What Causes Nash And How It Damages Your Liver?

Nonalcoholic steatohepatitis, or NASH, is a form of fatty liver disease in which fat accumulation is accompanied by active inflammation and liver cell injury. The word “steato” refers to fat, and “hepatitis” means inflammation of the liver. Unlike simple fatty liver, where fat builds up but the liver is largely quiet, NASH involves ongoing damage that can lead to scarring. Over time, that scarring can progress to cirrhosis, a condition where healthy liver tissue is replaced by stiff scar tissue that impairs the organ’s ability to function.

What Causes NASH And How Does It Develop?

NASH is not caused by a single factor. It develops when multiple stresses on the liver converge. The most common underlying driver is insulin resistance, a condition in which cells respond poorly to insulin and the body compensates by producing more of it. Insulin resistance is strongly associated with excess weight, particularly fat stored around the abdomen, and with type 2 diabetes.

Here is what happens at the cellular level. Insulin resistance increases the flow of free fatty acids from fat tissue into the liver. The liver takes up more fat than it can export or burn. Fat accumulates inside liver cells. This alone is steatosis — simple fatty liver. In some people, the process does not stop there. The excess fat triggers oxidative stress and the release of inflammatory signals. Liver cells begin to die. The immune system responds, and the cycle of injury and inflammation becomes self-sustaining. That is when simple fatty liver becomes NASH.

Researchers have identified several processes that appear to push the disease forward, including dysfunction in the mitochondria (the energy-producing structures inside cells), changes in how the liver handles iron, and alterations in the gut microbiome. The evidence for each of these is at different stages. Insulin resistance and fat accumulation are the best-established initiating factors.

It is important to be clear about what does not cause NASH. Alcohol-related liver disease can look similar under a microscope, but NASH by definition occurs in people who drink little or no alcohol. The distinction matters because the two conditions have different causes and different management approaches.

Who Is Most at Risk for NASH?

The strongest risk factors for NASH are conditions that promote insulin resistance. These include:

• Obesity, especially when fat is concentrated around the abdomen
• Type 2 diabetes — a large share of people with type 2 diabetes have some form of fatty liver disease
• Metabolic syndrome, which combines high blood pressure, abnormal cholesterol or triglycerides, elevated blood sugar, and excess abdominal fat
• Prediabetes and other conditions involving impaired glucose handling

But NASH also occurs in people without these conditions. Some people with a healthy body weight develop NASH, a pattern sometimes called “lean NASH.” This suggests that genetics, diet quality, and other factors independent of body weight play a role. Research into genetic variants associated with fatty liver disease has identified several gene variations that influence how the liver handles fat. This is an active area of study, and the picture is not complete.

Certain medications, including some corticosteroids and certain drugs used for irregular heart rhythms, have been associated with fatty liver. Rapid weight loss and poor nutrition can also contribute. These are less common causes than insulin resistance, but they matter in individual cases.

How Does NASH Damage the Liver Over Time?

The damage in NASH follows a recognizable pattern, though the speed varies widely from person to person. The initial injury is fat accumulation. Then inflammation sets in. Then the liver attempts to repair itself by laying down collagen, the same protein found in scar tissue.

In the early stages, this collagen is laid down in a fine meshwork around liver cells. If the injury continues, the scar tissue accumulates and begins to bridge across the liver. This stage is called fibrosis. Fibrosis is graded on a scale from F0 (no scarring) to F4 (cirrhosis). The difference between F1 and F3 is significant — at F1, the scarring is mild and may be reversible if the underlying cause is addressed. At F3, the scarring is extensive and the risk of progression to cirrhosis is substantially higher.

Cirrhosis, or F4, is the end stage of scarring. At this point, the liver’s architecture is distorted. Blood flow through the liver is impaired, which can lead to portal hypertension — increased pressure in the veins that carry blood from the digestive organs to the liver. Portal hypertension can cause fluid accumulation in the abdomen (ascites), enlarged veins in the esophagus that may bleed, and confusion or cognitive changes (hepatic encephalopathy).

Cirrhosis from NASH also increases the risk of liver cancer, even in people who have not developed severe liver failure. This is one reason why NASH is taken seriously. It is not simply a benign accumulation of fat.

Not everyone with NASH progresses. Some people remain stable for years. Others develop cirrhosis within a decade. Researchers are still working to understand what determines the rate of progression. The presence of type 2 diabetes, ongoing alcohol use, and certain genetic factors appear to make faster progression more likely.

What Are the Symptoms of NASH?

Most people with NASH have no symptoms. The liver does not have pain receptors in its interior, so early damage is silent. When symptoms do occur, they are often vague — fatigue, a general sense of not feeling well, or mild discomfort in the upper right abdomen.

Because symptoms are absent or nonspecific, NASH is usually discovered incidentally. A routine blood test may show mildly elevated liver enzymes, such as ALT or AST. An imaging study done for another reason may reveal fat in the liver. Sometimes NASH is found during an evaluation for an unrelated condition.

When NASH has progressed to cirrhosis, symptoms become more apparent. These can include swelling in the legs and abdomen, easy bruising or bleeding, yellowing of the skin and eyes (jaundice), and confusion. By the time these symptoms appear, the disease is advanced. This is why NASH is often called a silent condition.

How Is NASH Diagnosed?

Diagnosis typically begins with blood tests and imaging. Elevated liver enzymes are a clue, but normal enzymes do not rule out NASH. Many people with NASH have completely normal liver enzyme levels. Imaging, such as ultrasound or a specialized MRI technique called elastography, can detect fat and estimate stiffness in the liver.

However, imaging cannot definitively distinguish simple fatty liver from NASH. The difference is inflammation and cell injury at the microscopic level. For years, the only way to confirm NASH was a liver biopsy — a procedure where a small sample of liver tissue is removed and examined under a microscope. Biopsy remains the most accurate test, but it is invasive, carries a small risk of complications, and samples only a tiny portion of the liver.

Newer blood-based tests and imaging techniques are being developed to estimate the likelihood of NASH and significant fibrosis without biopsy. Some are already used in clinical practice to identify people who should be referred for further evaluation. None has fully replaced biopsy for definitive diagnosis, but the field is moving in that direction.

Can NASH Be Treated or Reversed?

There is no medication approved specifically for NASH in the United States as of now, though several drugs are in late-stage clinical trials. The most effective approach currently available is weight loss through diet and physical activity. Research has shown that losing a meaningful amount of body weight — generally in the range of 7 to 10 percent of starting weight — can reduce liver fat, decrease inflammation, and in some cases improve fibrosis.

That said, the evidence on how much fibrosis can reverse is mixed. Some studies show improvement, particularly at earlier stages. Reversal of established cirrhosis is uncommon. The goal in advanced disease is often to prevent further progression and manage complications.

Other management strategies include controlling blood sugar in people with diabetes, lowering cholesterol and triglycerides when elevated, and avoiding alcohol entirely. Some medications used to treat diabetes, particularly certain classes, have shown promise in reducing liver fat and inflammation in clinical trials. This is an area where guidance is evolving.

Bariatric surgery can lead to significant and sustained weight loss in people with severe obesity, and some studies indicate it may improve NASH. It is not a first-line treatment for NASH alone, but it may be considered in appropriate candidates.

What does not work: no supplement, detox, or herbal product has been shown in rigorous clinical trials to reverse NASH. The evidence for milk thistle, for example, is mixed and generally considered insufficient to recommend it as a treatment. Anyone considering a supplement for liver health should discuss it with a doctor, because some supplements can actually harm the liver.

What Is the Difference Between NAFLD and NASH?

NAFLD stands for nonalcoholic fatty liver disease. It is an umbrella term that includes two main conditions: simple fatty liver (also called nonalcoholic fatty liver, or NAFL) and NASH. In simple fatty liver, fat is present but there is little or no inflammation or cell injury. In NASH, inflammation and injury are present.

The distinction matters because the risk of progression is different. Simple fatty liver generally has a lower risk of advancing to cirrhosis. NASH has a higher risk, though it is still not guaranteed to progress. Some people with simple fatty liver remain stable for life. Some with NASH also remain stable. But on average, NASH carries a greater likelihood of fibrosis progression.

Recently, some medical organizations have proposed replacing the term NAFLD with a new term, metabolic dysfunction-associated steatotic liver disease, or MASLD. The goal is to use language that better reflects the metabolic roots of the condition and reduces stigma. The terminology is still transitioning, and you may see both terms used.