"3. Alanine aminotransferase (ALT), Aspartate aminotransferase (AST)
The liver has many roles in the body. For one, it plays a role in the conversion of amino acids. Two enzymes that are involved in the conversion of amino acids are ALT and AST. These enzymes are commonly used as a surrogate marker for damage to the liver.
ALT is found mostly in the liver so it's more specific to hepatocellular damage (liver damage). AST is found in the cytosol of many other cells like muscle, brain, lung and pancreas, which makes it less specific to the liver. AST and ALT have both been found to be elevated after acute bouts of training. One study found that levels can be elevated for up to 7 days after a bout of lifting in men not used to weight training.
The muscle damaged incurred during heavy endurance training or lifting releases the enzymes into the bloodstream which will then cause the blood tests to be elevated. High-protein diets have also been shown to increase the concentration of these enzymes. The rise doesn't occur from damage but because the body upregulates the production of these enzymes as you consume more protein.
You can see how combining hard training with a high-protein diet can cause elevations in AST and ALT. Now, of course, there can be a disease process occurring as well. So you shouldn't just write off elevations, especially in conjunction with other symptoms like abdominal pain, jaundice, change in stool color, and darkening of the urine. (Of note, alcohol and medications can also cause increases in liver values.)"