Aspartate Transferase

AST (Aspartate Transferase, sometimes written ASAT) is an enzyme found inside the cells of several tissues, most significantly the liver, heart, and skeletal muscle, as well as the kidneys and red blood cells. Unlike ALT, which is almost exclusive to the liver, AST is multi-source. That makes it less specific on its own but very informative when interpreted alongside other markers. A raised AST tells you something in the body is releasing the enzyme, the job is to work out which tissue.

Inside the cell, AST plays a role in amino acid metabolism, helping to convert amino acids into building blocks for energy and other molecules. It works alongside ALT in the same pathway. When cell membranes are damaged by inflammation, injury, or overuse, AST leaks into the bloodstream. Because it is particularly concentrated in heart and muscle cells, an intense workout, a long run, heavy resistance training, or a muscle injury can raise AST for a day or two after the event, even in someone with a perfectly healthy liver.

The real diagnostic power of AST comes from the AST:ALT ratio. Because ALT is liver-specific and AST is multi-source, their relationship gives a pattern. In non-alcoholic fatty liver disease (NAFLD), the ratio is usually less than 1, ALT runs higher than AST. In alcoholic liver disease, the ratio flips: AST is typically more than twice ALT (a ratio above 2), because alcohol preferentially damages mitochondria where a lot of AST is stored. Acute viral hepatitis tends to raise both roughly equally. A marked AST rise with normal or mildly raised ALT often points to muscle rather than liver, for example after a heavy gym session, prolonged exercise, or in rhabdomyolysis.

If your AST is raised, your GP will consider the context: recent exercise, alcohol, medications, and the AST:ALT ratio alongside GGT and ALP. In many cases, mild elevations reflect lifestyle factors and resolve on their own. Persistent or marked elevations warrant further investigation to identify the source and underlying cause.