What is non‑alcoholic steatohepatitis (NASH)?

What is Non-Alcoholic Steatohepatitis (NASH)?

Non-alcoholic steatohepatitis (NASH) is a progressive inflammatory form of fatty liver disease characterized by the presence of ≥5% hepatic steatosis (fat accumulation), hepatocyte injury with ballooning, and lobular inflammation, with or without fibrosis, occurring in individuals who consume minimal to no alcohol (≤30g/day for men, ≤20g/day for women). 1, 2, 3

Key Defining Features

NASH is distinguished from simple non-alcoholic fatty liver (NAFL) by three critical histologic findings that must be present on liver biopsy 2, 4:

• Macrovesicular steatosis (≥5% of hepatocytes)
• Hepatocyte ballooning (cellular injury and enlargement)
• Lobular inflammation (mixed inflammatory infiltrate including neutrophils)
• Zone 3 perisinusoidal fibrosis (may or may not be present initially)

Simple steatosis (NAFL) lacks the hepatocyte injury and inflammation that define NASH, and carries minimal risk of progression 2, 4.

Clinical Significance and Disease Progression

NASH represents the aggressive subset of NAFLD with substantial risk for progression to cirrhosis, liver failure, and hepatocellular carcinoma. The disease trajectory is concerning 4:

• Approximately 30-40% of NASH patients develop fibrosis
• 20% progress to cirrhosis over time
• 2-3% of NASH cirrhosis patients develop hepatocellular carcinoma annually
• NASH is now the third leading cause of cirrhosis and third most common indication for liver transplantation in the United States

Importantly, recent evidence challenges the traditional view that NAFL and NASH are separate entities—sequential biopsy studies demonstrate that even patients with bland steatosis can progress to NASH and fibrosis, particularly when metabolic risk factors are present 4. The fibrosis progression rate from stage 0 to stage 1 is approximately 7 years for NASH versus 14 years for NAFL 4.

Epidemiology and Risk Factors

NASH affects an estimated 3-12% of the US population (approximately 15-25% of those with NAFLD) 4, 5. The disease is strongly associated with metabolic dysfunction 1, 3:

• Obesity (most common association)
• Type 2 diabetes mellitus (2-3 fold increased risk)
• Insulin resistance (central pathophysiologic mechanism)
• Metabolic syndrome (hypertension, dyslipidemia, central adiposity)
• Hispanic ethnicity (genetic predisposition via PNPLA3 I148M variant)

Diagnostic Requirements

Definitive diagnosis of NASH requires liver biopsy—this is the only accepted diagnostic method 1, 2, 3. Non-invasive tests and scoring systems can assess risk and stage fibrosis, but cannot definitively distinguish NASH from NAFL. The histologic diagnosis uses standardized scoring systems:

• NAFLD Activity Score (NAS): Composite of steatosis, inflammation, and ballooning scores
• Fibrosis staging: Separate assessment from F0 (no fibrosis) to F4 (cirrhosis)

Mortality and Outcomes

Patients with NASH have significantly increased mortality compared to the general population and those with simple steatosis 2, 5:

• All-cause mortality: 25.56 per 1000 person-years
• Liver-specific mortality: 11.77 per 1000 person-years
• Cardiovascular disease remains the leading cause of death (not liver disease)
• Liver-related mortality is substantially higher in NASH versus NAFL

Exclusion Criteria

The diagnosis requires exclusion of 1, 6:

• Significant alcohol consumption (≥30g/day men, ≥20g/day women)
• Secondary causes of steatosis (steatogenic medications: corticosteroids, amiodarone, methotrexate, tamoxifen, valproic acid)
• Other chronic liver diseases (viral hepatitis, autoimmune hepatitis, Wilson's disease, hemochromatosis)

Clinical Context

NASH should be understood as part of a multi-systemic metabolic disease rather than an isolated liver condition 3. The disease is closely intertwined with insulin resistance, and the liver in NASH is programmed toward lipogenesis rather than glycogenesis 7. This metabolic reprogramming, combined with mitochondrial dysfunction, oxidative stress, and inflammatory cascades, drives disease progression from simple steatosis to steatohepatitis and ultimately fibrosis.