High ALT and AST: fatty liver, MASLD and FIB-4

High ALT and AST liver enzymes: common causes, fatty liver (MASLD), the FIB-4 score and useful weight loss goals from AASLD and European guidelines.

High ALT and AST: fatty liver, MASLD and FIB-4

9 min - BioSense Team - 2025-01-09

  • level:understanding
  • ALAT
  • ASAT
  • transaminases
  • foie gras
  • MASLD
  • FIB-4

High ALT and AST: fatty liver, MASLD and the FIB-4 score

Liver enzymes (ALT, AST) slightly above the reference range are one of the most common findings on a routine blood test. Most of the time they do not point to severe hepatitis or heavy drinking, but often to a liver that stores too much fat, a condition renamed MASLD in 2023. The real question is not the number itself but the risk of fibrosis, meaning scarring of the liver, which a simple calculation, the FIB-4 score, helps to sort out. This guide explains how to read your liver enzymes and what US and European liver societies recommend.

What ALT and AST measure

ALT and AST, also called transaminases, are enzymes found inside cells. When cells are damaged, they release these enzymes into the blood and the measured level rises.

  • ALT (alanine aminotransferase, also called SGPT) is found mainly in the liver. It is the more liver-specific marker of liver cell injury.
  • AST (aspartate aminotransferase, also called SGOT) is found in the liver, but also in the heart, muscles, kidneys and brain. It can therefore rise without the liver being involved.

ALT and AST measure injury to liver cells at a given moment, not the amount of fat in the liver, nor fibrosis. People with MASLD and normal enzymes can still have significant liver disease. That is why a liver panel also includes alkaline phosphatase, bilirubin and albumin, sometimes GGT, read with the platelet count. For GGT, often raised at the same time, see our dedicated article: high GGT, far beyond alcohol.

Normal ALT and AST levels and how to read them

The reference range printed on your report, which depends on the lab, remains the reference. MedlinePlus gives 4 to 36 U/L for ALT and 8 to 33 U/L for AST. In the US, the American College of Gastroenterology (ACG, 2017) states that a truly healthy ALT is lower: 29 to 33 IU/L in males and 19 to 25 IU/L in females, and that levels above this should be assessed by a physician. The 2024 European guidelines use similar cutoffs (above 33 U/L in males and 25 U/L in females). This explains why a result "within range" can coexist with a fatty liver.

| Level (compared with the upper limit of normal) | ACG category | What it can suggest | Usual approach | |---|---|---|---| | Less than 2 times | borderline | fatty liver, alcohol, a medication, recent intense exercise | repeat test later, look for simple causes, calculate FIB-4 | | 2 to 5 times | mild | same causes, but also viral hepatitis or another liver disease | more complete workup ordered by your doctor | | 5 to 15 times | moderate | same causes, more active liver injury | prompt evaluation by your doctor | | More than 15 times | severe | acute injury: acute viral hepatitis, drug toxicity (including acetaminophen overdose), reduced blood supply to the liver | prompt medical care, the same day if there are symptoms |

Two further points help the reading:

  • The AST/ALT ratio. In fatty liver, ALT is usually higher than AST. The ACG notes that an AST at least twice the ALT is commonly associated with alcohol-related liver injury.
  • The trend. A value stable for years and a value that doubles in six months do not carry the same weight. Comparing with your previous results is often more telling than today's value: see how to compare two blood tests.

High ALT and AST: the most common causes

Fatty liver (MASLD)

This is a leading cause of moderately raised liver enzymes. The American Association for the Study of Liver Diseases (AASLD) estimates that 25% to 30% of adults are affected, with 30% to 75% among people with type 2 diabetes. The typical profile: a larger waist, borderline blood sugar, high triglycerides, slightly high blood pressure. Insulin resistance is often the driver.

Alcohol

Alcohol directly damages liver cells and often adds to fatty liver. See our article on the impact of alcohol and tobacco on blood tests.

Medications and supplements

Many medications can raise ALT and AST, as can high doses of acetaminophen, over-the-counter products, herbal products and dietary supplements. Keep in mind that the FDA does not approve dietary supplements before they go on the market. Tell your doctor about everything you take, and never stop a treatment on your own.

Intense exercise

Muscles also contain these enzymes. In a Swedish study, one hour of weightlifting in men not used to it raised AST and ALT for at least seven days, while GGT stayed normal. If you did intense exercise in the week before the blood test, tell your doctor.

Other causes to look for

  • hepatitis A, B and C, tested with a simple blood test;
  • iron overload (hereditary hemochromatosis), detected with ferritin and transferrin saturation;
  • celiac disease, autoimmune hepatitis, Wilson disease, alpha-1 antitrypsin deficiency.

NAFLD is now MASLD: what the name change means

In June 2023, an international consensus of the major liver societies, including the AASLD, replaced the former term NAFLD (nonalcoholic fatty liver disease) with MASLD, metabolic dysfunction-associated steatotic liver disease. The inflammatory form, formerly called NASH, becomes MASH.

The former definition relied on an exclusion (no alcohol); the new one is positive: MASLD means steatosis (fat in the liver) plus at least one of five cardiometabolic risk factors:

  1. overweight or increased waist circumference;
  2. raised fasting glucose, raised A1C or type 2 diabetes;
  3. high or treated blood pressure;
  4. high or treated triglycerides;
  5. low or treated HDL cholesterol.

A new category also appears: MetALD, when MASLD coexists with higher alcohol intake, 140 to 350 g per week for women and 210 to 420 g per week for men (about 10 to 25 and 15 to 30 US standard drinks of 14 g per week). Above that, the condition is called alcohol-associated liver disease.

The FIB-4 score: how to estimate liver fibrosis risk

The outlook of a fatty liver depends mainly on fibrosis, not on ALT and AST levels. The AASLD (2023) recommends a first risk assessment with FIB-4, and the 2024 EASL-EASD-EASO European guidelines recommend looking for fibrosis in three groups: people with type 2 diabetes, people with abdominal obesity and at least one other metabolic risk factor, and people with abnormal liver tests.

FIB-4 is calculated from four values already present on most blood work:

FIB-4 = (age × AST) / (platelets in 10⁹/L × square root of ALT)

Platelets in 10⁹/L is the same number as in K/µL on US reports. Example: age 55, AST 40 U/L, ALT 50 U/L, platelets 220 K/µL. The calculation gives (55 × 40) / (220 × 7.07), about 1.41.

| FIB-4 | Interpretation (AASLD 2023 and EASL-EASD-EASO 2024) | |---|---| | Below 1.3 | low risk of advanced fibrosis; reassess every 1 to 2 years with several risk factors (AASLD), every 1 to 3 years (EASL) | | 1.3 to 2.67 | intermediate range: second test (elastography), or lifestyle and risk factor intervention, then retest | | Above 2.67 | higher risk: second test and specialist opinion |

After age 65, the European guidelines raise the lower cutoff to 2.0, because age pushes the score up. Before age 35, FIB-4 performs poorly. It is not always printed on the report: ask your doctor for it.

After FIB-4: FibroScan and other second-line tests

When FIB-4 is not reassuring, the next step is vibration-controlled transient elastography (FibroScan), a painless test lasting a few minutes that measures liver stiffness. According to the AASLD, stiffness below 8 kPa helps rule out advanced fibrosis, and a value above 12 kPa is associated with a higher likelihood of it. A specialized blood test (ELF) can be used instead. Liver biopsy is reserved for cases where these tests are inconclusive.

How to lower ALT and AST: what guidelines recommend for fatty liver

Lifestyle is the foundation of MASLD treatment, and its effects are measurable. The European guidelines set these goals in people with overweight:

| Sustained weight loss | Expected effect on the liver | |---|---| | 5% or more | less fat in the liver | | 7 to 10% | improved inflammation | | 10% or more | improved fibrosis |

The AASLD is close: 3 to 5% improves steatosis, and more than 10% is generally needed to improve MASH and fibrosis.

  • Physical activity: preferably more than 150 minutes a week of moderate-intensity activity, or 75 minutes of vigorous activity, tailored to each person. Physical activity helps reduce liver fat.
  • Diet: the AASLD finds that different eating patterns (low-carbohydrate, low-fat, Mediterranean) appear comparable when they help reduce calories. Limit ultra-processed foods and sugar-sweetened drinks. Triglycerides often respond to the same changes.
  • Alcohol: the AASLD says heavy drinking should be avoided with fatty liver and calls for complete abstinence with significant fibrosis (stage F2 or higher); the European guidelines discourage alcohol in everyone with fatty liver and support complete abstinence in cirrhosis.
  • Coffee: here the societies differ. The AASLD says that 3 or more cups a day could be recommended in the absence of contraindications, while the European guidelines note only an association in observational studies.
  • Associated risk factors: diabetes, blood pressure, cholesterol and smoking are managed at the same time, because the AASLD considers cardiovascular disease an important cause of death in people with this condition.

For adults with MASH and moderate to advanced fibrosis, without cirrhosis, the FDA has approved two medications under accelerated approval: resmetirom (Rezdiffra) in March 2024 and semaglutide (Wegovy) in August 2025, used alongside lifestyle changes. Whether they are appropriate is a decision for you and your liver specialist.

Frequently asked questions

My ALT and AST are normal: is my liver fine?

Not necessarily. MASLD, including with inflammation, can exist with normal ALT and AST. If you have type 2 diabetes or abdominal obesity with another risk factor, calculating FIB-4 remains relevant even with a normal liver panel.

High ALT or AST: should the blood test be repeated?

A moderate, isolated rise is usually rechecked later, after ruling out intense exercise, a medication or recent alcohol intake. The timing is set by your doctor, often a few weeks to a few months.

High AST with normal ALT: what can it mean?

The cause is not necessarily the liver: the ACG notes that an AST increase without a rise in ALT suggests heart or muscle disease. Intense exercise is a common explanation, worth mentioning to your doctor, who then decides on a repeat test or further tests.

Can thin people have fatty liver?

Yes, particularly with abdominal fat, diabetes or high triglycerides, even at a normal weight.

*This article is for information only and does not replace medical advice. Jaundice, dark urine, abdominal pain or severe fatigue call for a prompt medical visit. Only your doctor can interpret your results and decide which tests to carry out.*

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Sources and methodology

International

United States

United Kingdom

Europe

*BioSense methodology: our content is based on guidance from US authorities and medical societies (such as the USPSTF, CDC, NIH, FDA, ACC/AHA and ADA), UK bodies (NICE, NHS), European societies and the WHO. Sources are listed by country, and differences between countries are stated when they matter.*