AST

What is AST?

AST (aspartate aminotransferase) is an enzyme present in the liver, heart muscle, and — in high concentrations — skeletal muscle. When cells in any of these tissues are damaged or break down, AST leaks into the bloodstream. It is reported in units per litre (U/L) and is a non-specific marker for tissue injury: a raised value tells you cells are being damaged somewhere, but not which tissue. The caveat for athletes is particularly relevant — after a heavy resistance session or intensive endurance effort, AST can double or triple from normal muscle breakdown, even with a fully healthy liver. The contrast with ALT is diagnostically useful. ALT (alanine aminotransferase) is found predominantly in liver tissue and rises more selectively with liver stress than AST does. When ALT and GGT are both elevated alongside AST, the picture shifts toward the liver; when AST rises most strongly with a normal ALT, a muscle source is more likely. The AST/ALT ratio — the De Ritis ratio — helps with that distinction, but remains a supporting tool: the full liver enzyme profile and clinical context remain primary.

Why is AST relevant?

AST is most useful as part of the complete liver enzyme profile, alongside ALT, GGT, bilirubin, and ALP. Liver conditions involving cell death — viral hepatitis, alcoholic liver disease, drug-induced injury, or ischaemic hepatitis — often produce a simultaneous rise in AST and ALT, sometimes with AST rising earlier and more steeply. For athletes and people who train intensively, AST has a different, less alarming relevance: it reflects the muscle breakdown that is part of normal training. An elevated AST the day after a heavy strength session or a marathon is entirely expected. Knowing this prevents unnecessary repeat tests or anxiety. It also makes clear why the blood sample gives the most useful liver picture when drawn after a rest day rather than immediately after effort.

AST high or low — what it means

Never interpret AST in isolation — always read it alongside ALT, GGT, bilirubin, and ALP, and in the context of training load, medication use, and symptoms. An isolated raised AST with a normal ALT, no symptoms, and recent heavy training is almost always muscle in origin. A simultaneous rise in AST, ALT, and GGT points more toward the liver and bile-duct compartment. For a fair liver assessment, wait at least 48–72 hours after strenuous effort before drawing blood — especially if strength or interval training occurred in the preceding days. Reference ranges are lab- and sex-specific; as a general orientation, most labs consider values up to about 40 U/L in women and 50 U/L in men as normal, but always check the reference range of your own laboratory. The trend across measurements taken under comparable conditions says more than any single value.

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