Liver enzymes: ALT, AST and what raises them

They are not a measure of liver function

ALT and AST are enzymes that live inside liver cells. When a cell is damaged, they leak into the blood. So the test measures leakage, not performance — a liver can be badly scarred and doing its job poorly while the enzymes sit in the normal range, because there are fewer cells left to leak.

Actual liver function is better read from albumin, bilirubin and clotting. The enzymes tell you something is irritating the liver now.

Why ALT and not AST

ALT is the more liver‑specific of the two. AST is also abundant in heart and skeletal muscle, so a raised AST can come from a hard workout, a muscle injury or a statin‑related muscle problem rather than the liver at all.

That is why our baseline includes ALT and not AST. It is ₡12,000, and adding AST would have added another ₡12,000 to answer a question ALT mostly answers. AST goes on when the ratio between them matters.

The ratio, when both are measured

Where both are available, their relationship carries information:

• ALT higher than AST — the usual pattern in fatty liver disease.
• AST roughly twice ALT or more — a pattern classically associated with alcohol‑related liver disease.

Both are patterns, not tests. Plenty of people do not read the textbook, and the ratio has been shifted by anything that raises AST from muscle.

What raises them

The list is longer than most people expect, which is why a mildly raised ALT is a reason to look further rather than to panic:

• Fatty liver — by far the commonest cause in a metabolic clinic's population, and often the first hint that insulin resistance is doing visible damage
• Alcohol
• Medicines — paracetamol, some antibiotics, statins, and many others
• Supplements, including several sold as liver tonics or for bodybuilding
• Viral hepatitis
• Strenuous exercise in the day or two before the draw — mostly AST, but ALT can rise too
• Coeliac disease, thyroid disease and haemochromatosis, all of which are easy to miss

Because of the exercise effect, an unexpected result after a hard week of training is worth repeating before it is investigated.

The argument about the normal range

Laboratory reference ranges for ALT were set from populations that, it turns out, included a great many people with undiagnosed fatty liver. Several groups have argued that the true healthy upper limit is meaningfully lower than most laboratories report, and that the thresholds should differ between men and women.

This has been debated for years without being settled into universal practice, and ranges still vary between laboratories. The practical consequence: an ALT at the top of the stated range is not automatically fine, particularly alongside central weight, raised triglycerides and a high HbA1c. Read as a group, those four describe one picture.

Why it is in our baseline

Fatty liver is common, usually silent, and sits directly on the path the rest of the panel is measuring. It is also the one thing on the baseline that can be improved substantially without any medicine at all, which makes it worth knowing about early.

And it matters for safety. Several of the medicines used in metabolic care are processed by the liver, and a baseline before starting is the only way to know later whether a change is the drug or the disease.

The boundary

One mildly raised enzyme is a prompt for a conversation and often a repeat — not a diagnosis and not a reason for a liver supplement. What it means depends on how high, what else is on the panel, what you take and what you drink, which is a discussion with a physician.