispanics[80].

Recent studies have found other PNPLA3 SNPs associated with NAFLD[85,86]. A recent GWAS of Japanese NAFLD patients identified rs2896019 and rs381062, SNPs that the investigators suggested are associated with the NAFLD grade and/or stage[86]. Additionally, this GWAS also found SAMM50 and PARVB SNPs, both of which were considered to be involved in the second hit of NAFLD, leading to a shift from simple steatosis to NASH. In contrast, a GWAS of Caucasian NAFLD patients identified FDFT1 and COL13A1 SNPs[87]. Another GWAS of Caucasian NAFLD patients identified NCAN and PPP1R3B SNPs, as well as a PNPLA3 SNP (rs738409), and demonstrated that these SNPs correlate with hepatic steatosis. Furthermore, it was found that SNPs in the NCAN, GCKR, LYPLAL1, and PNPLA3 genes correlated with histological lobular inflammation/fibrosis[88]. In addition, accumulated data have suggested that some SNPs, other than the PNPLA3 SNP, may be associated with the pathogenesis of ALD. A meta-analysis revealed a close relationship between a TNFA SNP (rs361525) and ALD[89]. Moreover, several studies have found a higher frequency of the CD14 -159 C/T SNP in patients with alcoholic cirrhosis than in those without this disease[76]. Despite extensive genome-wide searching, the PNPLA3 rs738409 G genotype is one of the few confirmed genetic factors contributing to a patient’s susceptibility to both ALD and NAFLD and to the progression of both diseases. Because this SNP may be useful in screening and managing high-risk patients and may constitute a therapeutic target, further studies are warranted.

PREDICTORS OF OUTCOMES

Predictors of hepatocarcinogenesis, hepatic decompensation, and mortality have been identified in patients with ALD and NAFLD (Table 3).

Table 3 Outcome predictors in patients with alcoholic liver disease and nonalcoholic fatty liver disease.

Outcomes	ALD	NAFLD	Ref.
Hepatocarcinogenesis
Underlying liver disease
Compensated cirrhosis	Not identified		[90]
Decompensated cirrhosis	Older age		[91]
Compensated/decompensated cirrhosis	PNPLA3 rs738409 G	Older age, any alcohol intake	[92-94]
	Not identified	Older age, elevated GGT, high CP score	[96]
Unknown stage		Older age, type 2 diabetes, elevated AST, low PLT	[95]
Hepatic decompensation
	Persistent alcohol intake		[90]
Mortality (overall)
Underlying liver disease
Simple hepatic steatosis	Low ALB, severe steatosis	Low ALB	[55]
Simple hepatic steatosis/steatohepatitis		Older age, type 2 diabetes	[101]
Advanced fibrosis/cirrhosis	Persistent alcohol intake, older age, smoking, low ALB	AST/ALT > 1, older age	[99,100]
Compensated cirrhosis	Persistent alcohol intake		[90]
Decompensated cirrhosis	Older age, alcohol abuse, elevated ALP		[97]
	Older age, persistent alcohol intake, low ALB, high MELD score		[98]
	Older age, poor liver function		[91]
Unknown stage		Metabolic syndrome, older age, smoking, Black	[102]
	Type 2 diabetes, insulin resistance	Type 2 diabetes, insulin resistance	[103]

ALD: Alcoholic liver disease; NAFLD: Nonalcoholic fatty liver disease; ALB: Albumin; ALP: Alkaline phosphatase; MELD: Model for end-stage liver disease; GGT: γ-glutamyl transpeptidase; CP: Child-Pugh; AST: Aspartate aminotransferase; PLT: Platelet; ALT: Alanine aminotransferase.

Hepatocarcinogenesis

Although a recent study of patients with compensated alcoholic cirrhosis failed to identify any risk factors significantly associated with hepatocarcinogenesis[90], a study of patients with decompensated alcoholic cirrhosis found that older age was significantly associated with hepatocarcinogenesis[91]. Moreover, the PNPLA3 SNP was reported to be closely associated with the susceptibility to hepatocellular carcinoma (HCC) in patients with ALD[92,93].

Older age and alcohol intake were found to be independent risk factors for hepatocarcinogenesis in patients with NASH-related cirrhosis[94]. A multivariate analysis of a large number of Japanese patients with NAFLD found that older age, type 2 diabetes, an elevated serum AST concentration, and a low platelet count were significantly associated with hepatocarcinogenesis[95].

A recent study compared hepatocarcinogenesis in patients with alcoholic liver cirrhosis and NASH-related cirrhosis. A multivariate analysis revealed that older age, an elevated serum GGT concentration, and a higher Child-Pugh score were risk factors for the development of HCC in patients with NASH-related cirrhosis, whereas no factor was found to be significantly associated with hepatocarcinogenesis in patients with alcoholic cirrhosis[96].

Hepatic decompensation

Persistent alcohol intake has been reported to predict hepatic decompensation in patients with alcoholic cirrhosis[90]. To our knowledge, however, no studies to date have assessed the risk factors for hepatic decompensation in patients with compensated NASH-related cirrhosis.

Mortality

A long-term cohort study analyzed the predictors of mortality in patients with ALD and simple hepatic steatosis and reported that a low serum albumin concentration and severe steatosis were significantly associated with increased mortality[55]. Moreover, persistent alcohol intake was found to be the only factor predictive of mortality in patients with compensated alcoholic cirrhosis[90]. A long-term follow-up study, in which most of the patients suffered from decompensated alcoholic cirrhosis, found that older age, alcohol abuse, and elevated alkaline phosphatase levels were risk factors for mortality[97]. In another long-term follow-up study involving patients with decompensated alcoholic cirrhosis, older age, persistent alcohol intake, a low serum albumin concentration, and a higher baseline model for end-stage liver disease (MELD) score were predictive of mortality[98]. Similarly, older age and poorer liver function were significantly associated with increased mortality in patients with decompensated alcoholic cirrhosis[91]. More recently, an analysis of patients with advanced, non-decompensated ALD revealed that smoking, a low serum albumin concentration, persistent alcohol intake, and older age were associated with increased mortality[99].

In patients with NAFLD and simple hepatic steatosis, only a low serum albumin concentration was found to be significantly associated with patient mortality[55]. A long-term cohort study involving patients with NAFLD found that an AST/ALT ratio > 1 and older age were associated with overall mortality, whereas higher serum bilirubin concentrations and stage 4 fibrosis were associated with liver-related mortality[100]. Older age and accompanying type 2 diabetes were significantly associated with increased mortality in patients with biopsy-proven NAFLD, whereas NASH, older age and type 2 diabetes were risk factors for liver-related mortality[101]. A recent, large population-based study revealed that metabolic syndrome was independently associated with overall mortality, liver-specific mortality, and cardiovascular mortality and that older age, smoking, and black race were risk factors for overall mortality[102].

A large cohort study found that type 2 diabetes and/or IR were independent predictors of overall mortality in patients with both ALD and NAFLD. Furthermore, type 2 diabetes, IR, obesity, and metabolic syndrome were independent predictors of liver-related mortality in both patient subsets[103].

COMORBIDITIES

ALD and NAFLD are frequently accompanied by extrahepatic complications. Therefore, we compared the comorbidities of patients with these two types of liver disease.

Metabolic syndrome and its related diseases

Excessive alcohol intake is highly associated with metabolic syndrome-related diseases, including hypertension, type 2 diabetes, and dyslipidemia[104-107], whereas NAFLD and metabolic syndrome are closely related[108]. Few studies to date have compared the associations of ALD and NAFLD with metabolic syndrome and/or related diseases. A recent study, however, found that an alcoholic fatty liver was more strongly associated with hypertension than a nonalcoholic fatty liver, whereas the latter was more strongly associated with dyslipidemia than the former[109]. Further studies are required to confirm these findings.

Cerebrovascular and cardiovascular diseases

Although little is known regarding the relationships between ALD and cerebrovascular and cardiovascular diseases, excessive alcohol intake has been reported to enhance the risks of these diseases, whereas light-to-moderate alcohol intake lowers these risks[110-112]. In comparison, a systematic review confirmed that NAFLD is an independent risk factor for cardiovascular disease[113], but it remains unclear whether NAFLD is a risk factor for cerebrovascular disease.

Extrahepatic malignancy

Although alcohol per se is not a carcinogen, excessive alcohol intake has been associated with HCC and other cancers, including oral cavity, pharyngeal, laryngeal, esophageal, colorectal, and female breast cancer[96,114]. A recent systematic review suggested a close relationship between NAFLD and colorectal cancer[113].

CHARACTERISTICS OF OUTCOMES

ALD and NAFLD are heterogeneous disorders that encompass a wide range of pathologies, from simple hepatic steatosis to liver cirrhosis and HCC. Furthermore, both diseases are frequently accompanied by extrahepatic complications. Patient survival can depend on various disease conditions. Although few studies to date have directly compared the outcomes of patients with ALD and NAFLD, we have delineated the characteristics of the reported outcomes (Table 4).

Table 4 Outcome characteristics of patients with alcoholic liver disease and nonalcoholic fatty liver disease.

Outcomes	ALD	NAFLD	Ref.
Hepatocarcinogenesis
Incidence rate
Simple hepatic steatosis/steatohepatitis		0%-0.2% per year	[115]
Steatohepatitis/cirrhosis		0.3%-4.3% per year	[115]
Compensated cirrhosis	0.7% per year		[90]
Decompensated cirrhosis	1.4% per year		[91]
Compensated/decompensated cirrhosis	2.5% per year	2.1% per year	[96]
Unknown stage		0.043% per year	[95]
Prevalence rate of non-cirrhotic liver as underlying liver disease	5.3%-12.0%	25.0%-58.3%	[116-118]
Comparison of survival	Similar to non-ALD, NBNC HCC	Better than ALD- or HCV-related HCC	[119,120]
Hepatic decompensation
Incidence rate
Compensated cirrhosis	4.4% per year	4.5% per year	[90,121]
Mortality (overall)
Incidence rate
Simple hepatic steatosis	3.3% per year		[52]
Steatohepatitis	5.0% per year		[52]
Steatohepatitis/cirrhosis		1.8% per year	[100]
Compensated cirrhosis	3.2% per year		[90]
Decompensated cirrhosis	5.7%-6.0% per year		[91,97]
Compensated/decompensated cirrhosis		5.0% per year	[124]
Compared with liver diseases due to other causes		Similar to HCV-related compensated liver disease	[100]
	Similar to HCV-related compensated cirrhosis	Better than HCV-related compensated cirrhosis	[90,122]
	Similar to HCV-related decompensated cirrhosis	Similar to HCV-related decompensated cirrhosis	[91,122]
		Similar to HCV-related cirrhosis	[124]
Improved survival	Abstinence		[123]
Causes of death
Simple hepatic steatosis			[55]
Liver-related causes	17%	2%
Arteriosclerosis	20%	38%
Extrahepatic malignancy	14%	17%
Infection	3%	8%
Cirrhosis			[90,97,98,122,124]
HCC	10%-13%	6.9%-47.4%
Liver failure	25%-60%	17.2%-31.6%
Cardiovascular disease	1%	27.6%
Cerebrovascular disease	1%-4%
Infection	8.9%-25%	41.4%
Extrahepatic malignancy	8%-25%

ALD: Alcoholic liver disease; NAFLD: Nonalcoholic fatty liver disease; HCC: Hepatocellular carcinoma; NBNC: Non-B, non-C; HCV: Hepatitis C virus.

Hepatocarcinogenesis

The cumulative incidence rates of HCC were 6.8% at 10 years in ALD patients with compensated cirrhosis[90] and 7.1% at 5 years in patients with decompensated cirrhosis[91]. In contrast, a systematic review found that the cumulative incidence rate of HCC at up to 20 years in patients with NAFLD or NASH, of whom few or none had cirrhosis, was 0%-3%[115]. In patients with NASH and cirrhosis, the cumulative rates of HCC ranged from 2.4% over 7 years to 12.8% over 3 years[115]. A large Japanese study of patients with NAFLD found that the annual rate of new HCC cases was 0.043%[95]. More recently, the 5-year cumulative rates of HCC were found to be comparable in patients with alcoholic (12.5%) and NASH-related (10.5%) cirrhosis[96].

Collectively, the incidence of HCC seems to be somewhat higher in patients with ALD than in patients with NAFLD. Most (88.0%-94.7%) ALD-related HCCs arise from cirrhosis, whereas 41.7%-75.0% of NAFLD-related HCCs arise from a non-cirrhotic liver[116-118]. Moreover, there was no difference in age between the patients with ALD-related HCC and those with NAFLD-related HCC, although the percentage of females was lower in the former than in the latter[116,119].

To our knowledge, no studies to date have compared the outcomes of patients with ALD- and NAFLD-related HCC. A study of patients who had non-hepatitis B/non-hepatitis C HCC found that the survival rates were similar between the patients with ALD-related HCC and those with non-ALD-related HCC, whereas the tumor recurrence rate was higher in the ALD group[120]. A comparison of the outcomes of curative treatment for HCC in patients with NASH and those with ALD or hepatitis C-related liver disease revealed that the cause of liver disease did not influence the recurrence-free survival according to a multivariate analysis, whereas NASH was significantly associated with an improved overall survival[119].

Hepatic decompensation

A study of the natural history of alcoholic cirrhosis found that the cumulative 10 year hepatic decompensation rate was 37.4%[90]. During the follow-up, only approximately one-fourth of these patients abstained from alcohol. In contrast, 45% of patients with compensated NASH-related cirrhosis developed hepatic decompensation during a follow-up of 10 years[121]. Ascites, a manifestation of hepatic decompensation, is frequently observed in patients with alcoholic cirrhosis[90,98,122], but the incidence of ascites is relatively low in patients with NASH-related cirrhosis[121].

Mortality and causes of death

The cumulative 5-year survival rates of patients with ALD were reported to be 83.3% in patients with simple hepatic steatosis and 74.9% in those with alcoholic steatohepatitis[52]. Furthermore, the cumulative 5-year survival rates in patients with compensated and decompensated alcoholic cirrhosis were reported to be 83.9%[90] and 71.3%[91], respectively. Abstinence from alcohol has the potential to increase the survival of patients with alcoholic cirrhosis[123]. In a study of the survival of patients with decompensated alcoholic and hepatitis C-related cirrhosis, a multivariate analysis revealed that the cause of liver disease did not affect survival[91].

A comparison of patients with NASH-related cirrhosis (mean Child-Pugh score 6.1) and hepatitis C-related cirrhosis (mean Child-Pugh score 6.1) revealed that the cumulative 5-year survival rates were 75.2% and 73.8%, respectively[124]. A similar study examining the natural history of advanced fibrosis or compensated cirrhosis due to either NASH or hepatitis C found that the 10-year cumulative survival rates were 81.6% and 82.0%, respectively[100].

A recent study of patients with NASH-related or alcoholic cirrhosis who underwent liver transplantation found no significant differences in the post-transplant survival and cardiovascular mortality rates. However, cardiovascular causes of post-transplant death were more frequent in the NASH group, while malignancies were more frequent in the alcoholic group[125]. In addition, studies have reported that the overall survival after liver transplantation was excellent in patients with HCC and ALD or NAFLD[126,127].

The causes of death have been found to differ in patients with ALD and NAFLD. A long-term follow-up of patients with simple hepatic steatosis revealed that many of the patients with ALD and NAFLD died of extrahepatic, rather than hepatic, causes. For example, the main causes of death in ALD patients were arteriosclerosis (20%), liver cirrhosis (17%), unknown causes (16%), and extrahepatic cancers (14%), whereas the main causes of death in NAFLD patients were arteriosclerosis (38%), unknown causes (19%), extrahepatic cancers (17%), and infections (8%)[55]. Similarly, studies of patients with alcoholic cirrhosis indicated that a significant percentage of individuals died of extrahepatic causes, with the leading causes being liver failure (25.0%-36.0%), bacterial infection (11.5%-25.0%), extrahepatic cancers (8.0%-25.0%), and HCC (12.5%-13.0%)[90,98]. A population-based study found that ALD contributed to liver-related but not cardiovascular mortality[128]. Moreover, recent studies of patients with histologically proven NAFLD, of whom 41.6%-59.2% had NASH, found that the leading causes of death were cardiovascular disease (27.8%-28.2%), liver-related causes (15.4%-26.1%), and extrahepatic malignancy (15.7%-17.9%)[101,129]. Similar patterns were observed in other studies of patients with NAFLD, with the leading causes of death being extrahepatic malignancy (28.0%-33.3%), liver-related complications (12.8%-19.0%), and cardiovascular disease (19.0%-25.0%)[130-133]. In one study of patients with NASH-related cirrhosis, the major causes of death were infection (41.4%), cardiovascular disease (27.6%), and liver-related complications (24.1%)[121], whereas the causes were HCC (47.4%) and liver failure (31.6%) in a second study[124].

In summary, the mortality rates of patients with ALD and NAFLD were found to be similar, although the causes of death differed somewhat. When compared with patients with hepatitis C, larger percentages of patients with ALD and NAFLD died of extrahepatic causes; many ALD patients died of infection and extrahepatic cancers, and many NAFLD patients died of cardiovascular disease.

CONCLUSION

In this review, we attempted to delineate the characteristics and outcomes of patients with ALD and NAFLD, particularly from a clinical aspect. Both liver diseases are generally related to unhealthy lifestyle habits, including excessive alcohol and food intake, and both are likely to be serious health problems in the future. In contrast to chronic viral liver diseases, ALD and NAFLD are frequently accompanied by extrahepatic diseases that can influence patient survival. A comprehensive understanding of these diseases is essential for their management.