ALT and AST: what liver enzymes on a routine panel tell you

Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are enzymes inside cells. When cells are damaged, the enzymes leak into the blood. ALT is concentrated in the liver, so it is the more liver-specific of the two. AST is also found in muscle, heart and red cells. Both are reported in units per litre (U/L or IU/L), and every lab sets its own reference range.

The ranges

The American College of Gastroenterology set its lower "healthy" ALT range in 2017 because elevated ALT, even within many labs' printed ranges, is associated with liver-related death. So for ALT, the guideline itself is stricter than the typical report. That is the reverse of most markers on these pages, where the "optimal" claims come from outside the guidelines.

FIB-4 combines age, AST, ALT and platelet count to estimate the chance of significant scarring in fatty liver disease. The AASLD uses it as the first step before imaging or elastography.

What raises and lowers them

• Fatty liver disease (now called MASLD) is the most common cause of mildly raised ALT.
• Alcohol, which typically raises AST more than ALT.
• Medicines and supplements: paracetamol (acetaminophen) at high doses, statins (usually minor), some antibiotics, and a long list of herbal and bodybuilding products.
• Viral hepatitis, autoimmune liver disease, haemochromatosis and rarer conditions, which is why the ACG work-up checks for them.
• Muscle: heavy resistance training, a marathon or a muscle injury can raise AST, and to a lesser extent ALT, for several days. Creatine kinase on the same draw shows whether muscle is the source.
• Lowers: weight loss, reducing alcohol, and treating the cause.

What it predicts

A Korean cohort in the BMJ (2004) followed 94,533 men and 47,522 women aged 35 to 59 for eight years. Compared with ALT under 20 IU/L, the adjusted relative risk of death from liver disease in men was 2.9 at 20 to 29 IU/L and 9.5 at 30 to 39 IU/L. For AST, the figures were 2.5 and 8.0. All of those values were inside the lab ranges of the time. The best cutoffs for predicting liver disease in men were about 30 IU/L for ALT and 31 for AST. This is observational, and the absolute risk of liver death in a middle-aged cohort is low, but it is the kind of evidence behind the ACG numbers.

On GLP-1 drugs and weight loss

Weight loss is the main treatment for fatty liver disease, and liver enzymes usually fall with it. The strongest evidence is from biopsies rather than enzymes. In ESSENCE (NEJM, 2025), an interim analysis of 800 people with MASH and moderate or advanced fibrosis, steatohepatitis resolved without worsening fibrosis in 62.9% on semaglutide 2.4 mg against 34.3% on placebo at 72 weeks, and fibrosis improved in 36.8% against 22.4%. The FDA approved Wegovy for this use in August 2025. The fatty liver study page covers ESSENCE and the tirzepatide data.

Rapid weight loss also raises the risk of gallstones, which can raise liver enzymes with pain under the right ribs. That is a call, not a note; see the side effects page.

How to test

• No hard training for 48 to 72 hours beforehand, especially new or eccentric lifting.
• No alcohol for a couple of days, and note any paracetamol or supplements.
• Fasting is not essential but keeps conditions consistent with a metabolic panel.
• Repeat a mild rise before investigating. Day-to-day variation in ALT is substantial.
• Ask for platelets on the same draw so FIB-4 can be calculated.

Tracking it in Peptide AI

In Peptide AI, ALT and AST sits in the metabolic panel. The app reads it from a lab PDF or manual entry, scores the panel from 0 to 100 alongside an overall grade, flags values that warrant prompt physician consultation, and plots each result against your dose timeline. Logging training in the training log makes a post-workout AST spike easy to spot. Log the draw date, whether you fasted, and anything unusual in the days before, such as illness, a hard session or a dose step, so the next result is compared like with like. Bloodwork tracking shows the series next to the dose log, weight and side effects, and the panels page lists every marker the app groups. More markers are in the bloodwork guides.