Liver Enzymes: What ALT and AST Actually Tell You

ALT and AST are enzymes that live inside liver cells. When those cells are irritated, inflamed or damaged, the enzymes leak into the bloodstream, so a raised level is a marker of liver cell stress, not of how well the liver is working. A liver can be badly scarred with near-normal enzymes, and a liver can have very high enzymes from a temporary insult and be entirely fine a month later.

Mild elevations are common. Most are explained without any dramatic diagnosis.

What each one is telling you#

ALT is found mainly in the liver, which makes it the more liver-specific of the two. AST is present in liver but also in muscle, heart and red blood cells, so it can rise from heavy exercise, a muscle injury, or an intramuscular injection.

The ratio between them is a rough clue. An AST that is roughly twice the ALT or higher raises the possibility of alcohol-related liver injury or a muscle source. An ALT higher than AST is the more usual pattern in fatty liver and viral hepatitis.

Two other enzymes usually appear on the same report. ALP and GGT rise more when the problem involves bile flow rather than liver cells. A raised GGT with a normal ALP often reflects alcohol or medicines.

The common causes of a mild rise#

Fatty liver is now the single most common cause worldwide, and it travels with excess weight around the middle, prediabetes, high triglycerides and high blood pressure.

Alcohol, in quantities many people would not describe as heavy.

Medicines and supplements. Paracetamol in excess, some antibiotics, statins, anti-tuberculosis drugs, and a long list of herbal and bodybuilding supplements. Green tea extract and anabolic steroids are frequent culprits people do not think to mention.

Viral hepatitis, particularly hepatitis B and C, which are common globally and often silent for years.

Celiac disease, thyroid disease and haemochromatosis account for a meaningful slice of otherwise unexplained elevations.

What a doctor typically does next#

For a mild, symptom-free rise: review alcohol, weight and medicines, then repeat the panel in four to eight weeks. If it persists, a standard non-invasive screen follows, usually hepatitis B and C testing, iron studies, glucose or HbA1c, lipids, celiac serology, thyroid function, and a liver ultrasound. In many health systems a fibrosis score such as FIB-4 is calculated from routine bloods to estimate scarring risk.

Very high levels, above roughly ten times the upper limit, indicate acute liver injury and are handled urgently.

What helps#

If fatty liver is the cause, weight loss of around seven to ten per cent has the strongest evidence for improving liver inflammation. Reducing alcohol, treating diabetes and high triglycerides, and regular activity all help. Hepatitis B vaccination is worth checking if you have not had it.

The wider picture#

Liver enzymes are best read as part of a whole panel, and the in-depth guide to understanding your blood test results shows how they fit with kidney, blood count and metabolic markers.