Fatty liver disease (MASLD): what a mild ALT flag means

Most people meet fatty liver as a surprise line on a lab report: a mildly high ALT (alanine aminotransferase, an enzyme that leaks from injured liver cells), or a note that an ultrasound showed a 'fatty' or 'bright' liver, in someone who feels completely fine. That is fatty liver in its usual form, extra fat packed into liver cells, and it is the most common liver condition in US adults. How high the enzyme reads tells you almost nothing about how much damage is there.

What is fatty liver?

Fatty liver is the buildup of fat, mostly triglycerides, inside liver cells. The medical word for it is hepatic steatosis (steatosis means fat accumulation). A small amount of liver fat is normal; the condition begins when fat reaches roughly 5% or more of the liver's weight and starts to matter.

In 2023 a multisociety panel renamed the disease. What used to be called NAFLD (nonalcoholic fatty liver disease) is now MASLD, metabolic dysfunction-associated steatotic liver disease, and the inflammatory form once called NASH is now MASH. The diagnosis needs two things: fat in the liver plus at least one of five cardiometabolic (heart-and-metabolism) risk factors (excess weight, high blood sugar, high triglycerides, low HDL, or raised blood pressure). ¹

The new name points at the cause. In the Delphi survey behind the change, 61% of the panel found the word 'nonalcoholic' stigmatizing and 66% found 'fatty' stigmatizing, so the group defined the disease by its metabolic drivers rather than by exclusion. ¹

What are the symptoms of fatty liver?

Usually none, at least early on. Simple fatty liver rarely produces symptoms you would notice. Some people report vague tiredness or a dull ache under the right ribs, but those are non-specific and easy to miss. Clear symptoms tend to appear only once the disease has advanced to cirrhosis (a scarred, hardened liver), which is why fatty liver is almost always found on a blood test or a scan rather than because someone felt ill.

What causes fatty liver, and who is at risk?

The engine is insulin resistance: the body's cells stop responding well to insulin, blood sugar and blood fats run high, and the liver stores the surplus as fat. That is why fatty liver clusters with excess weight, type 2 diabetes, high triglycerides, and high blood pressure. You do not need all of them; the diagnosis requires liver fat plus at least one of the five cardiometabolic factors below. ¹

Alcohol is a separate but overlapping cause. Heavy drinking produces alcohol-related liver disease, which looks similar under the microscope, and the two can occur together (labeled MetALD when metabolic fat sits alongside more than moderate drinking). In a US adult, a mild rise, under five times the upper limit, is most often fatty liver. ³ As the ALT page puts it plainly: "a mildly raised ALT in a US adult is fatty liver until proven otherwise." When alcohol is the driver, the enzyme pattern tends to shift, and that pattern lives on the sibling pages.

How fatty liver progresses, the part the enzyme hides

The height of the enzyme does not follow the stage of the disease. The AASLD guidance is explicit that ALT can be normal in people with significant MASLD, and even with advanced fibrosis. ² A reassuring enzyme is not the same as a clear liver.

What tells you the risk: FIB-4

Because the enzyme is a weak guide, clinicians estimate scarring a different way. FIB-4 (Fibrosis-4) is a simple score calculated from your age, AST, ALT, and platelet count. The AGA care pathway uses it as the first triage step: a score under 1.3 makes advanced scarring unlikely, a score above 2.67 makes it likely and prompts specialist referral, and the band in between calls for a stiffness scan (elastography, a special ultrasound that measures how firm the liver is). ⁴

Is fatty liver serious?

For most people with simple fatty liver, the near-term risk is low, and it stays that way. The risk concentrates in the minority who develop fibrosis. At population scale, though, MASLD is now the most common chronic liver disease worldwide and a leading and rising cause of cirrhosis and liver transplant, and it independently tracks with heart disease through the same metabolic drivers. ⁵ It is common and usually mild, but not nothing, and worth staging rather than ignoring.

Can fatty liver be reversed?

Yes, often, especially early. Weight loss is the first-line treatment, and the benefit follows a dose-response: the more weight people lose, the more of their liver disease reverses. Roughly 3 to 5% of body weight lost cuts liver fat, about 7% can resolve steatohepatitis (the inflammation), and about 10% can regress early fibrosis. ⁶² This is a clinician-guided plan, not a crash diet.

No supplement is a proven substitute for this. The core of treatment is bringing down weight and managing the metabolic drivers: blood sugar, blood pressure, and blood fats. Medications for the inflammatory form now exist, but whether any medication fits you is a decision for your clinician, made alongside the lifestyle plan, not something to start on your own.

How is fatty liver diagnosed?

1. 1A flag appearsA routine blood panel shows a mildly high ALT or AST, or an ultrasound done for another reason shows fat in the liver.
2. 2Other causes get ruled outYour clinician checks history and bloods for alcohol, viral hepatitis, and medicines that can mimic the picture.
3. 3Fibrosis risk is scoredFIB-4 is calculated from your age, AST, ALT, and platelet count to sort you into low, intermediate, or high risk.
4. 4Imaging confirms the stageIntermediate or high scores get elastography (a stiffness scan) or other imaging to measure fat and scarring more directly.
5. 5Biopsy, rarelyA liver biopsy is used only when the diagnosis or stage stays uncertain after the non-invasive steps.

On practicalities: ALT and AST come as part of a standard liver panel or comprehensive metabolic panel, usually inexpensive and often already bundled into routine bloods, and the enzymes themselves do not require fasting (a paired glucose or lipid check often does). An ultrasound is a separate imaging order. Screening is risk-based: US guidance points to checking a FIB-4 in higher-risk groups such as people with type 2 diabetes or obesity with metabolic risk, then rechecking periodically rather than once, because fatty liver builds slowly and silently. ⁴²

How the enzymes connect

The blood tests that flag fatty liver each answer a different question, and the marker pages read each one in full rather than repeating it here:

• ALT is the most liver-specific of the common enzymes, and a mild isolated rise is fatty liver until proven otherwise.
• AST is not liver-specific; it also leaks from muscle, heart, and red blood cells, so it is only informative read next to ALT.
• GGT confirms that a raised alkaline phosphatase (ALP) is coming from the liver rather than bone, and it joins the alcohol pattern.
• For the whole panel and where each test sits, see how to read liver enzymes; for why 'normal' is a 95% band, see reference ranges explained.

Fatty liver is the most common finding behind all three, and it sits under the wider liver topic hub.

LifeFrom membershipWant your own liver panel read in context instead of just flagged as 'high'? Start with a full LifeFrom panel and get every marker, ALT, AST, GGT, and your fibrosis picture, explained in plain language against the cutoffs that matter, to review with your clinician.Get started