Aspartate Aminotransferase

What is AST?

Aspartate aminotransferase (AST, historically SGOT) is an enzyme present in many tissues, not only the liver: heart muscle, skeletal muscle, kidney, brain and red blood cells all contain substantial amounts. Healthy cells keep it inside, so what circulates in blood has escaped from cells whose membranes were damaged. That breadth of origin is the whole story of how AST is read — a raised value confirms that cells are injured somewhere, and the tests drawn alongside it (ALT for liver, creatine kinase for muscle) say where.

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

A protein enzyme of the aminotransferase family that exists as two isoenzymes: a cytoplasmic form and a mitochondrial form. In liver cells roughly four fifths of the AST is mitochondrial, which matters because mild injury releases mainly the cytoplasmic form while alcohol and severe cell death release the mitochondrial one. Like ALT, it needs pyridoxal-5'-phosphate (active vitamin B6) as a cofactor, so severe B6 deficiency lowers the measured activity.

What does AST do in the body?

AST moves an amino group from aspartate to alpha-ketoglutarate, producing oxaloacetate and glutamate. That reaction links amino-acid metabolism to the citric acid cycle and is one arm of the malate-aspartate shuttle, which is how muscle and heart move reducing power into mitochondria to make energy — the reason these tissues carry so much of it. Its diagnostic use has nothing to do with this job: AST is simply a marker that spills into blood in proportion to how many AST-rich cells are breaking. It clears from blood with a half-life of roughly 17 hours, about a third of ALT's, so after a one-off injury AST falls back faster than ALT does.

What does a high AST mean?

A raised AST means cells rich in the enzyme are being damaged, and the pattern of the tests around it identifies which tissue. AST above ALT with a raised GGT points towards alcohol; ALT above AST fits fatty liver or viral hepatitis; AST raised with creatine kinase and a normal ALT points to muscle. The ACG 2017 guideline on abnormal liver chemistries grades elevations by multiples of the laboratory's upper limit — borderline below twice, mild two to five times, moderate five to fifteen times, severe beyond that — and values in the moderate range or above call for prompt medical assessment rather than a wait-and-repeat approach.

Common causes:

• Fatty liver and the metabolic syndrome — the most common reason overall, though here ALT usually leads and AST follows.
• Alcohol. It damages mitochondria, releasing the mitochondrial AST isoenzyme, and depletes vitamin B6, which blunts ALT more than AST — together this produces the classic AST-dominant pattern.
• Muscle injury of any kind: hard or unaccustomed exercise, a fall, an intramuscular injection, seizures, muscle disease, or statin-related muscle damage. Creatine kinase rises alongside and ALT stays comparatively low.
• Heart muscle injury. AST was the original 'cardiac enzyme' in the 1950s; troponin has replaced it for that purpose, but a heart attack or myocarditis still raises AST.
• Acute liver injury from viruses, medicines, herbal products or a temporary loss of blood supply. These produce the largest rises, typically together with ALT.
• Haemolysis — either in the body or in the sample tube. Red cells contain far more AST than ALT, so a haemolysed sample inflates AST specifically.
• Macro-AST: the enzyme bound to an antibody into a complex too large to clear. It causes a persistent, isolated, entirely harmless AST elevation and is identified with a specific laboratory test.

What does a low AST mean?

A low AST is almost never a clinical problem and laboratories rarely flag it. The exceptions are worth knowing rather than worrying about: severe vitamin B6 deficiency lowers measured activity, people on long-term dialysis tend to run low, and in older adults a persistently low AST has been linked in cohort studies to low muscle mass — an association that describes groups, not a finding to act on individually.

Common causes:

• Vitamin B6 deficiency.
• Long-term haemodialysis.
• Low muscle mass in older age.

What moves AST apart from disease?

Strenuous exercise and muscle injury raises it

Muscle is the single biggest non-liver source of a raised AST. In healthy volunteers, one hour of unaccustomed weightlifting raised AST for a week, with the peak two to four days after the session. Any AST measured within a few days of hard training, a marathon, a fall or an intramuscular injection is at risk of being misread as liver disease.

Haemolysis of the sample raises it

Red cells hold a large reservoir of AST, so a difficult draw, a tube shaken hard, or a sample left too long before separation raises AST while ALT stays about the same. Laboratories usually mark haemolysed samples on the report; an unexplained isolated AST rise is a reason to check for that note.

Alcohol in the preceding days and weeks raises it

Alcohol raises AST more than ALT, which is why an AST/ALT ratio above 2 is read as an alcohol signal. Even a single heavy evening can shift the ratio, and habitual intake keeps it shifted. Several alcohol-free weeks before a repeat test remove this variable.

Sex, age and body composition either direction

Men run somewhat higher than women, partly because of greater muscle mass. Values rise slightly with body weight and visceral fat, and drift downward in old age as muscle mass falls. Newborns and young children run higher than adults.

Vitamin B6 status lowers it

AST needs vitamin B6 to work, so a marked deficiency lowers the measured activity even when cells are leaking. Some laboratory methods add B6 to the reaction to remove this effect; others do not, which is one reason results differ between laboratories.

Medicines and supplements raises it

Statins, some antibiotics, anti-tuberculosis drugs, antifungals, valproate, high-dose niacin, anabolic steroids and many herbal products can raise AST — either by injuring liver cells or, for statins, occasionally muscle. A full list of everything taken, including over-the-counter and herbal items, is essential context for any raised value.

Which other tests are read together with AST?

Alanine Aminotransferase read as a ratio

The AST/ALT ratio (De Ritis ratio) is how the two are read together. Below 1 fits fatty liver or viral hepatitis; above 2 with a raised GGT points to alcohol; a ratio that climbs above 1 during chronic liver disease is a marker of advancing fibrosis. AST raised with a normal ALT usually means the source is not the liver at all.

Gamma-Glutamyl Transferase same panel

GGT decides whether an AST-dominant pattern is alcohol-related. AST above ALT with a normal GGT makes muscle or heart a more likely source than alcohol, because GGT is absent from muscle.

Alkaline Phosphatase same panel

Alkaline phosphatase separates a hepatocellular pattern (aminotransferases dominant) from a cholestatic one (ALP dominant). The two patterns lead to different causes and different next tests.

Total Bilirubin downstream

Bilirubin says whether the liver is still doing its job. Aminotransferases rising together with bilirubin — the pattern behind Hy's law — signals a much more serious injury than aminotransferases rising alone.

Platelet Count downstream

AST and platelet count form the APRI score and, with ALT and age, the FIB-4 score — the two most widely used blood tests for estimating liver fibrosis without a biopsy.

Triglycerides upstream

Triglycerides, insulin resistance and liver fat rise together. A mild AST elevation with ALT slightly higher, in a person who feels well, is usually part of that metabolic cluster.

Which foods affect AST?

• Alcohol — raises strong evidence
• A Mediterranean-style pattern — olive oil, fish, legumes, vegetables, nuts — lowers moderate evidence
• Replacing sugar-sweetened drinks and fruit juice with water — lowers moderate evidence
• Coffee, 2-3 cups a day, caffeinated or not — lowers moderate evidence
• Vitamin B6-rich foods — fish, poultry, potatoes, chickpeas, bananas — when intake is low — supports moderate evidence

Which supplements are studied for AST?

Vitamin B6 (pyridoxine) moderate evidence

AST is a B6-dependent enzyme, so correcting a real deficiency restores measured activity. Supplementing when B6 status is normal does nothing to the result, and B6 has no protective effect on the cells that release AST.
Typical dose: no general dose given
Cautions: Long-term intake above about 100 mg a day causes sensory nerve damage — numbness and unsteadiness — that can take months to resolve. There is no reason to take high doses for a liver enzyme result. Only worth considering when deficiency is likely: heavy alcohol use, certain medications (isoniazid, some anti-epileptics) or a restricted diet.

Omega-3 (EPA/DHA) moderate evidence

Reduces liver triglyceride content, which is the driver when AST is mildly raised as part of fatty liver. Trials show reasonably consistent reductions in liver fat; the effect on AST itself is small and inconsistent.
Typical dose: 1-2 g/day combined EPA + DHA
Cautions: May increase bleeding tendency at higher doses or together with anticoagulants. Does nothing for an AST that comes from muscle, alcohol or medicines.

How does exercise affect AST?

Moderate aerobic exercise strong evidence

How much: 150-240 minutes a week at an intensity where talking is still possible, spread over at least three sessions.
What to expect: Lowers liver fat and, with it, a metabolically raised AST, whether or not weight changes. Moderate steady exercise does not itself raise AST in any meaningful way.

Resistance training moderate evidence

How much: Two or three sessions a week covering the major muscle groups, with load increased gradually over weeks rather than in one jump.
What to expect: Reduces liver fat over months. In the days after an unaccustomed or unusually heavy session, though, AST rises from muscle — sometimes to several times the upper limit — and stays up for about a week. Gradual progression keeps that spike small.

Which lifestyle factors affect AST?

Alcohol reduction strong evidence

The highest-yield change when AST leads ALT and GGT is also raised. Because alcohol shifts the AST/ALT ratio directly, an alcohol-free stretch of several weeks before a repeat test shows whether alcohol was the explanation.

Timing blood tests around training strong evidence

Blood drawn within a few days of hard exercise reports muscle, not liver. Booking routine tests for a rest week, or at least 72 hours after the last heavy session, avoids a spurious result and an unnecessary follow-up.

Reviewing medicines and supplements strong evidence

A complete list — prescription, over-the-counter, herbal and sports supplements — belongs in every consultation about a raised AST. Drug-induced liver injury is under-recognised, and bodybuilding products in particular are a frequent, rarely volunteered cause.

Weight loss of 5-10% where fatty liver is the cause strong evidence

Losing 5% of body weight typically reduces liver fat, and 7-10% is associated with improvement in liver inflammation on biopsy. Aminotransferases usually follow liver fat down.

How should you prepare for a AST test?

• Avoid hard or unaccustomed exercise for at least 72 hours before the draw — after an unusually heavy session, AST can stay raised for about a week.
• Fasting is not required for AST itself, although it usually is for tests ordered on the same panel.
• Check the report for a haemolysis note whenever AST is raised and ALT is not; a repeat sample often resolves it.
• A persistently raised AST with a normal ALT, GGT and creatine kinase is the situation in which laboratories test for macro-AST — a harmless cause worth ruling out before any further investigation.
• Report recent alcohol intake, intramuscular injections, falls and all medicines and supplements; each changes how the result is read.

References

1. 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
2. Giannini EG, Testa R, Savarino V. Liver enzyme alteration: a guide for clinicians. CMAJ 2005 — https://doi.org/10.1503/cmaj.1040752
3. Pettersson J et al. Muscular exercise can cause highly pathological liver function tests in healthy men. Br J Clin Pharmacol 2008 — https://doi.org/10.1111/j.1365-2125.2007.03001.x
4. Nathwani RA, Pais S, Reynolds TB, Kaplowitz N. Serum alanine aminotransferase in skeletal muscle diseases. Hepatology 2005 — https://doi.org/10.1002/hep.20548