ALT: what is a good level?
Upper limits of about 40–55 U/L for men and 33–45 U/L for women are typical, varying widely between laboratories. These were derived from reference populations that included many people with undiagnosed fatty liver, so the "normal" range partly describes the disease it should detect.
This is the range used by the organ dial in the Longevity Score calculator. It is deliberately tighter than the range printed on a standard lab report — the reasoning is below.
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A result anywhere in the pale band is reported as normal and generates no comment. The brighter band is where risk is lowest. The distance between them is the part a standard report does not tell you.
What ALT actually measures
ALT is an enzyme concentrated in liver cells. When those cells are stressed or damaged, ALT leaks into the blood, so the serum level is an indirect measure of ongoing hepatocyte injury.
It is more liver-specific than AST, which is also abundant in muscle. A raised AST with a normal ALT after heavy training is usually muscle, not liver.
Why it matters
Metabolic dysfunction-associated fatty liver disease now affects roughly a quarter to a third of adults in many countries, and it is usually silent. A mildly raised ALT is frequently its only laboratory sign.
Liver fat is not a local problem. It drives hepatic insulin resistance, raises VLDL output and therefore triglycerides, and tracks closely with visceral adiposity — which is why ALT often moves in step with the metabolic and atherogenic dials.
Within the standard normal range, higher ALT is associated with higher cardiometabolic risk and mortality. The information in a result of 35 U/L is being discarded by reporting it as normal.
Why it's on the dashboard
ALT made the panel because it is frequently the only laboratory sign of a condition — fatty liver — that now affects roughly a quarter to a third of adults silently. The lifespan case comes directly from the population this dial's thresholds are built on: Kim et al. (BMJ, 2004) followed over 142,000 Korean adults for 8 years and found that even within the conventional normal range, higher aminotransferase levels predicted substantially higher liver-disease mortality — the relative risk for liver-related death was roughly 3-fold higher at 20-29 U/L and nearly 10-fold higher at 30-39 U/L compared with under 20 U/L, the exact distinction this engine's stricter thresholds are drawing. The healthspan case is about where fatty liver spends its damage beyond the liver itself: Sinn et al. (Hepatology, 2022), following over 52,000 adults, found those with fatty liver disease lost appendicular skeletal muscle mass significantly faster over 5 years than those without it, with more severe disease predicting faster loss — a direct link between this marker and the same muscle mass this dashboard's structural dial is built to protect. See the Organ dial for the broader context.
Why this range is stricter than your lab's
Reference intervals are conventionally set as the central 95% of a reference population. When a third of that population has fatty liver, the resulting upper limit encodes the condition into normality.
Studies that excluded people with viral hepatitis, raised BMI and abnormal lipids found healthy upper limits closer to 30 U/L for men and 19 U/L for women — far below most printed ranges.
The optimised thresholds follow that work. They are sex-specific because the healthy distributions genuinely differ, and the engine scores them separately rather than applying a single cutoff.
This distinction applies across the whole panel, not just this marker — see optimal versus standard lab ranges for the general case.
What moves ALT
Educational information, not a treatment plan. Discuss changes to medication or supplementation with a clinician who knows your history.
How fast it responds
Weeks to months. ALT responds to reduced liver fat within about 8–12 weeks of sustained change, which makes it a reasonable way to confirm that a metabolic intervention is working. See response times across all markers for how this compares.
What can distort the result
- Strenuous or unaccustomed exercise in the days before a draw raises ALT, and raises AST more. Avoid heavy training for 48–72 hours before testing.
- Optimal thresholds are sex-specific; women's are roughly 5 U/L lower.
- A normal ALT does not exclude liver disease. Advanced fibrosis and cirrhosis can present with normal enzymes, and ALT can fall as the liver loses functioning cells.
- Persistently raised ALT needs a cause, not a lifestyle guess. Viral hepatitis, haemochromatosis, autoimmune hepatitis, coeliac disease and thyroid dysfunction all present this way and all need specific treatment.
- Muscle injury, including rhabdomyolysis, raises ALT alongside a much larger AST and creatine kinase rise.
Frequently asked questions
What is a good ALT level?
The longevity-optimised target used in this calculator is under 20 U/L (men), under 15 U/L (women). Upper limits of about 40–55 U/L for men and 33–45 U/L for women are typical, varying widely between laboratories. These were derived from reference populations that included many people with undiagnosed fatty liver, so the "normal" range partly describes the disease it should detect.
Why is this target stricter than the range on my lab report?
Reference intervals are conventionally set as the central 95% of a reference population. When a third of that population has fatty liver, the resulting upper limit encodes the condition into normality.
How fast does ALT respond to change?
Weeks to months. ALT responds to reduced liver fat within about 8–12 weeks of sustained change, which makes it a reasonable way to confirm that a metabolic intervention is working.
Related markers
ALT feeds the organ dial, and read it alongside Ferritin · Triglycerides.
Ready to act on it? How to lower ALT ranks the interventions by leverage and says what does not work.