Fatty Liver Disease:
Hepatic Steatosis and MASLD

1. Introduction: A Silent Epidemic

Hepatic steatosis — the accumulation of fat in the liver exceeding 5% of the organ's weight — is the most prevalent liver disease in the world. It is estimated that between 25% and 38% of the global population is affected, totaling approximately 2 billion people. In Brazil, population-based studies suggest a prevalence of 20–35%, with higher rates in urban populations and those with greater access to ultra-processed diets.

The disease is often called a "silent killer" because most patients remain completely asymptomatic for decades. The liver has no nerve endings in its parenchyma; therefore, even with significant inflammation and fibrosis, the patient rarely feels pain. When symptoms do appear — ascites, jaundice, bleeding from esophageal varices — the disease is already at an advanced stage of cirrhosis.

MASLD is closely linked to metabolic syndrome: type 2 diabetes, visceral obesity, dyslipidemia, and arterial hypertension form a constellation of mutually reinforcing factors. Understanding fatty liver is therefore understanding the global metabolic crisis of the 21st century.

2. Nomenclature: From NAFLD to MASLD

For decades, the classification of this disease was based on a definition of exclusion: it was "non-alcoholic" because it required the absence of significant alcohol consumption. This approach was problematic for several reasons.

2.1 Why the Change?

The term "non-alcoholic" carried a double stigma. First, it implied that patients needed to "prove" they did not drink, generating clinical discomfort. Second, it defined the disease by what it is not — rather than by what it is — obscuring its true metabolic etiology. Furthermore, the binary exclusion criterion did not account for patients with moderate alcohol consumption who simultaneously present metabolic dysfunction, an extremely common clinical reality.

The 2023 Delphi consensus, published in the Journal of Hepatology and Hepatology, proposed an approach based on inclusion criteria: the diagnosis of MASLD requires evidence of hepatic steatosis plus at least one of five cardiometabolic risk factors (overweight/obesity, type 2 diabetes, insulin resistance, dyslipidemia, or hypertension).

2.2 The New Taxonomy

3. Pathophysiology: The Multiple-Hit Theory

The former "two-hit hypothesis" — proposed by Day and James in 1998 — suggested that steatosis ("first hit") made the liver vulnerable to a "second hit" (oxidative stress) that triggered inflammation. Although elegant, this theory was deemed overly simplistic.

The current model is the multiple-hit hypothesis, which recognizes parallel and simultaneous insults to the hepatocyte, all converging toward disease progression.

3.1 Mechanisms of Injury

This integrated model explains why patients with the same degree of steatosis can have radically different outcomes: the combination and intensity of "hits" is unique to each individual. Insulin resistance remains the central mechanism, acting as the "conductor" orchestrating all other processes.

4. Risk Factors and Epidemiology

The prevalence of MASLD mirrors the global pandemic of obesity and diabetes. Data from the Global Burden of Disease study show a 50% increase in cases between 1990 and 2019, with alarming projections for the coming decades.

4.1 Major Risk Factors

• Obesity: Between 70–80% of individuals with obesity present some degree of hepatic steatosis. Visceral (abdominal) fat is more predictive than BMI alone.
• Type 2 Diabetes: Up to 70% of patients with type 2 diabetes have MASLD, and the presence of steatosis accelerates diabetic complications.
• Metabolic Syndrome: Hypertension, elevated triglycerides, low HDL, and increased waist circumference form the constellation of maximum risk.
• Sedentary Lifestyle: Physical inactivity reduces fatty acid oxidation by skeletal muscle, redirecting lipids to the liver.
• Diet: Excessive consumption of fructose (soft drinks, industrialized juices), saturated fats, and ultra-processed carbohydrates promotes hepatic de novo lipogenesis.
• Genetics: The PNPLA3 I148M polymorphism (prevalent in Hispanics) increases the risk of progression by 3–4×. The TM6SF2 E167K variant confers hepatic risk but paradoxically protects against cardiovascular events.

5. Diagnosis: Non-Invasive Methods

Historically, liver biopsy was the gold standard for diagnosis and staging. However, it is an invasive procedure, subject to sampling error (the specimen represents only 1/50,000 of the liver) and carries a risk of complications. Modern hepatology prioritizes non-invasive tests (NITs — Non-Invasive Tests), reserving biopsy for selected cases.

5.1 Comparison of Diagnostic Methods

The current EASL 2024 guideline recommendation is a stepwise approach: first, calculate the FIB-4 in all at-risk patients. If ≥ 1.30, proceed with elastography. If liver stiffness is ≥ 8 kPa, refer to a hepatologist for specialized evaluation and possible biopsy.

6. Disease Stages

MASLD is not a static entity. It represents a continuous spectrum of histological severity, from harmless fat accumulation to hepatocellular carcinoma.

6.1 Natural Progression

1. Simple Steatosis (MASL): Fat accumulation in > 5% of hepatocytes, without significant inflammation. Present in most patients. Excellent prognosis when isolated — the majority never progress.
2. Steatohepatitis (MASH): Steatosis + lobular inflammation + hepatocellular ballooning. This is the "tipping point" of the disease, representing 20–30% of MASLD cases. Significant risk of progression to fibrosis.
3. Fibrosis (F0–F4): Progressive collagen deposition in the liver. F0 = no fibrosis; F1 = perisinusoidal; F2 = significant portal; F3 = bridging fibrosis; F4 = cirrhosis. The fibrosis stage is the strongest predictor of mortality.
4. Cirrhosis: End-stage fibrosis with complete architectural remodeling of the liver. Risk of hepatic failure, portal hypertension, esophageal varices, and need for transplantation.
5. Hepatocellular Carcinoma (HCC): Can arise in the setting of established cirrhosis, but — unlike viral hepatitis — up to 20–50% of HCC cases due to MASLD occur in non-cirrhotic livers, making surveillance more challenging.

7. Cardiovascular and Metabolic Impact

A finding that frequently surprises both patients and non-hepatologist physicians: the leading cause of death in patients with MASLD is not liver disease, but cardiovascular disease. Cohort studies with 10–20-year follow-up demonstrate that cardiovascular events (myocardial infarction, stroke, heart failure) account for 40–45% of deaths, while hepatic complications represent 10–15%.

7.1 Mechanisms of the Cardiovascular Association

The steatotic liver is not a passive bystander. It becomes a "factory" of atherogenic and pro-inflammatory mediators:

• Increased VLDL production: Greater secretion of triglyceride-rich particles, raising atherogenic cholesterol (small dense LDL).
• Systemic inflammation: Hepatic release of CRP, IL-6, fibrinogen, and other acute-phase reactants that promote atherosclerosis.
• Pro-thrombotic coagulopathy: Increased PAI-1 (Plasminogen Activator Inhibitor), factor VIII, and fibrinogen.
• Systemic insulin resistance: Hepatic steatosis amplifies insulin resistance, accelerating progression to type 2 diabetes.

7.2 Other Associated Comorbidities

MASLD is independently associated with chronic kidney disease (1.5–2× higher risk), obstructive sleep apnea, polycystic ovary syndrome, and depression. Increasingly, hepatic steatosis is understood not as a "liver disease" but as the hepatic manifestation of a systemic metabolic disease.

8. Treatment: Lifestyle as Medicine

Until recently, no medication had been specifically approved for MASLD/MASH. The cornerstone of treatment has always been — and continues to be — lifestyle modification. The good news: the results are extraordinary when implemented consistently.

8.1 Weight Loss: The Dose-Response Relationship

The relationship between weight loss and histological improvement is dose-dependent and well established by the landmark study by Vilar-Gomez et al. (2015):

• ≥ 3–5% of body weight: Significant reduction in hepatic steatosis.
• ≥ 7% of body weight: Resolution of steatohepatitis (MASH) in 64% of patients.
• ≥ 10% of body weight: Fibrosis regression in 45% of patients — including advanced fibrosis (F3).

8.2 Mediterranean Diet

The Mediterranean diet has the strongest scientific evidence for MASLD. Rich in extra-virgin olive oil, fatty fish (omega-3), nuts, vegetables, whole grains, and polyphenols, it reduces steatosis independently of weight loss. Mechanisms include modulation of de novo lipogenesis, improved insulin sensitivity, and anti-inflammatory effects.

Foods to avoid: added fructose (soft drinks, industrialized juices, high-fructose corn syrup — a potent inducer of hepatic lipogenesis), trans fats, ultra-refined carbohydrates, and alcohol (even in "moderate" amounts, the interaction with MASLD amplifies liver damage).

8.3 Physical Exercise

Exercise reduces hepatic fat even without significant weight loss, by increasing fatty acid oxidation and improving muscular insulin sensitivity. Evidence-based recommendations include:

• Aerobic exercise: 150–300 minutes per week of moderate intensity (brisk walking, cycling, swimming) or 75–150 minutes of vigorous intensity.
• Resistance training: 2–3 sessions per week with progressive loading. Increased muscle mass improves glucose uptake and resting lipid oxidation.
• Reducing sedentary time: Beyond structured exercise, reducing sitting time and increasing NEAT (non-exercise activity thermogenesis) contributes significantly.

9. Pharmacological Perspectives

The pharmacological landscape of MASLD/MASH changed dramatically in 2024 with the first approval of a disease-specific medication, inaugurating a new therapeutic era.

9.1 Resmetirom (Rezdiffra®) — A Historic Milestone

In March 2024, resmetirom (Rezdiffra®) became the first medication approved by the FDA specifically for the treatment of MASH with moderate to advanced fibrosis (F2–F3). It is a selective agonist of the thyroid hormone receptor beta (THR-β), predominantly expressed in the liver.

The MAESTRO-NASH clinical trials demonstrated that resmetirom promoted resolution of MASH without worsening fibrosis in 26–30% of patients (vs. 10–15% with placebo) and improvement of fibrosis by at least one stage in 24–26% of patients after 52 weeks. Additionally, it significantly reduced LDL cholesterol, triglycerides, and liver markers.

9.2 GLP-1 Agonists: Semaglutide and Tirzepatide

Semaglutide (Ozempic®/Wegovy®) — originally developed for diabetes and obesity — has demonstrated impressive results in MASH. In the phase 2b trial, subcutaneous semaglutide achieved MASH resolution in 59% of patients (vs. 17% with placebo). Phase 3 trials are currently underway.

Tirzepatide (Mounjaro®), a dual GIP/GLP-1 agonist, has also shown significant reduction in hepatic fat in preliminary studies, potentiated by robust weight loss (15–22% of body weight). Both drugs act through weight loss, improved insulin resistance, and possible direct anti-inflammatory effects on the liver.

9.3 Other Therapies Under Investigation

• Pioglitazone: A thiazolidinedione with robust evidence for MASH resolution, but with side effects (weight gain, fluid retention) that limit its routine use. Recommended off-label by AASLD guidelines in patients with MASH and diabetes.
• Vitamin E (800 IU/day): An antioxidant with modest efficacy in MASH resolution in non-diabetic patients. Long-term safety concerns (cardiovascular risk?) limit its use to selected cases.
• FXR agonists (obeticholic acid): Improved fibrosis in clinical trials, but with significant pruritus and adverse lipid changes.
• ASK1 inhibitors, ACC inhibitors, FGF21 analogs: Multiple molecular targets in phase 2–3 development, reflecting the richness of pathogenic pathways amenable to intervention.

10. Prevention and Monitoring

Prevention of MASLD is inseparable from prevention of metabolic syndrome. Public health and individual strategies overlap.

10.1 Screening in At-Risk Groups

The EASL/AASLD 2024 guidelines recommend active screening for hepatic fibrosis in the following groups:

• All patients with type 2 diabetes (prevalence of significant fibrosis: 15–20%).
• Patients with obesity and metabolic syndrome.
• Individuals with persistently elevated liver enzymes (ALT above the upper limit of normal).
• Patients with incidental hepatic steatosis on imaging studies.
• High genetic risk populations (Hispanics, family history of cirrhosis).

10.2 Monitoring Protocol

For patients with established MASLD, longitudinal monitoring is essential:

• FIB-4 every 1–2 years in patients with simple steatosis and low risk.
• Annual elastography in patients with fibrosis ≥ F2 or confirmed MASH.
• HCC screening (ultrasound ± alpha-fetoprotein every 6 months) in all patients with cirrhosis (F4) and consider in patients with advanced fibrosis (F3).
• Periodic cardiovascular assessment: cardiac risk should be evaluated and treated with the same intensity as hepatic risk.

10.3 Final Message

Fatty liver disease is both an epidemic and an opportunity. An epidemic because its prevalence grows in parallel with the global metabolic crisis. An opportunity because, unlike many chronic diseases, MASLD in its early stages is completely reversible with accessible lifestyle interventions: balanced nutrition, regular exercise, and moderate weight loss. The greatest challenge is not the science — it is the sustained implementation of these changes in daily life.

Selected References