NASH stands for nonalcoholic steatohepatitis, a form of liver disease in which fat builds up in the liver and causes inflammation and cell damage. It’s more serious than simple fatty liver because the inflammation can eventually lead to scarring (fibrosis) and, in some cases, cirrhosis. As of 2023, the medical community officially renamed NASH to MASH, which stands for metabolic dysfunction-associated steatohepatitis.
Why the Name Changed to MASH
The original term “nonalcoholic steatohepatitis” was always something of a placeholder. It defined the condition by what it wasn’t (not caused by alcohol) rather than what it was. As researchers learned more, it became clear that the vast majority of cases are driven by metabolic factors: insulin resistance, visceral obesity, and abnormal blood lipids. The name didn’t reflect any of that.
There were also concerns about stigma. Both “nonalcoholic” and “fatty” in the older naming convention were seen as potentially stigmatizing to patients. In 2023, a global initiative led by the American Association for the Study of Liver Diseases replaced NAFLD (nonalcoholic fatty liver disease) with MASLD (metabolic dysfunction-associated steatotic liver disease) and NASH with MASH. The goal was an affirmative diagnosis that describes the actual biology of the disease, not just the absence of alcohol use. You’ll still see “NASH” used widely in older literature and everyday conversation, but clinical guidelines now use MASH.
What Happens in the Liver
A diagnosis of NASH (or MASH) requires three things happening simultaneously in liver tissue: excess fat accumulation, inflammation, and a specific type of cell injury called hepatocyte ballooning, where liver cells swell and become damaged. Simple fatty liver, by contrast, involves fat deposits without significant inflammation or damage. That distinction matters because inflammation is what drives scarring.
Progression to cirrhosis occurs in roughly 3 to 5% of patients and often takes more than 20 years. One large analysis found that about 3% of people with fatty liver disease developed cirrhosis within 15 years. So while NASH is a serious condition, most people with it will not reach end-stage liver disease, particularly if the underlying metabolic drivers are managed.
Symptoms Are Rare Until Late Stages
NASH is largely a silent disease. Most people have no symptoms at all, even after years of ongoing liver inflammation. Some people feel fatigue or a vague discomfort in the upper right side of the abdomen, but these are easy to overlook or attribute to other causes. You may not develop noticeable symptoms even if cirrhosis has already set in, which is why the condition is often caught incidentally during blood work or imaging done for unrelated reasons.
When symptoms do appear in advanced disease, they reflect significant liver damage: yellowing of the skin and eyes, fluid accumulation in the abdomen, easy bruising, and confusion caused by toxins the liver can no longer filter. By that point, the disease has progressed well beyond the NASH stage.
How It’s Detected
Because symptoms are unreliable, doctors use a combination of blood tests and imaging to identify the disease and gauge its severity. Screening typically starts with a simple blood-based score called FIB-4, which uses age, platelet count, and liver enzyme levels to estimate the likelihood of significant scarring. A FIB-4 score below 1.3 rules out advanced fibrosis in about 89% of patients, making it a useful first filter.
If the FIB-4 score is elevated, the next step is usually a specialized ultrasound called vibration-controlled transient elastography (often known by the brand name FibroScan). This measures liver stiffness, which correlates with the degree of scarring. It’s painless and takes a few minutes, though its accuracy decreases in people with obesity or who haven’t fasted beforehand. MRI-based elastography is more precise but also more expensive and less widely available.
Liver biopsy remains the most definitive way to confirm a NASH diagnosis, since it’s the only test that can directly visualize fat, inflammation, and cell ballooning at the same time. But the trend in clinical practice is toward using non-invasive tests first and reserving biopsy for cases where the diagnosis is uncertain or treatment decisions hinge on the exact stage of fibrosis.
Treatment Options
For most people with NASH, the core treatment is addressing the metabolic factors behind it: losing weight, improving diet, and increasing physical activity. Even a modest weight loss of 5 to 10% of body weight can significantly reduce liver fat, inflammation, and fibrosis. This remains the most effective intervention for the majority of patients.
In March 2024, the FDA approved the first medication specifically for NASH. Sold under the brand name Rezdiffra, it’s indicated for adults with moderate to advanced liver scarring (stages F2 to F3 fibrosis) who have not yet developed cirrhosis. It’s meant to be used alongside diet and exercise, not as a replacement. It is not approved for people with decompensated cirrhosis, where the liver has already lost significant function. The approval marked a milestone after decades in which no drug had been specifically authorized for this condition.
Who Is at Risk
The metabolic factors that drive NASH overlap heavily with those behind type 2 diabetes and cardiovascular disease. Insulin resistance is central to the process. People who carry excess weight, particularly around the midsection, who have elevated blood sugar, high triglycerides, or low HDL cholesterol are at the highest risk. Type 2 diabetes is one of the strongest individual predictors: people with diabetes develop NASH at significantly higher rates than the general population.
That said, NASH can also occur in people who are not overweight, particularly in certain ethnic groups or in people with specific genetic predispositions. The new MASH criteria explicitly tie the diagnosis to the presence of at least one metabolic risk factor, which helps clinicians identify the condition even in patients who don’t fit the stereotypical profile.