What Are ALT and AST?
ALT (alanine aminotransferase) and AST (aspartate aminotransferase) are enzymes found primarily in liver cells. When liver cells are damaged or stressed, they release these enzymes into the bloodstream — so elevated ALT and AST in a blood test signal that something is happening in the liver.
They are the core markers in a liver function panel, alongside GGT, bilirubin, and albumin.
ALT vs. AST: What's the Difference?
ALT is more liver-specific. It's found predominantly in liver cells, so elevation is a more direct indicator of liver inflammation or damage.
AST is found in the liver but also in skeletal muscle, heart, kidneys, and red blood cells. It's less specific — an elevated AST can reflect liver pathology or muscle damage (including from intense exercise).
The AST/ALT ratio provides additional diagnostic information. A ratio above 2:1 (AST more than twice ALT) is a classic pattern in alcoholic liver disease. A ratio below 1 (ALT higher than AST) is more typical of non-alcoholic fatty liver disease (NAFLD) and viral hepatitis.
What Elevated ALT and AST Indicate
Mild, persistent elevation (up to 3x the upper limit of normal) is most commonly associated with:
- Non-alcoholic fatty liver disease (NAFLD) — the most common cause of elevated liver enzymes in the developed world; driven by insulin resistance, obesity, and metabolic syndrome
- Alcohol-related liver disease — even at moderate intake levels
- Medication effects — statins, acetaminophen at high doses, certain supplements
- Thyroid dysfunction — both hypo- and hyperthyroidism affect liver enzymes
- Coeliac disease — chronic mucosal damage elevates liver enzymes in a subset of patients
- Recent intense exercise — particularly resistance training (raises AST from muscle)
Higher elevations (above 5–10x normal) suggest more acute liver pathology — viral hepatitis, autoimmune hepatitis, or drug-induced liver injury — and require prompt investigation.
ALT and AST Reference Ranges
ALT upper limit: ~56 U/L (men), ~45 U/L (women). AST upper limit: ~40 U/L (men), ~32 U/L (women).
Note: Some researchers argue that current "normal" ranges for ALT are set too high, and that optimal ALT for lowest liver disease risk is below 25 U/L in men and below 20 U/L in women.
Non-Alcoholic Fatty Liver Disease (NAFLD)
NAFLD deserves specific attention because it's common, often asymptomatic, and frequently missed until it has progressed. It affects an estimated 25–30% of adults in developed countries and is driven by the same metabolic dysfunction (insulin resistance, excess visceral fat) that drives type 2 diabetes and cardiovascular disease.
In early NAFLD, ALT and AST may be only mildly elevated — or even normal. Without intervention, NAFLD can progress to fibrosis, cirrhosis, and liver failure over years to decades.
How to Protect Liver Health
- Limit alcohol — even moderate intake contributes to liver stress
- Reduce fructose and refined sugar — fructose is metabolized almost entirely in the liver
- Address insulin resistance — the primary driver of NAFLD
- Review medications and supplements
- Exercise — aerobic exercise directly reduces hepatic fat
Is ALT/AST Tested in a Standard Physical?
Yes — liver enzymes are typically included in basic preventive bloodwork. The Core Health Assessment tests ALT, AST, GGT, bilirubin, and albumin as a complete liver panel.
