NASH, or nonalcoholic steatohepatitis, is a progressive form of fatty liver disease in which fat buildup in the liver triggers inflammation and damage to liver cells. It affects roughly 16% of adults worldwide and differs from simple fatty liver because the liver is actively being injured, not just storing excess fat. Left unmanaged, NASH can lead to permanent scarring and, eventually, liver failure.
How NASH Differs From Simple Fatty Liver
Having fat in your liver cells (a condition called steatosis) is extremely common and, on its own, relatively harmless. Stored fat in the form of triglycerides isn’t directly toxic to liver cells. The problem starts when other types of lipids that accumulate alongside triglycerides, including fatty acids, cholesterol, and certain oxidized fats, begin damaging those cells. This process, called lipotoxicity, is what separates NASH from simple fatty liver.
In NASH, the rate of liver cell death is significantly higher. Fatty acids overwhelm the cell’s energy-producing structures (mitochondria), generating harmful molecules that deplete the liver’s natural defenses against oxidative stress. When mitochondria become severely damaged, they can no longer produce energy, and the cell dies. That wave of cell death triggers inflammation, which in turn activates scarring. The inflammation also worsens insulin resistance, which drives even more fat into the liver, creating a self-reinforcing cycle of damage.
The Name Is Changing to MASH
In 2023, the global liver disease community officially renamed NASH to MASH, which stands for metabolic dysfunction-associated steatohepatitis. The broader umbrella term NAFLD (nonalcoholic fatty liver disease) was also renamed to MASLD (metabolic dysfunction-associated steatotic liver disease). The change happened for two reasons: the old name defined the disease by what it wasn’t (“nonalcoholic”) rather than what caused it, and surveys found that both “nonalcoholic” and “fatty” were considered stigmatizing by a majority of patients and clinicians.
The updated name also comes with clearer diagnostic criteria. A MASLD diagnosis now requires evidence of fat in the liver plus at least one of five cardiometabolic risk factors, such as elevated blood sugar, high blood pressure, or a large waist circumference. You’ll still see “NASH” widely used in medical literature and by patients, but expect “MASH” to gradually replace it.
Symptoms Are Often Absent
NASH is frequently called a “silent” disease. Many people have no symptoms at all, and the condition is often discovered incidentally through blood tests or imaging done for another reason. When symptoms do appear, the most common ones are persistent fatigue and a dull ache or fullness in the upper right side of the abdomen. A doctor may notice that the liver is mildly enlarged during a physical exam.
More noticeable symptoms typically don’t surface until the disease has progressed to advanced scarring or cirrhosis. At that stage, signs can include yellowing of the skin and eyes, swelling in the legs or abdomen, easy bruising, and confusion caused by toxins the damaged liver can no longer filter.
Who Is Most at Risk
NASH is tightly linked to metabolic syndrome, the cluster of conditions that includes high blood sugar, excess belly fat, high blood pressure, high triglycerides, and low HDL cholesterol. Having metabolic syndrome is associated with more than three times the odds of developing NASH with significant scarring. Each additional component of metabolic syndrome roughly doubles the risk.
Among the individual risk factors, elevated blood sugar or type 2 diabetes carries the strongest association, with about 3.5 times the odds of progressing to dangerous NASH. Large waist circumference and low HDL cholesterol are also independent risk factors. About 65% of adults with fatty liver disease are obese, and a similar percentage have insulin resistance. Men have a slightly higher overall prevalence of NASH than women (roughly 16% versus 16.5% globally), though the gap is expected to narrow. BMI, age, and time all increase risk, and the disease is most common in high-income Western countries among men and in Central Asian, Middle Eastern, and North African countries among women.
How NASH Is Diagnosed
There is no single blood test that definitively confirms NASH. The process usually starts with a simple scoring tool called FIB-4, which combines your age, liver enzyme levels, and platelet count to estimate your risk of significant liver scarring. A low FIB-4 score (below 1.3) generally means advanced fibrosis is unlikely. An intermediate score (1.3 to 2.67) typically prompts a referral for further evaluation.
The next step is often transient elastography, a specialized ultrasound that measures liver stiffness as a proxy for scarring. Combining FIB-4 with elastography reduces the chance of missing advanced fibrosis. Newer blood tests are also improving early detection. One measures fragments of a protein released when liver cells die, which are significantly elevated in NASH compared to simple fatty liver. Another, called the enhanced liver fibrosis score, tracks markers of scar tissue formation and can reduce unnecessary referrals by up to 71% when used alongside FIB-4.
A liver biopsy, where a small tissue sample is examined under a microscope, remains the gold standard for confirming NASH and precisely staging fibrosis. It’s the only way to definitively distinguish active inflammation and cell damage from simple fat accumulation. However, because it’s invasive, it’s generally reserved for cases where the diagnosis is uncertain or when staging the disease will change treatment decisions.
Stages of Liver Scarring
Fibrosis in NASH is graded on a scale from F0 to F4. F0 means no scarring. F1 and F2 represent mild to moderate scarring, sometimes grouped together as “early fibrosis.” F3 is advanced fibrosis, where scar tissue has built up substantially but the liver still functions. F4 is cirrhosis, where scarring is so extensive that the liver’s structure is fundamentally altered and its ability to function is compromised.
The distinction matters because outcomes diverge sharply at higher stages. Patients with F0 to F2 fibrosis generally have a much better prognosis and more time to intervene with lifestyle changes. Once fibrosis reaches F3 or F4, the risks of liver failure, liver cancer, and death increase significantly. Importantly, fibrosis at earlier stages can be slowed or even reversed with the right interventions.
Weight Loss Is the Most Effective Treatment
Losing weight through diet and exercise remains the most powerful intervention for NASH. A landmark study found that losing at least 5% of total body weight led to NASH resolution in 58% of patients, with 82% showing meaningful improvement in their liver inflammation scores. The results were even more striking at higher levels of weight loss: among patients who lost 10% or more of their body weight, 90% had complete resolution of NASH and 45% saw their fibrosis actually regress.
Those thresholds give you a practical target. If you weigh 200 pounds, losing 10 pounds (5%) can meaningfully reduce inflammation, and losing 20 pounds (10%) gives you the best chance of reversing both the inflammation and the scarring. The type of diet matters less than the sustained calorie reduction, though Mediterranean-style eating patterns rich in vegetables, whole grains, fish, and olive oil are commonly recommended for their overall metabolic benefits.
The First Approved Medication
In March 2024, the FDA approved the first medication specifically for NASH. Called Rezdiffra, it works by partially activating a thyroid hormone receptor in the liver, which reduces fat accumulation in liver cells. It’s approved for adults with NASH who have moderate to advanced scarring (F2 or F3) but have not yet progressed to cirrhosis where the liver is failing. It’s meant to be used alongside diet and exercise, not as a replacement.
To qualify for the drug in clinical trials, patients needed a biopsy confirming both active inflammation and moderate or advanced fibrosis. The approval represents a significant shift for a disease that, until now, had no pharmacological treatment options beyond managing related conditions like diabetes and high cholesterol.