Liver Function Tests — ALT, AST and GGT
The liver is one of the body's most important organs. It processes nutrients, regulates blood glucose, produces essential proteins, helps digest fats, and breaks down medications and toxins.
Blood tests can provide clues about liver health, sometimes identifying problems before symptoms appear.
Three commonly discussed tests are ALT, AST, and GGT. These enzymes can increase when liver cells are injured or stressed.
However, there is an important distinction: ALT, AST, and GGT are primarily markers of liver injury, not direct measurements of how well the liver functions.
Understanding these tests can be especially useful for people concerned about metabolic health, insulin resistance, and fatty liver disease.
ALT — Alanine Aminotransferase
ALT is an enzyme found primarily in liver cells.
When liver cells are damaged, ALT can leak into the bloodstream, causing blood levels to rise.
Common causes of elevated ALT include:
Metabolic dysfunction-associated steatotic liver disease (MASLD), previously called nonalcoholic fatty liver disease
Viral hepatitis
Alcohol-related liver injury
Certain medications and supplements
Other liver diseases
Laboratories often report upper limits around 35–55 U/L, although reference ranges vary.
Some liver specialists believe these limits are too high, particularly when screening for metabolic liver disease.
A normal ALT does not necessarily mean the liver is healthy. People with significant liver fat or even advanced fibrosis can sometimes have ALT values within the laboratory reference range.
AST — Aspartate Aminotransferase
AST is another enzyme that can increase when liver cells are injured.
Unlike ALT, AST is also found in muscles and other tissues.
This means an elevated AST does not always indicate liver disease.
Strenuous exercise, muscle injury, and certain medical conditions can raise AST.
Doctors often evaluate AST together with ALT.
The relationship between the two can sometimes provide clues about the cause of liver injury.
For example, an AST-to-ALT ratio greater than 2 may suggest alcohol-associated liver disease in the appropriate clinical setting, although it is not diagnostic.
GGT — Gamma-Glutamyl Transferase
GGT is an enzyme found primarily in the liver and bile ducts.
It can rise with liver or bile duct disease and may also increase with alcohol consumption, certain medications, and metabolic dysfunction.
GGT is sometimes used to help determine whether an elevated alkaline phosphatase (ALP) level originates from the liver rather than bone.
Some observational studies have found associations between higher GGT levels and increased risk of type 2 diabetes and cardiovascular disease.
However, GGT is not a specific test for insulin resistance, and a higher result does not establish a diagnosis of metabolic disease.
What Is Fatty Liver Disease?
Fatty liver disease occurs when excess fat accumulates in liver cells.
The newer term metabolic dysfunction-associated steatotic liver disease (MASLD) describes liver fat accumulation associated with cardiometabolic risk factors, such as abdominal obesity, elevated blood glucose, high blood pressure, or abnormal triglycerides and HDL cholesterol.
MASLD can develop even in people who are not significantly overweight.
It is closely associated with insulin resistance and type 2 diabetes.
Some people develop inflammation and liver cell injury, called metabolic dysfunction-associated steatohepatitis (MASH).
Over time, a proportion of patients develop liver fibrosis, cirrhosis, or other complications.
Many people have no symptoms during the early stages.
How Are Insulin Resistance and Fatty Liver Connected?
The liver plays a central role in regulating blood glucose and fat metabolism.
When insulin resistance develops, the liver may continue producing glucose even when the body already has sufficient energy.
At the same time, increased fatty acid delivery and fat production can contribute to fat accumulation within liver cells.
Elevated triglycerides, abdominal fat, and abnormal glucose regulation often accompany fatty liver disease.
Fatty liver can be both a consequence of metabolic dysfunction and a contributor to worsening insulin resistance.
This is why liver health is relevant even for people who have never been diagnosed with liver disease.
Can a Low-Carbohydrate Diet Improve Fatty Liver?
Clinical trials have found that reducing refined carbohydrates and excess calorie intake can help decrease liver fat.
Low-carbohydrate diets, Mediterranean-style diets, and other dietary approaches can improve MASLD, particularly when they lead to sustained weight loss and better metabolic health.
Some studies suggest that carbohydrate restriction can reduce liver fat relatively quickly, even before substantial weight loss occurs.
However, longer-term outcomes depend on overall dietary quality, adherence, and other factors.
There is not yet convincing evidence that one dietary pattern is best for every person with fatty liver disease.
Regular exercise also helps reduce liver fat, sometimes independently of weight loss.
What About Normal Liver Enzymes?
One of the most important things to understand is that normal ALT and AST levels do not rule out fatty liver disease or liver fibrosis.
Someone can have considerable fat accumulation in the liver while routine blood tests remain within normal ranges.
When fatty liver disease is suspected, doctors may consider additional evaluation, including ultrasound, elastography, or other noninvasive tests.
The decision depends on metabolic risk factors, medical history, and laboratory findings.
FIB-4 — Estimating the Risk of Liver Fibrosis
FIB-4 is a calculation that uses four routinely available measurements:
Age
AST
ALT
Platelet count
It helps estimate the likelihood of advanced liver fibrosis.
For adults approximately 35–65 years old, a FIB-4 below 1.3 is generally considered low risk, while higher scores may warrant further assessment.
Age is especially important when interpreting FIB-4.
Because age is included in the calculation, older adults can have elevated FIB-4 scores even without advanced liver disease.
For people over 65, a higher cutoff of approximately 2.0 is commonly used when deciding whether additional testing is appropriate.
FIB-4 is a screening tool, not a diagnosis. An elevated result may lead to further evaluation, such as liver elastography.
What Tests Actually Measure Liver Function?
Although ALT, AST, and GGT are commonly included in liver panels, they do not directly measure the liver's ability to perform its essential jobs.
Other tests provide different information:
Albumin: A protein made by the liver. Low levels can occur with advanced liver disease but also with malnutrition, kidney disease, and other conditions.
Bilirubin: A substance processed by the liver. Elevated levels can result from liver disease, bile duct problems, or increased red blood cell breakdown.
Prothrombin time (PT/INR): Measures blood clotting and can help assess the liver's ability to produce clotting factors, although anticoagulant medications and other conditions also affect results.
Doctors interpret these measurements together with liver enzymes and the person's medical history.
Why Do These Tests Matter for Older Adults?
Liver test results can be affected by medications, supplements, alcohol, muscle injury, and underlying health conditions.
Older adults may be taking several medications, increasing the importance of reviewing possible drug-related liver effects.
For people in their 70s and 80s, a mildly elevated enzyme level may not indicate serious disease, but persistent abnormalities deserve evaluation.
People with diabetes, abdominal obesity, high triglycerides, or other metabolic risk factors may benefit from assessment for fatty liver disease and fibrosis.
It is also important to recognize that advanced liver disease can occasionally exist despite normal liver enzyme results.
What Can We Do to Protect Liver Health?
Several approaches have evidence supporting liver and metabolic health:
Maintain healthy blood glucose and blood pressure.
Reduce excess abdominal fat when appropriate.
Limit refined carbohydrates and highly processed foods.
Exercise regularly, including resistance training.
Limit or avoid alcohol, especially when liver disease is present.
Review medications and supplements for possible liver effects.
Address hepatitis and other treatable liver conditions.
People with established MASLD or MASH may need more specific medical evaluation and treatment.
The Bottom Line
ALT, AST, and GGT provide useful clues about liver injury, but they do not tell the whole story.
Elevated levels can have many causes, and normal results do not guarantee the absence of fatty liver disease or fibrosis.
For people concerned about metabolic health, the relationship between insulin resistance and liver fat is especially important.
Additional tools such as FIB-4 and liver elastography can help identify people who may need closer evaluation.
As with other laboratory tests, trends over time and the overall health picture are usually more informative than a single number.
Learn More
In this Metabolic Classroom lecture, Bikman explains how insulin resistance, elevated insulin, fructose, and alcohol can contribute to fat accumulation in the liver. He also discusses why liver enzymes may not reveal the full extent of fatty liver disease and how dietary changes may help.
A clear explanation from the National Library of Medicine of ALT, AST, GGT, bilirubin, albumin, and other tests used to evaluate liver health.
Understanding GGT — MedlinePlus
Explains what GGT measures, why levels may be elevated, and how it can help distinguish liver and bile duct problems from other conditions.
Clinical guidance explaining how FIB-4 and other noninvasive tests help identify people at risk for advanced liver fibrosis, including special considerations for adults over 65.