Non-alcoholic fatty liver disease and cardiovascular risk

International Journal of Cardiology 167 (2013) 1109–1117

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International Journal of Cardiology journal homepage: www.elsevier.com/locate/ijcard

Review

Non-alcoholic fatty liver disease and cardiovascular risk Angel Brea a, José Puzo b,⁎ a b

Unidad de Lípidos, Servicio de Medicina Interna, Hospital San Pedro, Logroño, Spain Unidad de Lípidos, Bioquímica Clínica, Hospital San Jorge, Huesca, Spain

a r t i c l e

i n f o

Article history: Received 12 January 2012 Received in revised form 26 May 2012 Accepted 15 September 2012 Available online 9 November 2012 Keywords: Fatty liver Cardiovascular disease Alcohol Non-alcoholic fatty liver disease

a b s t r a c t The term “Non-alcoholic fatty liver disease” (NAFLD) covers a series of liver lesions similar to those induced by alcohol, but not caused by alcohol use. The importance of NAFLD lies in the high prevalence in Western societies and, from the point of view of the liver, in its progression from steatosis to cirrhosis and liver cancer. More recently, NAFLD has been found to be associated with lipid metabolism disorders, the deposition of fat outside of the adipocytes, insulin resistance and Metabolic Syndrome. Also attributed to NAFLD is a heightened systemic pro-inflammatory state, which accelerates arteriosclerosis, thereby increasing cardiovascular risk and associated cardiovascular events. Here we provide an update to the etiopathogenesis of NAFLD, its influence on cardiovascular disease, and the treatment options. © 2012 Elsevier Ireland Ltd. All rights reserved.

1. Introduction

2. Epidemiology

The term “Non-alcoholic fatty liver disease” (NAFLD) is used to describe a wide range of liver disorders which, histologically, are very similar to those caused by alcohol. NAFLD develops in patients who do not drink alcohol, or who drink less than 20 g/day, which is the figure considered being the limit above which alcohol can cause liver damage [1]. NAFLD was first described almost 60 years ago [2], although it would be another thirty years before it was recognised as such by pathologist Jurgen Ludwig [3]. NAFLD refers to a series of liver conditions that range from simple fatty infiltration of the hepatocytes (steatosis) to inflammation and the development of fibrosis: “Non-alcoholic steatohepatitis” (NASH), with potential for progression to cirrhosis and liver cancer [4]. The high prevalence of NAFLD in Western societies was described at the end of the last century, but more recently it has been confirmed that not only can it be fatal due to development of chronic liver disease, but that it may also be associated with lipid metabolism disorders and increased cardiovascular risk. This review provides an update to available literature on etiopathogenesis of NAFLD, its influence on development of cardiovascular disease (CVD), and current treatment options.

NAFLD is an asymptomatic disease, which does not cause significant changes in transaminase levels in two thirds of those who suffer it. Its prevalence is determined through abdominal ultrasound, although this technique requires at least a fatty infiltration of a third of the liver parenchyma to detecting steatosis. Even with these limitations, it is estimated that NAFLD affects 20–30% of the population in the West [5]. NASH, diagnosed by liver biopsy would then affect 2–3% of the population [6]. The prevalence of NAFLD varies according to race, ranging from 45% in Hispanics to 33% in Caucasian Americans, 24% in African Americans [7] and 25% in Asians [8]. It tends to be more frequent in men than in women (42% versus 24% respectively), although may be higher among postmenopausal women [9]. The prevalence of NAFLD increases with age, from less than 20% in people under the age of 20 to more than 40% in over 60s. Nevertheless, NAFLD has also been described in the paediatric population with a prevalence of 2.6%, which may rise to anywhere in the range of 10–80% in obese children [10]. NAFLD is more prevalent in patients with diabetes mellitus type 2 (40–75%) or obesity (33–76%) and affects 99% of people undergoing bariatric surgery [11].

⁎ Corresponding author at: Hospital General San Jorge, Unidad de Lípidos, Bioquímica Clínica, Pº Martinez de Velasco 36, Huesca 22004, Spain. Tel.: +34 974247078; fax: +34 974247099. E-mail address: [email protected] (J. Puzo). 0167-5273/$ – see front matter © 2012 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijcard.2012.09.085

3. Pathogenesis The liver plays a central role in lipid metabolism with the uptake of free fatty acids (FFAs) from the plasma. If the FFAs are not oxidised and used as a source of energy, after the synthesis of lipids and lipoproteins, they are stored or exported. A series of abnormalities in local and systemic factors which control the balance between flow, oxidation and export of lipids leads to the accumulation of triglycerides (TG) in the liver.

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Insulin resistance (IR) [12] and obesity [13] are two important elements in the pathogenesis of NAFLD. Both increase the inflow of FFAs to the liver from subcutaneous and visceral fat and “de novo” intrahepatic synthesis of FFAs – de novo lipogenesis (DNL) – by overexpressing of lipogenic transcription factors such as protein-bound sterol regulatory element (SREBP-1c) or the bound protein response element to carbohydrates [14,15]. In NAFLD, DNL is a key factor in TG formation as, in contrast to what happens in healthy subjects, it is even activated during fasting periods, contributing 26% to TG synthesis [16]. After a diet high in sugars, in the IR state associated with NAFLD, the uptake of glucose by the muscles ends up being diverted towards the liver where it then contributes to DNL [16]. This is known as partial or selective IR, with dissociation between gluconeogenesis (cancelled out) and lipogenesis (stimulated). The hyperinsulinaemia then activates the mechanisms which desensitize hepatocytes to the effects of insulin. Postprandial gluconeogenesis is not suppressed and the postprandial hyperglycaemia

typical of a pre-diabetic state occurs. At the same time, lipogenesis is promoted, causing these cells to become filled with fat [17]. The FFAs, which are not incorporated into the TG, are metabolised by oxidation in mitochondria, peroxisomes and microsomes (Fig. 1A). However, activation of SREBP-1c increases malonyl-CoA, which inhibits oxidation of the FFAs, thereby halting the activity of carnitine palmitoyltransferase-1, the gateway into mitochondria for the Acyl-CoA, in order to be oxidised. The net result of these three changes – greater supply of FFAs, increased intrahepatic synthesis and reduction in mitochondrial activity – is a greater availability of FFAs in the liver as substrate for TG synthesis [18]. In obese patients with NAFLD, adipocytes secrete less adiponectin. This leads to greater production of pro-inflammatory factors, i.e. tumour necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) [19], which can contribute to IR and accumulation of fat in hepatocytes. In intrahepatic synthesis of TG, enzyme Acyl-CoA:diacylglycerol acyltransferase (DGAT) catalyses esterification from FFAs to TG. DGAT is composed of two

A) Normal hepatic triglyceride synthesis and VLDL

VLDL NEFA

apoB Insulin Glucose

MTP

SREBP-1c ChREBP

FFA synthesis

FFA DGAT2

TG

VLDL

Oxidation

The liver lipid content is determined by the balance of several processes: a) Import of non-esterified fatty acids (NEFA) from adipose tissue, b) de novo synthesis of NEFA in hepatocytes, c) Beta-oxidation of NEFA, d) esterification of FFA-mediated TG DAGT2 e) Export of triglycerides and VLDL Abbreviations: VLDL, very low density lipoproteins, FFA free fatty acids or NEFA non-esterified fatty acids, SREBP-1c: protein attached to the sterol regulatory element; ChREBP: protein response element linked to carbohydrates, TG: triglycerides, ApoB: apolipoprotein B, MTP: microsomal triglyceride transfer protein Fig. 1. A. Normal hepatic triglyceride synthesis and VLDL. The liver lipid content is determined by the balance of several processes: a) import of non-esterified fatty acids (NEFA) from adipose tissue, b) de novo synthesis of NEFA in hepatocytes, c) beta-oxidation of NEFA, d) esterification of FFA-mediated TG DAGT2, and e) export of triglycerides and VLDL. Abbreviations: VLDL: very low density lipoproteins, FFA: free fatty acids or NEFA: non-esterified fatty acids, SREBP-1c: protein attached to the sterol regulatory element, ChREBP: protein response element linked to carbohydrates, TG: triglycerides, Apo B: apolipoprotein B, and MTP: microsomal triglyceride transfer protein. B. Pathophysiology of NAFLD. Influence of insulin resistance in NAFLD. Abbreviations: ↑ = increased, ↓ = decreased; NAFLD: non-alcoholic fatty liver disease, NASH: non-alcoholic steatohepatitis, FFA: free fatty acids, SREBP-1c: protein attached to the sterol regulatory element; ChREBP: element binding protein response to carbohydrates, TG: triglycerides, VLDL: very low density lipoproteins, Apo B: apolipoprotein B; MTTP: microsomal triglyceride transfer protein, and VLDL: very low density lipoprotein.

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B) Pathophysiology of NAFLD. Influence of insulin resistance in NAFLD

VLDL FFA Insulin Resistance

apoB

Insulin

SREBP-1c

Glucose

ChREBP

MTTP FFA synthesis

FFA

TG

VLDL

DGA T2 Oxidation Fat vacuoles

Abbreviations = increased, decreased; NAFLD: Non-alcoholic fatty liver disease, NASH: Non-alcoholic steatohepatitis FFA: free fatty acids, SREBP-1c: protein attached to thesterol regulatory element; ChREBP: element binding protein responseto carbohydrates, TG: triglycerides, VLDL: very low density lipoproteins, Apo B: apolipoprotein B; MTTP: microsomal triglyceride transfer protein, VLDL: very low density lipoprotein. Fig. 1 (continued).

primary isoforms: DGAT1 and DGAT2. DGAT2 is hepatocyte specific. Experiments in mice have shown that over-expression leads to steatosis, while inhibition reverts it [20,21]. However, the cancelling out of DGAT2 promotes liver fibrosis, which means that its role is to synthesise TG to protect it from the damage caused by an increase in oxidisable or oxidised FFAs [22]. Following esterification, FFAs may be stored as TG vacuoles in the hepatocytes or, after binding to apolipoprotein B (apoB) – mediated by microsomal triglyceride transfer protein (MTP) – they may be externalised in the form of very low density lipoproteins (VLDL). Although SREBP-1c inhibits the formation of MTP and there may be also a reduction in apoB synthesis [23], or a reduced postprandrial apoB response [24] the dominant characteristic is an over-stimulation of MTP and apoB. This is typical of IR and responsible for the increase in plasma VLDL-TG in this condition [25]. There are other circumstances which negatively affect the excretion of apoB-VLDL: a) endoplasmic reticulum stress, which leads to altered apoB synthesis, greater degradation of apoB and apoptotic cell death [26]. This situation is more common in people with morbid obesity, being reversible through weight loss [27]; b) oxidative stress, which leads to an increase in apoB degradation and a reduction in MTP, with the resulting problems for assembly of VLDL-TG; and lastly, c) physical limitations in the liver in terms of secreting large particles of VLDL [28]. In all the above cases, the end result is difficulty in sending the excess TG to the plasma. When the rate of TG synthesis exceeds the capacity for production of VLDL or for exporting them, TG

accumulate within the hepatocytes, leading to steatosis, a definitive marker for NAFLD (Fig. 1B). 3.1. Progression from steatosis to cirrhosis The “two-hit” hypothesis to explain the pathogenesis and progression of NAFLD is widely accepted [29]. First hit is the accumulation of TG in hepatocytes (simple steatosis). Second hit, includes an uncontrolled production of cytokines (TNF-α, IL-6 and others), resulting from an attempt to compensate for the change in lipid homeostasis, and metabolicoxidative stress, with generation of reactive oxygen species (ROS) which leads to hepatocyte toxic damage and necrosis, rapid increase in inflammation, fibrosis, and cell death [30]. There are some excellent publications which explain in detail the many mechanisms involved in progression to NASH and cirrhosis, and their clinical implications [17,31]. 4. Aetiology Until recently, the principal known aetiological factor in hepatic steatosis was excessive alcohol use. As mentioned previously, alcohol use in NAFLD must, by definition, be nil or minimal. Nevertheless, there are other factors which can promote the development of steatosis (Table 1). The aetiology of NAFLD has both environmental and genetic factors. One of the environmental factors is the so-called Western

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Table 1 Secondary causes of hepatic steatosis. Modified from [49] Angulo P. N Engl J Med 2002; 346: 1221–31. Nutritional

Drugs

Metabolic

Other

Protein-energy malnutrition Famine Total parenteral nutrition Quick weight loss Bariatric surgery Jejunoileal resection Refeeding syndrome

Glucocorticoids synthetic Estrogens Aspirin Calcium Amiodarone Tamoxifen Methotrexate Valproic acid Cocaine Antiretrovirals Zidovudine Didanosine Tetracyclines Hypervitaminosis A

Weber–Christian disease Dysbetalipoproteinemia Lipodystrophy Wolman disease Cholesterol deposits Pregnancy Gaucher disease Alpha-1-antitrypsin deficiency Reye syndrome

Inflammatory bowel disease HIV infection Environmental hepatotoxins Phosphorus Poisonous mushrooms Petrochemical Organic solvents Toxin from Bacillus cereus Bowel diverticulosis Overgrowth bacterial Celiac disease

lifestyle, characterised by the combination of a plentiful diet and sedentary lifestyle that in most cases leads to weight gain. Both in adults and children, being overweight and obesity are associated with NAFLD [10,13,32]. In fact, in Ludwig's early description, all but one of the patients were either obese or overweight [33]. When the euglycaemic clamp – the gold standard for measuring sensitivity to insulin in vivo – was used in NAFLD patients, it was possible to demonstrate the association between the histological findings and the IR [12,34,35]. Another element which may be involved is intestinal flora. This also reflects the diet, acting as a facilitator for both intestinal permeability, allowing endotoxins to pass through, and immune system stimulation, which can contribute to a chronic inflammatory state [36,37]. In some people, there is also a familial component in the incidence of NAFLD [38]. Apart from the variants of the gene that encodes PNPLA3, there are also certain polymorphisms resulting from the change in a single nucleotide (SNPs) in some genes such as those in MTP [39], apolipoprotein C3 (apoC3) in slim Asian people without diabetes [40], the protein phosphatase 1 regulatory subunit 3B (PPP1R3B) and the one near the neurocan (NCAN) [41]. It has also been demonstrated in NAFLD that there is over-expression of proinflammatory and pro-apoptotic genes [42], and in people whose biopsies exhibit fibrosis, SNPs of the NCAN gene, the glucokinase regulatory protein (GCKR) and the lysophospholipase-like 1 protein (LYPLAL1) [41], as well as over-expression of pro-fibrogenic genes, including that of the leptin receptor [42].

5. Diagnosis Despite recent advances, there are still no biomarkers for the accurate diagnosis of NAFLD, or that differentiate between mild stages and the most advanced stages of steatohepatitis and fibrosis [43–45]. The liver biopsy therefore continues to be the gold standard for the diagnosis of NAFLD [46]. However, it is an invasive procedure with a complication rate of 0.5% and interpretation of the results is subjective and open to variability depending on the sample. In view of these factors and the high prevalence of NAFLD, the use of liver biopsy should be limited [47]. For routine medical management, a combination of non-invasive diagnostic tests and their corresponding decision algorithms is recommended [48,49]. Elevated transaminase levels are the most common abnormality observed in NAFLD (50–90% of cases). Gamma glutamyl transpeptidase (GGT) and alkaline phosphatase can be normal or up to three times their usual values. A “Fatty Liver Index” (FLI) was developed to predict fatty liver in the general population. The FLI uses four variables of BMI, waist circumference, GGT and serum triglyceride levels, and achieved an accuracy of 0.84 in detecting fatty liver [50]. Ultrasound scan (US) is the preferred technique for the diagnosis of NAFLD in people with elevated transaminases. US showed a

sensitivity of 64% and specificity of 85%, rising to 90–100% in patients with at least 30% steatosis [51,52]. Computed tomography is of less diagnostic use in diffuse steatosis. The radiation exposure further limits its use. Magnetic Resonance Imaging (MRI) can offer fairly accurate images and spectroscopic methods for quantifying the fat and differentiating between NAFLD and NASH [53]. Elastography (Fibroscan® Echosens, Paris, France) is a non-invasive method of assessing liver fibrosis which can be performed at the bedside or in the out-patient clinic. Fibroscan has now been validated in NAFLD, with a sensitivity and specificity of 0.94 and 0.95 respectively, and represents a useful tool for determining the need for biopsy [54,55]. 6. NAFLD and lipids Deriving from the presence of IR in NAFLD, the over-production of VLDL results in a lipid profile characterised by high TG levels, low cholesterol bound high density lipoprotein (HDLc) and an increase in small, dense, low-density lipoprotein particles (LDL) [56–58]. Hypertriglyceridaemia is the most common abnormality to show up in blood tests. In the general population, the findings of hypertriglyceridaemia or mixed dyslipidaemia multiply the likelihood of fatty infiltration on liver ultrasound by 5.9 and 5.1, respectively [59]. Although NAFLD is associated with obesity, the typical lipid findings are unrelated to that [60]. The presence of hepatic steatosis in patients with diabetes mellitus type 2 (DM2) aggravates diabetic dyslipidaemia, irrespective of the hyperglycaemia [61]. The levels of apolipoprotein A1 are lower in NAFLD, especially in patients with liver fibrosis. Although the apolipoprotein B concentrations are high, this is not related to the degree of steatosis in diabetics [61]. The Lp(a) levels are no different from general population [56–58]. 7. NAFLD and Metabolic Syndrome Metabolic Syndrome (MS) consists of a set of clinical and biochemical abnormalities in which visceral obesity and IR are the essential substrates [62]. MS is associated with an increased risk of DM2 and CVD [63]. Approximately 90% of patients with NAFLD have more than one component of MS; 35–75% meets the diagnostic criteria [64]. Compared with the general population, the prevalence of MS increases two- to three-fold in NAFLD [65]. MS is also predictive of a greater risk of developing NAFLD in both males and females, making regression of fatty infiltration in the liver unlikely and promoting progression towards NASH and cirrhosis [66]. Because of multiple shared pathophysiological mechanisms, NAFLD is considered a hepatic manifestation of MS [35,65,67,68]. NAFLD has also recently been described as an independent factor (independent even from MS) for the development of DM type 2 [69].

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8. NAFLD and cardiovascular risk NAFLD is not an innocuous condition. Disease progression can lead to liver-related death: cirrhosis, liver cancer, and complications of chronic liver disease. Let's take a look at the relationship between NAFLD and CVD. 8.1. NAFLD, risk factors and estimation of cardiovascular risk We know that NAFLD is associated with IR, MS and its components, and pro-atherogenic lipid abnormalities. The fact that lipid ratios (total cholesterol/HDLc; LDL cholesterol/HDLc; TG/HDLc) are powerful predictors of cardiovascular risk (CVR) is well established [70,71]. Using the estimation provided by the lipid ratios, it has been determined that CVR not only is increased in patients with NAFLD, but also progresses similarly to NASH [72]. Whether calculated from equations or risk tables, our group demonstrated some time ago that patients with NAFLD have a greater likelihood of suffering cardiovascular events [73]. This has been corroborated by larger series [74–76]. The risk also rises with the severity of the liver histology [75]. A number of different studies have shown how many of the new risk factors or markers (C-reactive protein (CRP), oxidised LDL, IL-6, plasminogen activator inhibitor-1(PAI-1)) are also associated with NAFLD [77]. In addition, the risk of progression to diabetes is high; at five years, 20–25% of patients with NAFLD will become diabetic [4,76,78,79]. In NASH the likelihood of developing diabetes is higher. Once the DM2 is established, the presence of NAFLD is a predictor of microvascular complications such as retinopathy and nephropathy [80]. 8.2. NAFLD and new markers of arteriosclerosis NAFLD causes an early increase in carotid intima-media thickness and higher prevalence of plaques, as we described, for the first time, in both sexes [81]. Other cohort studies, population studies, and one meta-analysis have corroborated these findings [82–86]. Moreover, through coronary angiography, it has been found that patients with hepatic steatosis have a higher prevalence and greater severity of coronary artery lesions [87,88]. Patients with NAFLD also suffer from endothelial dysfunction – measured by both brachial artery flow-mediated vasodilation and the presence of adhesion molecules – which is not dependent on obesity, MS, or any of its components [89,90]. Other techniques for detecting CVR in asymptomatic patients have found a greater prevalence of abnormalities in NAFLD [91,92].

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incidences of both CVD and CVD-related mortality. In the majority of cases, this was independent from other traditionally-considered factors, including age and weight [98–104]. Similar results have been obtained in studies in which the presence of NAFLD was defined with liver ultrasound scans, with follow-up periods of between 5 and 12 years [105–108]. Finally, all the series [4,109–114] except one [115], using liver biopsy as the undisputed diagnostic procedure for NAFLD, found shorter survival in these patients after observation periods between 7 and 18 years. The main cause was the higher incidence of cardiovascular events and death related to such events. From the above publications, it is possible from the population cohort-based studies [110–112] to define the relevance of the morbidity/mortality rates from arteriosclerotic causes in NAFLD, even in the diabetic patients [115]. Taken together, all these results confirm the hypothesis that patients with NAFLD have a high risk of suffering or even dying from a CVD. Nevertheless, as pointed out in an excellent review on this subject [116], larger prospective studies are needed to determine whether or not NAFLD carries independent risk beyond the traditionally-considered cardiovascular risk factors [117]. The opportunistic detection of hepatic steatosis on a conventional ultrasound scan should alert clinicians to the probable co-existence of multiple underlying cardiovascular risk factors which need to be investigated and treated, while at the same time closely monitoring for possible progression of the liver disease [81,117]. 9. Treatment Although IR is the principal pathogenic factor of NAFLD, there are many potential therapeutic objectives (Fig. 1B). Some managements combine improvements in liver abnormalities with reduction in vascular risk. We will review those treatments that improve the biochemical or histological spectrum of NAFLD and also influence CVR. 9.1. Changes in lifestyle as treatment of NAFLD

Although prevalence studies cannot provide us with cause–effect relationships, they can point to the possible interaction between concomitant conditions. A study of workers in Taiwan revealed that those whose ultrasound scans were compatible with hepatic steatosis had a higher prevalence of ischaemic heart disease, irrespective of obesity and other factors [93]. The mortality rate of NASH patients was higher than that in the general US population, coronary artery disease being the main cause of death [94]. Diabetic patients with NAFLD have a higher prevalence of coronary artery disease, cerebrovascular disease and peripheral arterial disease, independent of MS [95,96]. Lastly, in a paediatric post-mortem examination series, the children with fatty liver had double the rate of coronary artery lesions [97].

Dietary intervention and exercise are the corner stone of NAFLD treatment. Efficacy is limited, however, by patient compliance. Losing 5–10% of body weight is associated with pathophysiological changes which lead to greater sensitivity to insulin, reduced FFAs in the liver, reduction in the inflammatory mechanisms and improvement in transaminase levels and the histological parameters [118–122]. Additionally, an increase in physical activity without weight loss reduces steatosis and transaminase levels in patients with NAFLD [123]. The ideal qualitative composition of the diet in NAFLD has not been analysed, but it seems that a low-fat diet and omega 3 fatty-acid supplements would reduce liver's lipid deposits [124,125]. The data on whether or not alcohol may be taken in NAFLD are contradictory [126,127]. Bariatric surgery has been shown to improve components of MS, with a dramatic reduction in hepatic steatosis and an improvement in the NAFLD stage [128,129]. Nevertheless, if the weight loss brought about by the diet or the surgery occurs too rapidly (more than 1.6 kg/ week), this can aggravate NASH and fibrosis [130]. Since diet, exercise and weight reduction play a well-established role in improving the lipid profile, components of MS and cardiovascular morbidity/mortality, applying these measures in patients with NAFLD will improve both the hepatic and cardiovascular processes with a good cost-effectiveness ratio.

8.4. NAFLD and increased incidence of cardiovascular disease

9.2. Pharmacological treatment for NAFLD

There is a higher incidence of cardiovascular morbidity/mortality in patients with NAFLD, independent of the diagnostic method. Authors who used ALT or GGT elevation as substitute markers for NAFLD, and who followed-up large populations over periods of between 10 and 19 years found that the patients with raised liver enzymes had higher

No prospective, controlled, pharmacological studies have been able to demonstrate the capacity to reduce the liver damage or improve NAFLD-related morbidity beyond weight reduction. There are no drugs with the approved indication for the treatment of NAFLD. However, some clinical trials have reported beneficial effects.

8.3. NAFLD and increased prevalence of cardiovascular disease

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9.2.1. Drugs to treat obesity Orlistat has only been tested in small series that demonstrate improvement in transaminase levels and histology [131,132]. 9.2.2. Insulin sensitizers and related drugs A number of studies with NAFLD patients diagnosed by biopsy have shown that metformin is associated with reductions in transaminases and improvement in fatty infiltration, necro-inflammatory phenomena and fibrosis [133–135]. Glitazones act as PPARγ agonists, increasing the oxidation of FFAs and reducing FFA synthesis in the hepatocytes. Their use has produced varied results [136–138]. A recent meta-analysis confirmed the benefits of glitazones in terms of blood values and histology, but not those of metformin [139]. In view of the lack of sufficient evidence of the cardiovascular or hepatic benefits of these drugs, their use as therapy in NAFLD is not recommended. Preliminary trials in animals with incretins point to an improvement in transaminases and steatosis [140]. A study demonstrated the efficacy of sitagliptin in NAFLD patients with DM2, improving liver tests and suggesting that a large-scale clinical trial is warranted in the future [141].

9.2.3. Lipid-lowering drugs Statins play a well-established role in cardiovascular prevention. They can also lower transaminase and achieve improvements in the histological grade of inflammation, but not fibrosis [142]. There is no risk to the liver when using statins in patients with NAFLD [143], even with underlying slightly-elevated transaminases, since these levels tend to return to normality once the statins are being taken [144]. According to the data from a recent post-hoc analysis of the GREACE study (The Greek atorvastatin and coronary-heart-disease evaluation study), patients with elevated transaminases – probable NAFLD or NASH patients – who took atorvastatin, significantly reduced their CVR to a greater extent than patients with normal enzyme activity. This supports the use of statins that contribute to lowering this risk inherent to NAFLD [145]. Fibrates act on the PPAR-α, regulating the intrahepatic lipid metabolism, reducing IR and improving the lipid profile. A small study using gemfibrozil reported a reduction in transaminases and GGT [146]. A prospective, randomised study in non-diabetic patients with MS and both biochemical and ultrasonographic evidence of NAFLD showed that at the end of treatment, 42% of patients on fenofibrate and 70% on combination fenofibrate plus atorvastatin treatment, no longer had both surrogate markers of NAFLD [147]. Ezetimibe inhibits cholesterol absorption through the Niemann– Pick C1-like 1 receptor and has been shown to increase sensitivity to insulin in obese mice. Three recent studies in humans with NAFLD or NASH found a lowering of liver enzymes or improvement in histological inflammatory phenomena, although one found no significant changes on the ultrasound [148–150]. 9.2.4. Vitamins, antioxidants and other drugs Some series of preliminary in vitro and in vivo studies seemed to conclude that vitamin E, as an antioxidant, was of use in improving NAFLD. However, trials conducted with humans showed contradictory results on improvement of clinical chemistry or histology parameters [151,152]. Ursodeoxycholic acid (UDCA) is used in the treatment of sclerosing cholangitis and primary biliary cirrhosis. Their possible efficacy in the treatment of NAFLD has been shown in animal trials. Unfortunately, the only human study, a multicentre trial which treated 107 patients with UDCA or placebo for 2 years, found no histological differences between the two groups [153].

9.2.4.1. Other possible drugs. There is a series of drugs which have shown interesting properties in experimentation animals which make them candidates for controlled trials as therapeutic agents for NAFLD. Among these are: angiotensin II receptor antagonists, which appear to reduce liver fibrosis [154]. Pentoxifylline (TNF-α inhibitor), cilostazol (as antioxidant and raiser of adiponectin levels) and the probiotic VSL#3, all three improve both liver enzymes and the histology [155–157]. 10. Conclusion NAFLD is a prevalent, potentially serious condition which is generally underestimated. The association of NAFLD with the development of progressive liver disease is well-established. In recent years, an association between NAFLD and CVD has also been recognised, as has its capacity to accelerate the progression of arteriosclerosis and cardiovascular morbidity/mortality. With this information, concentrating solely on the pathological hepatic component of NAFLD may be a short-sighted approach. The chance detection of hepatic steatosis on a conventional US should alert clinicians to the probable co-existence of cardiovascular risk factors which require investigation and treatment. Although treating NAFLD could reduce CVR, very few clinical trials have studied this subject. Studies to investigate the impact of NAFLD on arteriosclerosis and its complications are required, along with clinical trials to evaluate new therapeutic approaches. Acknowledgement The authors of this manuscript have certified that they comply with the Principles of Ethical Publishing in the International Journal of Cardiology. References [1] Neuschwander-Tetri BA, Caldwell SH. Nonalcoholic steatohepatitis: summary of an AASLD Single Topic Conference. Hepatology 2003;37:1202-19. [2] Zelman S. The liver in obesity. AMA Arch Intern Med 1952;90:141-56. [3] Ludwig J, Viggiano TR, McGill DB, Oh BJ. Nonalcoholic steatohepatitis: Mayo Clinic experiences with a hitherto unnamed disease. Mayo Clin Proc 1980;55:434-8. [4] Adams LA, Lymp JF, St Sauver J, et al. The natural history of nonalcoholic fatty liver disease: a population based cohort study. Gastroenterology 2005;129:113-21. [5] Williams CD, Stengel J, Asike MI, et al. Prevalence of nonalcoholic fatty liver disease and nonalcoholic steatohepatitis among a largely middle-aged population utilizing ultrasound and liver biopsy: a prospective study. Gastroenterology 2011;140:124-31. [6] Bedogni G, Miglioli L, Masutti F, et al. Incidence and natural course of fatty liver in the general population: the Dionysos Study. Hepatology 2007;46:1387-91. [7] Browning JS, Szczepaniak LS, Dobbins LS, et al. Prevalence of hepatic steatosis in an urban population in the United States: impact of ethnicity. Hepatology 2004;40:1387-95. [8] Nomura H, Kashiwaqi S, Hayashi J, Kajiyama W, Tani S, Goto M. Prevalence of fatty liver in a general population of Okinawa Japan. Jpn J Med 1988;27:142-9. [9] Carulli L, Lonardo A, Lombardini S, Marchesini G, Loria P. Gender, fatty liver and GGT. Hepatology 2006;44:278-9. [10] Papandreou D, Rousso I, Mavromichalis I. Update on non-alcoholic fatty liver disease in children. Clin Nutr 2007;26:409-15. [11] Lazo M, Clark JM. The epidemiology of nonalcoholic fatty liver disease: a global perspective. Semin Liver Dis 2008;28:339-50. [12] Marchesini G, Brizi M, Morselli-Labate AM, et al. Association of nonalcoholic fatty liver disease with insulin resistance. Am J Med 1999;107:450-5. [13] Dixon JB, Bhathal PS, O'Brien PE. Nonalcoholic fatty liver disease: predictors of nonalcoholic steatohepatitis and liver fibrosis in the severely obese. Gastroenterology 2001;121:91–100. [14] Liu Q, Bengmark S, Qu S. Role of hepatic fat accumulation in pathogenesis of non-alcoholic fatty liver disease (NAFLD). Lipids Health Dis 2010;9:42. [15] Tamura S, Shimomura I. Contribution of adipose tissue and de novo lipogenesis to nonalcoholic fatty liver disease. J Clin Invest 2005;115:1139-42. [16] Donnelly KL, Smith CI, Schwarzenberg SJ, Jessurun J, Boldt MD, Parks EJ. Sources of fatty acids stored in liver and secreted via lipoproteins in patients with nonalcoholic fatty liver disease. J Clin Invest 2005;115:1343-51. [17] Jou J, Choi SS, Diehl AM. Mechanisms of disease progression in nonalcoholic fatty liver disease. Semin Liver Dis 2008;28:370-9. [18] Duvnjak M, Lerotić I, Barsić N, Tomasić V, Virović Jukić L, Velagić V. Pathogenesis and management issues for non-alcoholic fatty liver disease. World J Gastroenterol 2007;13:4539-50.

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