ALT

What is ALT?

ALT (alanine aminotransferase, also written ALAT) is an enzyme that lives mainly inside your liver cells, where it helps process amino acids. When those cells are irritated or damaged, ALT leaks out into the bloodstream and the blood level rises — which makes it one of the most commonly used routine markers of liver-cell stress. It is reported in units per litre (U/L); the exact upper limit of the reference range is lab- and method-dependent, and typically runs a little lower for women than for men. ALT is fairly specific to the liver, but it is not specific about the cause: a raised value tells you liver cells are under strain, not why. It is also worth knowing what ALT does not do — a normal ALT does not rule out liver disease, because meaningful fat, inflammation, or even fibrosis can be present while the value still reads in range. That is one more reason it is almost never read on its own. It travels with AST (aspartate aminotransferase), GGT, and bilirubin, and the pattern between them — which enzyme is highest, and by how much — is what points toward a likely driver. The Optimize Baseline measures ALT as part of this liver panel so you can see the picture rather than a single isolated number. For most people, ALT is most useful as a trend you watch over time rather than a one-off pass/fail line. A mildly raised value is common and frequently reversible; the question is whether it stays up.

Why is ALT relevant?

ALT matters because the most common reason for a mildly elevated value in otherwise healthy adults is metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD) — fat accumulating in the liver, usually alongside excess weight, insulin resistance, or an unfavourable metabolic profile. This is exactly the kind of slow, symptom-free process that preventive testing is designed to catch early, while lifestyle change can still reverse it. Because a normal result does not exclude disease, ALT is best treated as one input into your overall metabolic picture rather than an all-clear on its own. Alcohol and viral hepatitis are the other classic drivers, and some medications can raise ALT too — statins, for instance, often cause mild, transient bumps that are rarely a sign of true liver injury, while some antibiotics and a paracetamol overdose can do far more. The relationship with AST adds context. ALT is more liver-specific, whereas AST is also found in muscle and red blood cells, so the ratio between them (the De Ritis ratio) carries information: an AST-dominant pattern can point toward alcohol-related liver injury, advanced fibrosis, or simply muscle as the source, while an ALT-dominant pattern fits the common fatty-liver picture. GGT helps separate alcohol and bile-flow effects from the rest. These reference ranges are general and vary between laboratories — they are a guide for reading your result in context, not a diagnosis. Markedly high values (many times the upper limit) point to acute liver injury and warrant prompt medical assessment rather than watchful waiting.

ALT high or low — what it means

Read ALT alongside AST, GGT, and bilirubin — only together do they form a pattern that points toward a cause. A mild, isolated rise (up to a couple of times the upper reference limit) is common and often harmless, especially as a one-off. What matters more is whether it persists across repeat tests and what the rest of the panel is doing. Keep in mind too that a value within range does not guarantee a healthy liver, so an unfavourable metabolic profile still deserves attention even with a normal ALT. A single reading is only a snapshot, and several everyday things can lift it temporarily. Intensive exercise in the days before the draw is the big one: muscle contains these enzymes too, so hard training can raise AST and ALT and skew the ratio in a way that mimics a liver signal — a normal GGT alongside the rise is a clue that muscle, not the liver, may be the source. Recent alcohol, a passing viral illness, or a new medication can also nudge ALT up. For a fair baseline, avoid testing right after heavy training, note your recent alcohol and medication, and use the trend across several measurements rather than one value. The levers that bring a high ALT down are mostly the fatty-liver levers, because that is the usual cause: losing excess weight (even 5–10% of body weight meaningfully reduces liver fat, with larger losses needed to improve inflammation and scarring), cutting back or pausing alcohol, reducing refined sugar and ultra-processed food, moving regularly, and reviewing any medication or supplement that could be contributing with your doctor. Improvement often shows within weeks to a few months. A low ALT is usually not a concern on its own, though a persistently low value in older adults can sometimes accompany muscle loss or frailty and is worth mentioning to your doctor. A persistently or markedly raised value — particularly with abnormal AST, GGT, or bilirubin, or with symptoms like fatigue, jaundice, or right-upper-abdominal discomfort — should always be worked up with a clinician, who may add hepatitis serology, an ultrasound, or further liver testing.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.