Alanine Aminotransferase

An enzyme packed inside liver cells. Because healthy cells keep it in, the amount circulating in blood is one of the most sensitive routine signals of liver-cell stress or injury.

Curated · human-reviewed · Last reviewed 2026-08-20

Typical adult reference range: ≤ 40 U/L
Ranges differ between laboratories, assays, sex and age. The range printed on your own report takes precedence over this one.

What is ALT?

Alanine aminotransferase (ALT, historically SGPT) is an enzyme that lives in the cytoplasm of liver cells, with much smaller amounts in kidney, heart and skeletal muscle. It is not secreted on purpose — what shows up in blood has leaked out of cells whose membranes were damaged. That is why ALT is read as a marker of hepatocellular injury rather than of liver function: a badly scarred liver can have a normal ALT, and a mildly fatty liver can have a raised one.

What is ALT made of, and where does it come from?

A protein enzyme of the aminotransferase family. It only works when bound to pyridoxal-5'-phosphate, the active form of vitamin B6 — so severe B6 deficiency can lower measured ALT activity even when liver cells are being damaged.

What does ALT do in the body?

ALT catalyses the transfer of an amino group from alanine to alpha-ketoglutarate, producing pyruvate and glutamate. That single reaction is the hinge between protein and carbohydrate metabolism: during fasting it lets the liver turn amino acids arriving from muscle into glucose. Its diagnostic value, though, comes from where it sits rather than what it does — concentrated inside hepatocytes, it spills into blood in proportion to how many of them are leaking.

What does a high ALT mean?

Raised ALT means liver cells are leaking. It says nothing on its own about why, or about how much liver function remains. Mild elevations (up to about twice the upper limit) are common and most often metabolic; values above roughly ten times the upper limit point to acute injury — viral, drug-induced or ischaemic — and warrant prompt medical assessment rather than lifestyle changes.

Common causes:

Metabolic dysfunction-associated fatty liver — by a wide margin the most common reason for a mildly raised ALT in people without symptoms.
Alcohol intake. Alcohol-related injury more often raises AST above ALT, so the two are read together.
Medications and supplements — paracetamol/acetaminophen, some antibiotics, antifungals, anti-tuberculosis drugs, high-dose niacin, anabolic steroids and a number of herbal products.
Viral hepatitis (B and C especially), and less often autoimmune hepatitis, haemochromatosis, Wilson disease or coeliac disease.
Muscle injury, including a hard unaccustomed workout in the preceding days. Muscle raises AST and creatine kinase more than ALT, which is how the pattern is told apart.

What does a low ALT mean?

A low ALT is usually of no clinical concern and most laboratories do not flag it. Two situations are worth knowing: marked vitamin B6 deficiency reduces measured enzyme activity, and in older adults a persistently low ALT has been associated in cohort studies with reduced muscle mass and frailty — an association, not a diagnosis.

Common causes:

Vitamin B6 deficiency.
Low muscle mass, more often seen with advancing age.

What moves ALT apart from disease?

Strenuous or unaccustomed exercise raises it: Muscle damage from heavy resistance training or a long run can raise ALT for several days. Leaving 48-72 hours between a hard session and a blood draw avoids a misleading result.
Body weight and visceral fat raises it: ALT tracks liver fat closely. Weight gain of even a few kilograms can move it, and losing 5-10% of body weight often brings it back down.
Alcohol raises it: Both recent drinking and habitual intake matter. Several alcohol-free weeks before retesting makes the result much easier to interpret.
Sex and age either direction: Men run higher than women, which is why some laboratories publish sex-specific upper limits. Values drift down in older age.
Biological day-to-day variation either direction: The same healthy person's ALT can vary by tens of percent between days, and tends to read higher in the afternoon. A single borderline value is worth repeating before drawing conclusions.
Haemolysis of the sample raises it: A difficult blood draw that breaks red cells releases enzyme into the serum and inflates the result. Laboratories usually note this on the report.

Which foods affect ALT?

Cutting added sugar and sugar-sweetened drinks, especially fructose — lowers strong evidence
A Mediterranean-style pattern — olive oil, fish, legumes, vegetables, nuts — lowers strong evidence
Coffee, including decaffeinated, at 2-3 cups a day — lowers moderate evidence
Alcohol — raises strong evidence

Which supplements are studied for ALT?

Vitamin E (alpha-tocopherol) moderate evidence: Reduces oxidative stress in hepatocytes. In the PIVENS trial it improved liver histology and lowered aminotransferases in non-diabetic adults with biopsy-proven steatohepatitis.
Typical dose: 800 IU/day, in trials of biopsy-confirmed steatohepatitis only
Cautions: This dose was studied in a specific biopsy-confirmed population, not as a general remedy for a mildly raised ALT. Discuss with a doctor before starting. Long-term high-dose vitamin E has been linked in other trials to a higher risk of haemorrhagic stroke and, in men, of prostate cancer. Increases bleeding risk alongside anticoagulants.
Omega-3 (EPA/DHA) moderate evidence: Lowers hepatic triglyceride content. Effects on liver fat are reasonably consistent; effects on ALT itself are smaller and less consistent.
Typical dose: 1-2 g/day combined EPA + DHA
Cautions: May increase bleeding risk at higher doses or with anticoagulants.

How does exercise affect ALT?

Moderate aerobic exercise strong evidence: How much: 150-240 minutes a week, spread over at least three sessions — brisk walking, cycling or swimming all qualify.
What to expect: Reduces liver fat and ALT even when body weight does not change, though the effect is larger when weight comes down too.
Resistance training moderate evidence: How much: Two or three sessions a week covering the major muscle groups.
What to expect: Lowers liver fat at a lower cardiorespiratory cost than aerobic work, which makes it a practical option when aerobic capacity is limited. Expect a transient ALT rise in the days right after an unaccustomed session.

Which lifestyle factors affect ALT?

Alcohol reduction strong evidence: The single highest-yield change when intake is above low-risk levels. Several alcohol-free weeks before a repeat test also makes the result far easier to interpret.
Reviewing medications and supplements strong evidence: Bring a full list — prescription, over-the-counter and herbal — to the consultation. Drug-induced liver injury is under-recognised, and herbal products are a frequent and often unmentioned contributor.
Weight loss of 5-10% strong evidence: Where fatty liver is the cause, losing 5% of body weight typically reduces liver fat and 7-10% is associated with histological improvement.

How should you prepare for a ALT test?

Avoid strenuous or unaccustomed exercise for 48-72 hours before the draw.
Fasting is not required for ALT, though it usually is for the other tests ordered alongside it.
A single mildly raised value should be repeated before acting on it — day-to-day variation is substantial.
Tell the laboratory or clinician about all supplements and herbal products; several affect the result.

References

Kwo PY, Cohen SM, Lim JK. ACG Clinical Guideline: Evaluation of Abnormal Liver Chemistries. Am J Gastroenterol 2017 — https://doi.org/10.1038/ajg.2016.517
Sanyal AJ et al. Pioglitazone, Vitamin E, or Placebo for Nonalcoholic Steatohepatitis (PIVENS). N Engl J Med 2010 — https://doi.org/10.1056/NEJMoa0907929
Romero-Gómez M, Zelber-Sagi S, Trenell M. Treatment of NAFLD with diet, physical activity and exercise. J Hepatol 2017 — https://doi.org/10.1016/j.jhep.2017.05.016

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Compiled from published medical literature and human-reviewed. For general health education only — it does not replace diagnosis or treatment advice from a doctor.