Background & Aims: Cross-sectional studies of patients with nonalcoholic fatty liver disease (NAFLD) have reported a lower prevalence of severe disease among modest drinkers compared with nondrinkers. We collected data from adult participants in the Nonalcoholic Steatohepatitis (NASH) Clinical Research Network to evaluate the longitudinal association between modest use of alcohol and histology findings in patients with NAFLD, using paired liver biopsies collected more than 1 year apart. Methods: We studied NASH Clinical Research Network participants 21 years or older, not receiving pharmacologic therapy, from whom 2 or more liver biopsies and data on alcohol use within 2 years of the initial biopsy were available. Alcohol consumption was evaluated at study entry using the Alcohol Use Disorders Identification Test and Skinner Lifetime Drinking History questionnaires. At each follow-up visit participants were asked about alcohol use frequency, number of drinks on a typical day, and frequency of heavy drinking. The association between baseline drinking status and changes in fibrosis stage, NASH histology, and the NAFLD Activity Score and its individual components were evaluated by analysis of covariance. The association between change in drinking status and change in histology was evaluated using adjusted logistic regression. Results: Of 285 participants (82% white, 70% female, mean age, 47 y) meeting entry criteria, 168 (59%) were modest alcohol users (≤2 drinks/d) and the remaining 117 were abstinent. At baseline, a higher proportion of modest alcohol users were white (86% vs 76% nonwhite) (P =.04) and a lower proportion of modest alcohol users were diagnosed with definite NASH (57% vs 74% without NASH; P =.01). During a mean follow-up period of 47 months between biopsies, nondrinkers had a greater mean reduction in steatosis grade (reduction, 0.49) than modest drinkers (reduction, 0.30; P =.04) and a greater reduction in mean level of aspartate transaminase (reduction, 7 U/L vs an increase of 2 U/L in modest drinkers; P =.04). Modest drinkers had significantly lower odds of NASH resolution compared with nondrinkers (adjusted odds ratio, 0.32; 95% CI, 0.11–0.92; P =.04) on adjusted analysis. Conclusions: In a longitudinal analysis of liver biopsies from patients with NAFLD not receiving pharmacologic therapy, modest alcohol use was associated with less improvement in steatosis and level of aspartate transaminase, as well as lower odds of NASH resolution, compared with no use of alcohol.

Purpose of review

Nonalcoholic fatty liver disease (NAFLD) is the most common cause of liver disease in the United States and has a strong heritable component. Advances in understanding the genetic underpinnings of NAFLD have revealed important insights into NAFLD pathogenesis, prognosis, and potential therapeutic targets. The purpose of this review is to summarize data on common and rare variants associated with NAFLD, combining risk variants into polygenic scores to predict NAFLD and cirrhosis as well as emerging evidence on using gene silencing as a novel therapeutic target in NAFLD.

Recent findings

Protective variants in HSD17B13, MARC1 and CIDEB have been identified and a confer 10-50% lower risk of cirrhosis. Together, these as well as other NAFLD risk variants, including those in PNPLA3 and TM6SF2, can be combined to create polygenic risk scores associated with liver fat, cirrhosis, and hepatocellular carcinoma. Genomic analysis of extreme phenotypes including patients with lean NAFLD without visceral adiposity may uncover rare monogenic disorders with pathogenic and therapeutic implications and gene silencing strategies targeting HSD17B13 and PNPLA3 are being evaluated in early phase human studies as treatments for NAFLD.

Summary

Advances in our understanding of the genetics of NAFLD will enable clinical risk stratification and yield potential therapeutic targets.

Background

Non-alcoholic fatty liver disease (NAFLD) is a clinicopathologic entity that requires a liver biopsy assessment to diagnose the progressive form of NAFLD called non-alcoholic steatohepatitis (NASH). Liver biopsy is invasive, subject to sampling and interobserver variability, and impractical to scale to the affected population of up to 1 billion affected individuals worldwide. Non-invasive imaging biomarkers have emerged as a key modality to address the major unmet need to diagnose, stage, and longitudinally monitor NAFLD.

Scope of review

In this review, we critically examine the use of non-invasive imaging biomarkers to diagnose NAFLD, NASH, and fibrosis stage.

Major conclusions

Ultrasound and magnetic resonance imaging (MRI) biomarkers of liver fat can diagnose NAFLD. MRI proton density fat fraction (MRI-PDFF) is better than liver biopsy, particularly for following longitudinal changes in liver fat in clinical trials. Imaging biomarkers to reliably diagnose NASH are under investigation, but when used alone, continue to have only modest diagnostic accuracy. However, the fibrosis stage has the strongest association with liver decompensation and mortality, and elastography has emerged as a reliable biomarker for liver fibrosis. We review the combination of biomarkers to risk stratify patients and identify individuals needing treatment and the implications of longitudinal changes in liver stiffness measurement.

Hepatic steatosis is a key histological feature of nonalcoholic fatty liver disease (NAFLD). The non-invasive quantification of liver fat is now possible due to advances in imaging modalities. Emerging data suggest that high levels of liver fat and its temporal change, as measured by quantitative non-invasive methods, might be associated with NAFLD progression. Ultrasound-based modalities have moderate diagnostic accuracy for liver fat content and are suitable for screening. However, of the non-invasive imaging modalities, MRI-derived proton density fat fraction (MRI-PDFF) has the highest diagnostic accuracy and is used for trial enrolment and to evaluate therapeutic effects in early-phase clinical trials in nonalcoholic steatohepatitis (NASH). In patients with NAFLD without advanced fibrosis, high levels of liver fat are associated with rapid disease progression. Furthermore, changes on MRI-PDFF (≥30% decline relative to baseline) are associated with NAFLD activity score improvement and fibrosis regression. However, an inverse association exists between liver fat and complications of cirrhosis. Liver fat decreases as liver fibrosis progresses towards cirrhosis, and the clinical importance of quantitative measurements of liver fat differs by NAFLD status. As such, patients with NAFLD should be stratified by fibrosis severity to investigate the utility of quantitative measurements of liver fat for assessing NAFLD progression and prognosis.

The gut microbiome, a diverse microbial community in the gastrointestinal tract, plays a pivotal role in the maintenance of health. The gut microbiome metabolizes dietary and host-derived molecules to produce bioactive metabolites, which have a wide array of effects on host metabolism and immunity. 'Dysbiosis' of the gut microbiome, commonly considered as perturbation of microbiome diversity and composition, has been associated with intestinal and extra-intestinal diseases, including nonalcoholic fatty liver disease (NAFLD). A number of endogenous and exogenous factors, such as nutritional intake and xenobiotic exposure, can alter the gut microbiome. We will review the evolving methods for studying the gut microbiome and how these profiling techniques have been utilized to further our understanding of the gut microbial community composition and functional potential in the clinical spectrum of NAFLD. We will highlight microbiome-host interactions that may contribute to the pathogenesis of NAFLD, with a primary focus on mechanisms related to the metabolic output of the gut microbiome. Finally, we will discuss potential therapeutic implications of the gut microbiome in NAFLD.

The gut microbiome, a diverse microbial community in the gastrointestinal tract, plays a pivotal role in the maintenance of health. The gut microbiome metabolizes dietary and host-derived molecules to produce bioactive metabolites, which have a wide array of effects on host metabolism and immunity. 'Dysbiosis' of the gut microbiome, commonly considered as perturbation of microbiome diversity and composition, has been associated with intestinal and extra-intestinal diseases, including nonalcoholic fatty liver disease (NAFLD). A number of endogenous and exogenous factors, such as nutritional intake and xenobiotic exposure, can alter the gut microbiome. We will review the evolving methods for studying the gut microbiome and how these profiling techniques have been utilized to further our understanding of the gut microbial community composition and functional potential in the clinical spectrum of NAFLD. We will highlight microbiome-host interactions that may contribute to the pathogenesis of NAFLD, with a primary focus on mechanisms related to the metabolic output of the gut microbiome. Finally, we will discuss potential therapeutic implications of the gut microbiome in NAFLD.

Moderate alcohol consumption in patients with nonalcoholic fatty liver disease (NAFLD) is common, yet the effects on cardiovascular and liver health are unclear. Moderate alcohol use is associated with improved insulin sensitivity and decreased cardiovascular mortality in the general population, but whether similar benefits would be observed in persons with NAFLD remains largely unstudied. There is significant overlap in the pathogenesis of alcoholic liver disease (ALD) and NAFLD, although studies of ALD have focused on pathological alcohol intake and few mechanistic studies of moderate alcohol use in NAFLD exist. We undertook a critical review of the effect of moderate alcohol use on cardiovascular and liver disease in patients with NAFLD. A total of seven observational studies met the criteria for inclusion (one for cardiovascular endpoints and six for liver endpoints). Insufficient studies have assessed the association of moderate alcohol use with cardiovascular outcomes. There was a positive association between moderate alcohol use and decreased NASH and fibrosis; however, heavy episodic drinking may accelerate fibrosis progression and moderate alcohol use may increase the risk of hepatocellular carcinoma in patients with advanced fibrosis. Significant methodological limitations were present, including incomplete adjustment for confounding factors and failure to measure lifetime use or the pattern of alcohol intake. Thus, a strong recommendation of benefit of moderate alcohol use in NAFLD cannot be made. There remains a need for additional high-quality longitudinal studies that evaluate both cardiovascular and liver outcomes among NAFLD patients with moderate or lesser degrees of alcohol use. (Hepatology 2017;65:2090-2099).

Whereas the rising prevalence of nonalcoholic fatty liver disease (NAFLD) is closely related with the global obesity epidemic, up to 10–20% of individuals with NAFLD are lean as defined by a body mass index of < 25 kg/m2, or < 23 kg/m2 in Asians. This entity designated as “lean NAFLD” is estimated to affect 8 to 10 million individuals in the United States alone. Here, we review the emerging data on the epidemiology, natural history and prognosis of lean NAFLD and put forward a diagnostic approach that combines detailed clinical phenotyping with genomic analysis. We propose two subtypes of lean NAFLD referred to as type 1: individuals with visceral adiposity and insulin resistance but normal BMI; and type 2: lean individuals with hepatic steatosis secondary to a known or unknown monogenic disease. We envision that incorporation of genomic analysis in the diagnostic algorithm of lean patients with NAFLD will elucidate the contribution of common genetic variants through the calculation of NAFLD polygenic risk score and also characterize the diverse array of rare monogenic diseases that can lead to triglyceride accumulation in the cytoplasm of hepatocytes. Collectively, the integration of a molecular diagnosis in the clinical evaluation of patients with lean NAFLD will provide an accurate diagnosis, with possible targeted therapies and may uncover novel molecular mechanisms with potential broader therapeutic implications.