ALT (alanine aminotransferase) and AST (aspartate aminotransferase) are enzymes found in liver cells. When liver cells are damaged or stressed, they release these enzymes into the bloodstream — which is why elevated ALT and AST are used as markers of liver function. The problem is that these enzymes are also found in muscle cells. And when you train hard — particularly with resistance training — your muscle cells sustain microdamage. That damage triggers the release of ALT and AST into the blood, just like the liver does.
Why standard ranges mislead athletes
Standard reference ranges were developed using populations that include a lot of sedentary people. The upper limit for ALT in most labs is around 56 U/L. But research consistently shows that trained athletes can have ALT values of 60–100 U/L as a normal baseline — driven by muscle, not liver damage. The key question isn't how high the number is — it's what's driving it.
The simple test
If your ALT is elevated and you've just completed a hard training week, the likely explanation is muscle damage. Take 5 days off training completely and retest. If your ALT comes back to normal, it was muscle. If it stays elevated at rest, then investigation is warranted.
How to tell the difference
CK (creatine kinase) is almost exclusively muscle-derived — if ALT and CK are both elevated, the ALT is muscle. If ALT is elevated and CK is normal, the liver warrants more attention. Bilirubin and albumin, which are genuinely liver-specific markers, are worth checking alongside ALT to build a fuller picture. GGT is another useful marker. Unlike ALT and AST, GGT is almost exclusively liver-derived and not significantly elevated by exercise. If your ALT is high but your GGT is normal, muscle damage is overwhelmingly the likely cause. MarkerX flags elevated ALT in athletes with a specific note explaining this distinction. Standard lab reports don't. That context is the difference between an unnecessary referral and a sensible protocol adjustment.