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ALT

ALT is a liver enzyme that leaks into the blood when liver cells are damaged and serves as the standard marker for your liver.

What is ALT?

ALT is an enzyme that lives mainly inside your liver cells. When those cells are irritated or damaged, ALT leaks into the bloodstream and the blood level rises. A raised value tells you liver cells are under strain — not why. A normal ALT does not rule out liver problems. Fat accumulation or inflammation can be present while the value still reads within range. That is why ALT is read alongside AST, GGT, and bilirubin — only the pattern across those markers points toward a likely cause.

Why is ALT relevant?

The most common reason for a mildly elevated ALT in an otherwise healthy person is fat accumulating in the liver — a process that goes hand in hand with excess weight, insulin resistance, or an unfavourable metabolic profile. It rarely causes symptoms. Catching it early, while lifestyle change can still reverse it, is exactly what this kind of test is for. Alcohol and certain medications can raise ALT too. AST adds context: ALT is more liver-specific, while AST is also found in muscle. When AST rises more than ALT, that pattern fits alcohol use or muscle damage better than fatty liver.

ALT high or low — what it means

Read ALT alongside AST, GGT, and bilirubin. A single, mild elevation is often temporary and harmless. What matters more is whether it persists across repeat tests and what the rest of your blood work shows. Hard training in the days before the blood draw can temporarily raise both ALT and AST — muscles contain these enzymes too. A normal GGT alongside the rise suggests muscle rather than liver as the source. To get a clean reading, avoid testing right after intense exercise. To bring a raised ALT down, the main levers are losing excess weight, cutting alcohol, reducing refined carbohydrates, and moving regularly. Improvement often shows within weeks.

ALT reference ranges

MenUpper limit in people without liver disease (NVKC); some labs use a lower cut-off (around 30-35 U/L).< 45 U/L
WomenUpper limit in people without liver disease (NVKC); some labs use a lower cut-off (around 19-25 U/L).< 35 U/L

Cut-offs vary by laboratory and method; the reference range on your own lab report is leading. ALT is always read alongside AST, GGT, and bilirubin.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.

Read about our scientific approach →

Frequently asked questions

What is a normal ALT (ALAT) level?

In people without liver disease, ALT is usually below 45 U/L for men and below 35 U/L for women. Some labs now use lower upper limits (around 30-35 U/L for men and 19-25 U/L for women). The limit varies by laboratory and method, so the reference range on your own report is always leading.

What does a high ALT level mean?

A raised ALT means liver cells are under strain, but not why. The most common cause in otherwise healthy people is fatty liver; alcohol, certain medications, a viral infection, or hard training can also lift ALT temporarily. A mild rise (up to about twice the upper limit) is often harmless, especially as a one-off. What matters more is whether it persists and what AST, GGT, and bilirubin are doing.

What does a low ALT level mean?

A low ALT is usually not a concern on its own. In older adults a persistently low value can sometimes accompany muscle loss or frailty, which is worth mentioning to your doctor.

When is a raised ALT a cause for concern?

A value more than twice the upper limit warrants investigation of the cause. A markedly high value (many times the upper limit, up to more than 15x in acute hepatitis), or a persistent rise together with abnormal AST, GGT, or bilirubin, or with symptoms like fatigue, jaundice, or right-upper-abdominal pain, should always be assessed promptly by a clinician.

How do I lower a high ALT level?

Because fatty liver is usually the cause, the main levers are: losing excess weight (even 5-10% of body weight meaningfully reduces liver fat), cutting back or pausing alcohol, reducing refined sugar and ultra-processed food, moving regularly, and reviewing any medication or supplement that could be contributing with your doctor. Improvement often shows within weeks to a few months. Avoid testing right after hard training, as muscle can lift ALT temporarily.

Related biomarkers

AST
AST is an enzyme in liver, heart and muscle that leaks into your blood when those cells are damaged.
Gamma-GT
Gamma-GT is a liver and bile-duct enzyme that reacts sensitively to alcohol, fatty liver, and medication.
Bilirubin
Bilirubin is the yellow breakdown product of red blood cells that your liver clears through bile.
AST / ALT Ratio
The AST/ALT ratio compares two liver enzymes and helps tell whether raised values come from your liver or your muscles.

ALT is one of the biomarkers in the Optimize blood test. Book a blood draw at any of 238+ partner labs in the Netherlands, or upload your existing results in the app.

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