Non-alcoholic fatty liver disease (NAFLD) is a clinicopathological syndrome, which encloses a spectrum ranging from pure steatosis to steatohepatitis, fibrosis and cirrhosis eventually with scarce steatosis [1–4], while the non-alcoholic steatohepatitis (NASH) definition relies 4on characteristic histological findings similar to those of alcoholic hepatitis . Both situations are defined by exclusion of alcohol consumption of more than 20 g/day .
The NAFLD frequency is increasing, now being recognized as one of the most frequent causes of hepatic disease , occurring in 20% of the general population, whereas NASH occurs in about 3% [8–10].
Primary NAFLD must be differentiated from secondary steatosis or steatohepatitis, since their pathogeneses and prognosis are different, with a worse outcome in the latter . NAFLD may be secondary to nutritional, drug, genetic and environmental causes, among others.
Many authors consider primary NAFLD as the hepatic manifestation of the insulin resistance (IR) syndrome [12–14]. IR is associated with hyperinsulinism, glucose intolerance and type 2 diabetes mellitus, hypertriglyceridaemia and low levels of high-density lipoproteins, hypertension, fibrinolysis, accumulation of visceral fat, hyperuricaemia and polycystic ovarian syndrome . Closely associated with IR is a constellation of manifestations now defined as the metabolic syndrome on the basis of three or more criteria out of five defined by the Third Report of the National Cholesterol Education Program Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (ATP III): waist circumference, glucose, high-density lipoprotein-cholesterol, triglycerides, and arterial pressure .
NASH shows a strong association with the features of the metabolic syndrome, namely obesity (which is present in more than one-half of the patients, reaching 90% in some series), dyslipidaemia (in 21–92%) and type 2 diabetes mellitus (in 20–55%) [7,17–20]. In fact, the Verona Diabetes Study, with 7148 patients, showed a higher increase in the liver disease-related mortality than in the cardiovascular mortality of patients with diabetes mellitus, when compared with the general population .
Frequency of insulin resistance in NAFLD
There is discrepancy in the literature concerning the real frequency of IR in NAFLD, probably depending on the different criteria used, and on whether the all NAFLD spectrum or only NASH is considered. In fact, some authors [12,13] evaluated IR using the homeostatic model assessment (HOMA), while others used the frequently sampled intravenous glucose tolerance test . Furthermore, Chitturi et al. considered IR if HOMA-IR>1.64 , while Marchesini et al. considered IR if HOMA-IR≥3 . Concerning the definition of the metabolic syndrome, different criteria were also used, such as the World Health Organization definition  or the ATP III . Although insulin sensitivity is a continuum in the general population, and therefore any attempt to define a cut-off for IR is arbitrary, uniform criteria should be used in future prospective studies of NAFLD and IR; HOMA>3 as criteria of IR and the ATP III criteria for defining the metabolic syndrome are probably the more consensual.
Taking into account these discrepancies, IR is very frequent in NAFLD patients (47–98%), even in the absence of diabetes mellitus [13,22,24]. However, despite NAFLD association with the features of the metabolic syndrome, only 36% of patients with NAFLD fulfil at least three criteria and thus can be classified as having that syndrome, which was associated with a higher risk of having NASH and severe fibrosis . It should be noticed that the prevalence of the metabolic syndrome as defined by the ATP criteria in the general adult population from the United States is about 22% . Therefore, although there is a strong association, there are still a percentage of patients that do not have evidence of IR at the time of diagnosis. This can be due either to the fact that there are other causes of secondary NASH not identifiable at the moment, or to the fact that in some patients liver disease precedes IR, or yet that the indirect methods used to evaluate IR such as the HOMA are not sensitive enough.
In the present issue, Sequeira et al. report IR to be present in one-third of patients with NAFLD (note that most series relate IR with NASH and not with NAFLD), which is much lower than usually described. Nevertheless, their population included primary as well as secondary NAFLD. In fact, in patients who had exclusively metabolic risk factors, IR occurred in 50%, in agreement with the previous literature. On the other hand, only 9.4% of patients exposed to chemicals with no other risk factor had IR. Diagnosis of the metabolic syndrome (at least three criteria present) was present in 29.7% of cases of NAFLD, less frequent than previously reported in the literature, probably because the series included primary and secondary NAFLD. It is also interesting to note that, in conformity with previous literature, IR was associated with advanced fibrosis, which suggests that IR is important not only in the development of NAFLD but also in the progression of the disease. These results confirm the association of primary NAFLD with IR, underscoring that this is probably not the case in forms of secondary NAFLD.
NAFLD/NASH and IR association is relevant, as they are linked in terms of pathogenesis. NASH pathogenesis has been interpreted as the result of two hits, the first leading to steatosis and the second to inflammation and necrosis . Recently it was suggested that a single hit, IR, could be enough to explain the whole spectrum of NAFLD . In fact, a four-step model was recently proposed in which the first step is steatosis facilitated by insulin, the second is necrosis induced by intracellular lipid toxicity or lipid peroxidation, the third is release of bulk lipid from hepatocytes into the interstitium leading to direct and inflammatory injury to hepatic veins, and the fourth step is venous obstruction with secondary collapse and, ultimately, fibrous septation and cirrhosis .
IR courses with resistance to some insulin actions and a compensatory increase of insulin levels. As a result, many metabolic pathways may be over-activated. In peripheral tissues, IR leads to a decrease of the insulin inhibitor effect on hormone-sensitive lipase, which remains active, enhancing triacylglycerol hydrolysis to glycerol and fatty acids, increasing free fatty acids in the plasma and reaching the liver. In the liver, IR leads to an increase of fatty acid oxidation and gluconeogenesis. On the other hand, hyperinsulinism leads to an increase of fatty acid synthesis and a decrease of triglycerides output as very low density lipoproteins. In conclusion, IR/hyperinsulinism is associated with triglyceride accumulation in the liver, and thus steatosis, as a result of an increase of the input and synthesis of fatty acids and a decrease in the output of triglycerides. Adipose tissue, and particularly visceral adipose tissue, has a very important role in the development of IR and NAFLD. Adipose tissue is now recognized as an endocrine organ and cytokine producer. It is able to produce tumour necrosis factor-alpha (TNF-α) and three hormones: leptin, adiponectin and resistin. TNF-α, leptin and resistin (the latter still controversial) further increase IR [29–31]. Adiponectin has opposite effects when compared with TNF-α, being protective against IR . As suggested by animal models, there is a self-perpetuating pathway between IR and inflammation. It was demonstrated in the murine model of diet-induced steatohepatitis that free fatty acids markedly stimulated TNF-α expression in an NF-κβ-dependent process because a super-repressor of I-kB blocked TNF-α up-regulation. TNF-α can promote IR by signalling IKK-β activation and c-jun-N-terminal kinase activation [33,34].
There was previous evidence of increased cytochrome P450 2E1 (CYP2E1) expression being implicated in the development of NAFLD . There is now recent evidence that IR and CYP2E1 expression may be interrelated through the ability of CYP2E1-induced oxidant stress to impair hepatic insulin signalling. It was shown in the methionine and choline-deficient diet mouse model of steatohepatitis with CYP2E1 overexpression that insulin-induced IRS-1, IRS-2 and Akt phosphorylation were similarly decreased. This inhibition of insulin signalling by CYP2E1 overexpression was partially c-jun-N-terminal kinase dependent. These findings indicate that increased hepatocyte CYP2E1 expression and the presence of steatohepatitis result in the down-regulation of insulin signalling, potentially contributing to the IR associated with NAFLD .
No treatment has scientifically proved to ameliorate NAFLD lesions or to avoid its progression. The various therapeutic alternatives are aimed at interfering with the risk factors involved in the pathogenesis of NASH, in order to prevent the progression to end-stage disease associated with the development of cirrhosis and liver failure.
Because of the importance of IR in the basis of NAFLD pathogenesis, it is easy to understand the importance of increasing insulin sensitivity to treat these patients. To do so, the most important therapeutic measure is an attempt to change lifestyle mostly by dieting and implementing physical activities in order to lose weight, when overweight or obesity are present. This seems to lead to an improvement in biochemical tests and in steatosis, although no benefit in terms of inflammation or fibrosis has been demonstrated . These benefits are coupled with an improvement in glucose tolerance.
In what concerns drugs, insulin-sensitizing agents are the more promising. The most used agents are metformin [38,39] and the thiazolidinediones such as pioglitazone  and rosiglitazone . They proved to be effective in reducing IR and aminotransferase levels with an improvement in hepatic histology in the short period. Unfortunately, the long-term response is still unknown and there are no controlled studies with these agents.
Conflict of interest
Both authors contributed to the writing of the paper. H.C.-P. also reviewed the text.
1. Caldwell SH, Oelsner DH, Iezzoni JC, Hespenheide EE, Battle EH, Driscoll CJ. Cryptogenic cirrhosis: clinical characterization and risk factors for underlying disease. Hepatology 1999; 29:664–669.
2. Teli M, James OF, Burt AD, Bennett MK, Day CP. The natural history of nonalcoholic fatty liver: a follow-up study. Hepatology 1995; 22:1714–1719.
3. Clark JM, Diehl AM. Nonalcoholic fatty liver disease: an underrecognized cause of cryptogenic cirrhosis. J Am Med Assoc 2003; 289:3000–3004.
4. Marrero JA, Fontana RJ, Su GL, Conjeevaram HS, Emick DM, Lok AS. NAFLD may be a common underlying liver disease in patients with hepatocellular carcinoma in the United States. Hepatology 2002; 36:1349–1354.
5. James OF, Day CP. Non-alcoholic steatohepatitis
(NASH): a disease of emerging identity and importance. J Hepatol 1998; 29:495–501.
6. Brunt EM. Nonalcoholic steatohepatitis: definition and pathology. Semin Liver Dis 2001; 21:3–16.
7. Bacon BR, Farahvash MJ, Janney CG, Neuschwander-Tetri BA. Nonalcoholic steatohepatitis: an expanded clinical entity. Gastroenterology 1994; 107:1103–1109.
8. Wanless IR, Lentz JS. Fatty liver hepatitis (steatohepatitis) and obesity
: an autopsy study with analysis of risk factors. Hepatology 1990; 12:1106–1110.
9. Hilden M, Christoffersen P, Juhl E, Dalgaard JB. Liver histology of a ‘normal’ population — examinations of 503 consecutive fatal traffic casualties. Scand J Gastroenterol 1977; 12:593–597.
10. Neuschwander-Tetri BA, Caldwell SH. Nonalcoholic steatohepatitis: summary of an AASLD Single Topic Conference. Hepatology 2003; 37:1202–1219.
11. Fromenty B, Robin MA, Igoudjil A, Mansouri A, Pessayre D. The ins and outs of mitochondrial dysfunction in NASH. Diabetes Metab 2004; 30:121–138.
12. Marchesini G, Bugianesi E, Forlani G, Cerrelli F, Lenzi M, Manini R, et al. Nonalcoholic fatty liver, steatohepatitis, and the metabolic syndrome
. Hepatology 2003; 37:917–923.
13. Chitturi S, Abeygunasekera S, Farrell GC, Holmes-Walker J, Hui JM, Fung C, et al. NASH and insulin resistance
: insulin hypersecretion and specific association with the insulin resistance
syndrome. Hepatology 2002; 35:373–379.
14. Cortez-Pinto H, Camilo ME, Baptista A, Oliveira AG, Moura MC. Nonalcoholic fatty liver — another feature of the metabolic syndrome
. Clin Nutr 1999; 6:353–358.
15. Reaven G. Syndrome X: 6 years later. J Internal Med 1994; 236:13–22.
16. Executive Summary of The Third Report of The National Cholesterol Education Program (NCEP). Expert Panel on Detection Evaluation And Treatment of High Blood Cholesterol In Adults (Adult Treatment Panel III). J Am Med Assoc 2001; 285:2486–2497.
17. Powell EE, Cooksley WG, Hanson R, Searle J, Halliday RW, Powell LW. The natural history of nonalcoholic steatohepatitis: a follow-up study of 42 patients followed for up to 21 years. Hepatology 1990; 11:74–80.
18. Ludwig J, Viggiano RT, McGill DB, Ott BJ. Nonalcoholic steatohepatitis. Mayo Clinic experiences with a hitherto unnamed disease. Mayo Clin Proc 1980; 55:434–438.
19. Diehl AM, Goodman Z, Ishak KG. Alcohollike liver disease in nonalcoholics. A clinical and histopathological comparison with alcohol-induced liver injury. Gastroenterology 1988; 95:1056–1062.
20. Cortez-Pinto H, Baptista A, Camilo E, Valente A, Saragoça A, Moura MC. Nonalcoholic steatohepatitis: clinicopathological comparison with alcoholic hepatitis in ambulatory and hospitalized patients. Dig Dis Sci 1996; 41:172–179.
21. Locatelli F, Zoppini G. Cause-specific mortality in type 2 diabetes, The Verona Diabetes Study. Diabetes Care 1999; 18:353–358.
22. Pagano G, Pacini G, Musso G, Gambino R, Mecca F, Depetris N, et al. Nonalcoholic steatohepatitis, insulin resistence and metabolic syndrome
: further evidence for an etiologic association. Hepatology 2002; 35:367–372.
23. Alberti KG, Zimmet PZ. Definition, diagnosis and classification of diabetes mellitus and its complications. Part 1: diagnosis and classification of diabetes mellitus provisional report of a WHO consultation. Diabet Med 1998; 15:539–553.
24. Marchesini G, Brizi M, Bianchi G, Tomassetti S, Bugianesi E, Lenzi M, et al. Nonalcoholic fatty liver disease: a feature of the metabolic syndrome
. Diabetes 2001; 50:1844–1850.
25. Ford ES, Giles WH, Dietz WH. Prevalence of the metabolic syndrome
among US adults: findings from the third National Health and Nutrition Examination Survey. J Am Med Assoc 2002; 287:356–359.
26. Day CP, James OFW. Steatohepatitis: a tale of two ‘hits’. Gastroenterology 1998; 114:842–845.
27. Loria P, Lonardo A, Carulli N. Relative contribution of iron burden, HFE mutations, and insulin resistance
to fibrosis in nonalcoholic fatty liver [Letter]. Hepatology 2004; 39:1748.
28. Wanless IR, Shiota K. The pathogenesis of nonalcoholic steatohepatitis and other fatty liver diseases: a four-step model including the role of lipid release and hepatic venular obstruction in the progression to cirrhosis. Semin Liver Dis 2004; 24:99–106.
29. Zhang B, Berger J, Hu E, Szalkowski D, White-Carrington S, Spielgelman BM, Moller DE. Negative regulation of peroxisome proliferator-activated receptor-gamma gene expression contributes to the antiadipogenic effects of tumor necrosis factor-alpha. Mol Endocrinol 1996; 10:1457–1466.
30. Halle M, Berg A, Northorff H, Keul J. Importance of TNF-alpha and leptin in obesity
and insulin resistance
: a hypothesis on the impact of physical exercise. Ex Immunol Rev 1998; 4:77–94.
31. Nagaev I, Smith U. Insulin resistance
and type 2 diabetes are not related to resistin expression in human fat cells or skeletal muscle. Biochem Biophys Res Commun 2001; 85:561–564.
32. Xu A, Wang Y, Keshal H. The fat-derived hormone adiponectin alleviates alcoholic and nonalcoholic fatty liver diseases in mice. J Clin Invest 2003; 112:91–100.
33. Yuan M, Konstantopoulos N, Lee J, Hansen L, Li ZW, Karin M, Shoelson SE. Reversal of obesity
- and diet-induced insulin resistance
with salicylates or targeted disruption of Ikkbeta. Science 2001; 293:1673–1677.
34. Hirosumi J, Tuncman G, Chang L, Gorgun CZ, Uysal KT, Maeda K, et al. A central role for JNK in obesity
and insulin resistance
. Nature 2002; 420:333–336.
35. Weltman MD, Farrell GC, Liddle C. Increased hepatocyte CYP2E1 expression in a rat nutritional model of hepatic steatosis with inflammation. Gastroenterology 1996; 111:1645–1653.
36. Schattenberg JM, Wang Y, Singh R, Rigoli RM, Czaja MJ. Hepatocyte CYP2E1 overexpression and steatohepatitis lead to impaired hepatic insulin signaling. J Biol Chem 2005; 280:9887–9894.
37. Ueno T, Sugawara H, Sujaku K, Hashimoto O, Tsuji R, Tamaki S, et al. Therapeutic effects of restricted diet and exercise in obese patients with fatty liver. J Hepatol 1997; 27:103–107.
38. Bugianesi E, Gentilcore E, Manini R, Natale S, Vanni E, Villanova N, David E, et al.A randomized controlled trial of metformin versus vitamin E or prescriptive diet in nonalcoholic fatty liver disease. Am J Gastroenterol 2005; 100:1082–1090.
39. Uygun A, Kadayifci A, Isik AT, Ozgurtas T, Deveci S, Tuzun A, et al. Metformin in the treatment of patients with non-alcoholic steatohepatitis
. Aliment Pharmacol Ther 2004; 19:537–544.
40. Promrat K, Lutchman G, Uwaifo GI, Freedman RJ, Soza A, Heller T, et al. A pilot study of pioglitazone treatment for nonalcoholic steatohepatitis. Hepatology 2004; 39:188–196.
41. Neuschwander-Tetri BA, Brunt EM, Wehmeier KR, Oliver D, Bacon BR. Improved nonalcoholic steatohepatitis after 48 weeks of treatment with the PPAR-gamma ligand rosiglitazone. Hepatology 2003; 38:1008–1017.