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Comprehensive Notes: NAFLD and MAFLD/MASLD

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1. NOMENCLATURE - Understanding the Name Change

• The term "non-alcoholic" was stigmatizing and misleading - it defined disease by what it is NOT
• "Fatty" was considered offensive to patients
• "MAFLD" (proposed 2020) and then "MASLD" (agreed 2023) better reflect the true metabolic nature
• MASLD recognizes alcohol as a contributing factor in a subset (MetALD)
• The shift reflects that this disease involves a "panoply of factors" - age, sex, ethnicity, environment, diet, genetics, microbiota, and even alcohol

According to epidemiologic data, there is almost complete (at least 98%) overlap between the population historically defined as NAFLD/NASH and MASLD/MASH using the new diagnostic criteria. - Harrison's 22e

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2. DEFINITION

1. Does NOT drink excessive alcohol (< 20 g/day for women, < 30 g/day for men)
2. Does NOT have another secondary cause of hepatic steatosis
3. Has at least ONE cardiometabolic risk factor (for MASLD definition)

Normal Liver → Simple Steatosis (MASL) → Steatohepatitis (MASH) → Fibrosis → Cirrhosis → HCC

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3. EPIDEMIOLOGY

• MASLD affects over 1 billion people worldwide
• Prevalence worldwide: ~25% of the general population
• In the United States: 20-30% of adults have MASLD
• MASH prevalence: 3-6% of the general population (but 14% in adults ≥50 years undergoing routine screening)
• MASH is now the 2nd most common indication for liver transplantation in the United States
• Only about 4% of patients with steatosis progress to cirrhosis
• BUT roughly 20% of patients who progress from steatosis to MASH will develop cirrhosis over their lifetime
• Incidence in the US: 77-329 cases per 100,000 person-years
• Expected to increase 60% by 2030

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4. RISK FACTORS AND ASSOCIATIONS

• Obesity (especially visceral/abdominal obesity) - the most important driver
• Type 2 Diabetes Mellitus (T2DM) / Insulin resistance
• Hypertension
• Dyslipidemia (high triglycerides, low HDL)
• Metabolic syndrome

• MASLD can occur in lean individuals (especially common in Asian populations)
• Even in lean NAFLD, there is increased visceral adiposity despite normal BMI
• Lean patients with NAFLD may paradoxically be at greater risk for severe liver disease (HR 2.69) compared to overweight/obese patients

• Hypothyroidism
• Polycystic ovarian syndrome (PCOS)
• Obstructive sleep apnea (causes intermittent hypoxia → disease progression)
• High-fructose diet
• Sedentary lifestyle

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5. PATHOGENESIS - How Does Fat Accumulate in the Liver?

The "Two-Hit" Model (Old)

The "Multiple Parallel Hits" Model (Current)

• Obesity → visceral adipose tissue becomes inflamed and dysfunctional
• Adipocytes produce excess free fatty acids (FFA) due to overactive lipolysis (insulin can't suppress it)
• Adipocytes produce less adiponectin (the protective hormone) → less fatty acid oxidation in muscle → more FFA going to liver
• Adipocytes produce more TNF-α and IL-6 (pro-inflammatory) → further impairs insulin signaling

1. Increased FFA uptake from circulation (from inflamed adipose tissue)
2. Increased de novo lipogenesis (DNL) - liver makes new fat from dietary carbohydrates (especially fructose and glucose)
3. Decreased fatty acid oxidation (mitochondrial dysfunction)
4. Decreased VLDL export (triglycerides get stuck in the liver)

• Lipid peroxidation from oxidative stress damages mitochondrial and plasma membranes
• Endoplasmic reticulum (ER) stress → unfolded protein response
• Gut-derived endotoxins (LPS) from dysbiotic microbiome reach liver via portal vein → activate Kupffer cells → release TNF-α, IL-6, IL-1β
• Kupffer cell activation → pro-inflammatory cytokines → hepatocyte apoptosis and necrosis
• Hepatocyte death → release of DAMPs (damage-associated molecular patterns) → further inflammation

• Hepatocyte injury → release of TGF-β and other cytokines
• Hepatic stellate cells get activated → transform into myofibroblasts
• Myofibroblasts synthesize collagen → scarring (fibrosis)
• Fibrosis begins in Zone 3 (perisinusoidal/perivenular) - this is characteristic of NASH
• Progressive fibrosis → bridging fibrosis → cirrhosis

Diet (excess refined sugars + fat) + Genetics
↓
Gut changes: ↓ GLP-1, altered enteroendocrine response
↓
Obesity → Visceral adiposity expansion
↓
Insulin resistance → ↑ FFA, ↑ TNF-α, ↓ Adiponectin
↓
Liver: ↑ DNL + ↑ FFA uptake + ↓ β-oxidation + ↓ VLDL export
↓
Hepatic Steatosis
↓
Oxidative stress + ER stress + Gut endotoxins + Dysbiosis
↓
MASH → Stellate cell activation → Fibrosis → Cirrhosis

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6. GENETICS

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7. HISTOPATHOLOGY - What the Liver Biopsy Shows

Features of Simple Steatosis:

• Macrovesicular steatosis (large fat droplets push nucleus to side) - mainly in Zone 3 or panacinar
• No significant inflammation
• No hepatocyte ballooning

Features of MASH (steatohepatitis):

1. Macrovesicular steatosis (≥5% hepatocytes)
2. Hepatocyte ballooning (hepatocytes enlarge, cytoplasm becomes irregularly clumped with optically clear areas)
   • Ballooned cells may contain Mallory-Denk bodies (MDB) - eosinophilic ropey cytoplasmic inclusions of hyperphosphorylated, misfolded cytokeratin filaments
3. Lobular inflammation (mixed, predominantly mononuclear cells)

Fibrosis Staging in NAFLD:

NAFLD Activity Score (NAS):

• NAS ≥ 5: correlates well with diagnosis of steatohepatitis
• NAS < 3: unlikely to be steatohepatitis
• Fibrosis is scored separately (0-4) and is the most important prognostic factor

Pediatric NAFLD:

• Type 1: Resembles adult NASH - ballooning + perisinusoidal fibrosis
• Type 2: Steatosis + portal inflammation + portal fibrosis (without ballooning or perivenular fibrosis) - more common in children

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8. CLINICAL FEATURES

• Incidentally elevated liver enzymes (ALT > AST in early disease; AST > ALT suggests advanced disease/cirrhosis)
• Incidental hepatic steatosis on imaging (ultrasound for unrelated reasons)
• Right upper quadrant discomfort or vague abdominal fullness
• Fatigue

• Hepatomegaly (common)
• Features of metabolic syndrome: central obesity, acanthosis nigricans
• In advanced cirrhosis: spider angiomata, palmar erythema, jaundice, ascites, splenomegaly, caput medusae

• ALT and AST mildly elevated (usually 1-4x normal)
• ALT typically > AST in early NAFLD
• AST:ALT ratio > 1 suggests cirrhosis (or alcoholic liver disease)
• Serum ferritin often elevated (not diagnostic but common)
• Fasting glucose, HbA1c elevated if T2DM present
• Dyslipidemia (high triglycerides, low HDL)
• In cirrhosis: low albumin, prolonged INR, thrombocytopenia, elevated bilirubin

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9. DIAGNOSIS

Step 1: Clinical Assessment

• History: rule out significant alcohol use (<20g/day women, <30g/day men), medications, other liver diseases
• Look for features of metabolic syndrome
• Rule out other causes: viral hepatitis (HBsAg, anti-HCV), autoimmune liver disease, Wilson's disease, hemochromatosis

Step 2: Non-Invasive Testing

FIB-4 = (Age × AST) / (Platelet count × √ALT)

• FIB-4 < 1.3: Low risk - can be followed in primary care, reassess every 2-3 years
• FIB-4 1.3-2.67: Indeterminate - needs further testing (elastography)
• FIB-4 > 2.67: High risk - refer to specialist, consider liver biopsy
• Note: Less accurate in patients <35 or >65 years old

• Fatty Liver Index (FLI): Uses BMI, waist circumference, triglycerides, GGT - score >60 has 86% specificity for steatosis
• NAFLD Fibrosis Score (NFS): Uses age, BMI, hyperglycemia, AST/ALT ratio, platelets, albumin
• BARD score: BMI, AST/ALT ratio, diabetes

Suspected MASLD (metabolic risk factors or elevated LFTs)
↓
FIB-4 score
↓
< 1.3 → Low risk → Primary care follow-up, FIB-4 every 1-2 years (T2DM/metabolic syndrome) or 2-3 years
≥ 1.3 → Intermediate/High risk → Add elastography (FibroScan or MRE)
↓
LSM < 8 kPa → Rules out advanced fibrosis
LSM 8-12 kPa → Indeterminate → Refer to hepatologist; consider ELF test
LSM > 12 kPa → High risk for advanced fibrosis → Refer + consider biopsy

Step 3: Liver Biopsy (Gold Standard)

• After excluding other causes of liver disease
• To distinguish simple steatosis from MASH
• To stage fibrosis (when non-invasive tests are indeterminate or discordant)
• For clinical trials
• When the result will change management

• Clinical evidence of advanced cirrhosis already present (AST>ALT, thrombocytopenia, hypoalbuminemia, portal vein >1.1 cm, varices, ascites)
• Routine screening

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10. NATURAL HISTORY AND PROGNOSIS

• Simple steatosis: only 1-4% progress to advanced fibrosis
• MASH: 25-50% develop progressive fibrosis; 15-25% progress to cirrhosis
• Once cirrhosis develops: standard complications of portal hypertension apply

• Fibrosis stage is the strongest predictor of liver-related mortality
• Higher fibrosis stage = higher risk of liver-related events and all-cause mortality

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11. EXTRAHEPATIC COMPLICATIONS

Cardiovascular Disease (Most Important Extrahepatic Complication)

• NAFLD is associated with increased CVD mortality - this is actually the leading cause of death in NAFLD patients (more than liver-related deaths in early disease)
• Mechanism:
  • ↑ Hepatic triglyceride and VLDL production
  • Large VLDL → slow metabolism → small dense LDL (sd-LDL) formation
  • sd-LDL has lower affinity for LDL receptors → stays in circulation longer → penetrates arterial intima
  • Oxidized sd-LDL triggers macrophage foam cell formation → atherosclerotic plaque
  • Upregulation of VCAM-1 and ICAM-1 → monocyte adhesion and migration → inflammation

Type 2 Diabetes

• Bidirectional relationship: insulin resistance drives both T2DM and NAFLD
• NAFLD independently predicts development of T2DM

Hepatocellular Carcinoma (HCC)

• NAFLD is now a major cause of HCC
• HCC can develop even without cirrhosis (unlike most other liver diseases) - this is a unique and important feature
• PNPLA3 I148M variant is associated with increased HCC risk

Chronic Kidney Disease (CKD)

• MASLD independently increases risk of CKD (reviewed in recent meta-analyses, PMID 38141808)

Extrahepatic Cancers

• Colorectal cancer: OR 1.87-3.08
• Also associated with prostate, breast, gastric, esophageal cancers (though data less robust)
• Mechanism: pro-inflammatory, pro-carcinogenic effects of insulin resistance, gut microbiome dysbiosis, adipokines

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12. TREATMENT AND MANAGEMENT

A. Lifestyle Modification (Foundation of All Treatment)

• 7% weight loss → resolution of steatohepatitis in many patients
• 10% weight loss → improvement in fibrosis
• Important caveat (Yamada's): These are averages - many achieving 7% loss do NOT resolve MASH, and many with fibrosis improvement had <10% weight loss. Weight loss is not a simple linear function.

• Mediterranean diet is the most evidence-based dietary approach
• Reduce refined sugars (especially fructose - high-fructose corn syrup is a major driver)
• Reduce saturated and trans-fats
• The cultural basis of eating habits is a major barrier to long-term dietary change

• Aerobic exercise reduces hepatic steatosis independently of weight loss
• Resistance training also beneficial
• Reduces hepatic fat content and insulin resistance

• Stress eating, binge eating, nocturnal eating disorders
• Social determinants: food access, work hours, family responsibilities
• Multidisciplinary approach recommended (psychologist + nutritionist + physician)
• Non-judgmental approach is crucial - treating patients as "lazy" is both ineffective and inappropriate

B. Pharmacological Treatment for Metabolic Comorbidities

• Pioglitazone (Thiazolidinedione/PPAR-γ agonist): Most studied drug for NASH
  • Improves insulin resistance
  • Reduces steatosis, lobular inflammation, hepatocellular ballooning
  • Antifibrotic effects (meta-analysis supported)
  • Limitations: weight gain (persists after stopping), fluid retention, increased fracture risk (especially older women)
  • Cardiovascular safety: neutral
  • Use: particularly beneficial in patients with T2DM + NASH

• Semaglutide (Ozempic, Wegovy): Significant evidence for MASH improvement
  • Recent systematic review and meta-analysis (PMID 40489581, 2025) confirms efficacy of GLP-1 based therapies for MASLD/MASH
  • Reduces steatosis, NAS, and potentially fibrosis
  • Beneficial for T2DM and obesity comorbidities
  • Also reduces cardiovascular events (major benefit in NAFLD patients who die of CVD)
• Other GLP-1 agonists: Liraglutide also studied

• Antioxidant; improves MASH in non-diabetic patients (PIVENS trial)
• Concern: possible increased risk of prostate cancer and stroke at high doses
• Not recommended in diabetes or cirrhosis

• NOT proven to treat liver histology in NAFLD
• BUT can and should be used to treat dyslipidemia (they are NOT contraindicated in NAFLD/MASH)

• Does NOT improve liver histology in NAFLD
• Can be used for T2DM management

C. FDA-Approved Treatment for MASH (NEW - 2024)

• Mechanism: Thyroid hormone receptor-β (THR-β) selective agonist
  • THR-β is expressed predominantly in the liver
  • Activating THR-β promotes fatty acid oxidation in hepatocytes
  • Avoids cardiovascular and bone side effects of systemic thyroid hormone action (those are mediated by THR-α)
• Phase III trial (MAESTRO-NASH): demonstrated:
  • Biopsy improvement of MASH (primary endpoint met)
  • Improvement of MAFLD/NAFLD activity scores
  • Significant reduction in LDL cholesterol
• Recent systematic review confirms efficacy (PMID 39187533)
• Indicated for: MASH with moderate-to-advanced fibrosis (F2-F3) in adults

D. Bariatric Surgery / Metabolic Surgery

• Most effective treatment for obesity-related NAFLD
• Roux-en-Y gastric bypass and sleeve gastrectomy both improve/resolve MASH and fibrosis
• Duodenal mucosal resurfacing: Endoscopic emerging option - removes the proximal duodenum from food contact → metabolic benefits
• Considerations: increased suicidal risk and alcohol addiction transfer post-bariatric surgery must be screened for

E. Treatment of Advanced Disease (Cirrhosis)

• Manage portal hypertension complications (varices, ascites, SBP, hepatic encephalopathy)
• HCC surveillance: Ultrasound every 6 months (± AFP) - note: HCC can occur even without cirrhosis in NAFLD
• Liver transplantation:
  • MASH is now the 2nd most common indication for liver transplantation in the US
  • MASH cirrhosis may recur post-transplant if metabolic syndrome not controlled
  • Growing incidence of MAFLD in donors is affecting organ availability (steatotic livers are less suitable for donation)

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13. SPECIAL POPULATIONS

Pediatric NAFLD/MASLD

• Increasingly common with rising childhood obesity
• Type 2 histology more common in children (portal-based fibrosis, no ballooning)
• Treatment: lifestyle modification, weight management; resmetirom not approved in pediatrics yet
• Systematic review on interventions in pediatric MASLD available (PMID 39574069)

Lean NAFLD

• BMI < 25 kg/m² (Asian cutoff: < 23 kg/m²)
• Particularly common in East Asians
• Associated with visceral adiposity despite normal BMI
• Higher proportional risk for severe liver disease (HR 2.69) vs obese NAFLD
• Mechanism: genetic predisposition + altered fat distribution

Diabetes + NAFLD

• Extremely common combination - T2DM dramatically accelerates NAFLD progression
• Insulin (used for glycemic control in cirrhosis) can cause weight gain - must be monitored carefully in transplant candidates

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14. KEY DIFFERENCES BETWEEN NAFLD/MASLD AND ALCOHOLIC LIVER DISEASE (ALD)

"It is not possible to reliably distinguish alcohol-associated steatohepatitis from MASH based on histologic findings alone." - Robbins & Cotran

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15. MONITORING AND FOLLOW-UP

• Repeat FIB-4 every 1-2 years if T2DM or metabolic syndrome; every 2-3 years if low-risk
• Screen for T2DM (fasting glucose, HbA1c) and dyslipidemia
• Cardiovascular risk assessment and management

• Close specialist follow-up
• Consider pharmacologic therapy
• Serial elastography for fibrosis monitoring

• 6-monthly HCC surveillance with ultrasound ± AFP
• Endoscopy for varices
• Manage portal hypertension

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16. SUMMARY TABLE - NAFLD vs MAFLD vs MASLD at a Glance

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Recent Evidence Updates (2024-2025)

• Resmetirom (PMID 39187533, Suvarna et al., 2024): Systematic review confirms efficacy and safety of resmetirom (THR-β agonist) as first-in-class treatment for MASLD with MASH histology improvement.
• GLP-1 agonists (PMID 40489581, Wang et al., 2025): Meta-analysis confirms GLP-1 based therapies (semaglutide, liraglutide) are effective for both MASLD and MASH - reducing steatosis and potentially improving fibrosis.
• MASLD and CKD (PMID 38141808, Bilson et al., 2024): Steatotic liver disease independently increases risk of chronic kidney disease - supporting multi-organ monitoring.
• Nomenclature (PMID 38653634, Portincasa & Baffy, 2024): The evolution from NAFLD → MAFLD → MASLD is reviewed, with MASLD now being the consensus international terminology with >98% overlap with the old NAFLD population.

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