MECHANISM ENGINE

What Is ALT (Liver Enzyme)?

ALT isn’t a number most people have heard of until a GP mentions it. I hadn’t, until it showed up on one of my own routine panels. Turns out it’s less about the liver being diseased and more about the liver being under strain, which is a very different thing to hear.

The Smoke Alarm in the Kitchen

ALT is best thought of as a smoke alarm fitted inside liver cells, not a description of the liver itself. When liver cells are stressed or damaged, some of that enzyme leaks out into the bloodstream, the same way smoke only sets the alarm off once something’s actually burning, not before. A raised ALT on a blood test doesn’t tell you what’s causing the smoke. It tells you there’s smoke, and it’s worth finding the source, whether that’s excess alcohol, a high-UPF diet, rapid weight change, or fat building up in the liver itself.

ALT, alanine transaminase, is an enzyme found mainly inside liver cells. When those cells are damaged or under metabolic stress, ALT leaks into the bloodstream in higher amounts, making it a useful marker of liver strain rather than a direct measure of liver disease. Common drivers of a raised ALT include non-alcoholic fatty liver disease, which is strongly linked to insulin resistance and UPF intake, alongside alcohol and certain medications.

NHS reference ranges for ALT are generally 7 to 56 units per litre, though ranges vary slightly by lab; NICE guidance identifies raised ALT as one of the first indicators used to investigate non-alcoholic fatty liver disease.

Articles referencing this concept will populate here automatically as the Library builds.

Related mechanisms

Educational content only, not medical advice. Never stop or change medication without your GP’s supervision. Neil stopped his own medications in February 2024 on his own initiative and told his GP afterwards, with NHS blood tests to show. That is not the approach recommended here. Speak to your GP before changing your fasting pattern, diet, or medication.

Get the Blood Test Decoder

What this is based on

NHS and NICE guidance, cross-referenced against Neil’s own NHS blood test record going back to April 2012. See the full timeline and current numbers.