The liver is the body's primary metabolic engine and detoxification center. It performs over 500 vital functions, including filtering toxins from the blood, producing bile for digestion, storing glycogen for energy, and synthesizing essential proteins. Despite its incredible regenerative capacity, the liver is highly susceptible to damage from modern lifestyle factors, particularly poor diet, alcohol consumption, and metabolic syndrome.
Because the liver has no pain receptors, liver disease is notoriously silent. Significant damage, including advanced fibrosis (scarring), can occur without any physical symptoms. In clinical practice, our primary window into the health of this vital organ is a panel of blood biomarkers collectively known as Liver Function Tests (LFTs).
Understanding the Enzymes: ALT and AST
The core of an LFT panel involves measuring specific enzymes. Enzymes are proteins that facilitate chemical reactions. Normally, these enzymes reside safely inside the liver cells (hepatocytes). However, when liver cells are stressed, inflamed, or damaged, they break open and leak these enzymes into the bloodstream.
- ALT (Alanine Aminotransferase): ALT is found almost exclusively in the liver. It is the most specific and sensitive marker for acute hepatocellular injury (direct damage to the liver cells). Elevated ALT is the classic hallmark of liver inflammation, whether caused by a virus (hepatitis), toxins, or fat accumulation.
- AST (Aspartate Aminotransferase): AST is also found in the liver, but it is also present in cardiac muscle, skeletal muscle, and red blood cells. Therefore, it is less specific to the liver than ALT. However, the ratio of AST to ALT is clinically highly significant. For example, an AST level that is significantly higher than ALT is a strong indicator of alcohol-induced liver damage or advanced cirrhosis.
Markers of Biliary Function: ALP and GGT
While ALT and AST indicate damage to the liver cells themselves, ALP and GGT provide information about the biliary system—the network of ducts that transport bile from the liver to the gallbladder and intestines.
- ALP (Alkaline Phosphatase): ALP is an enzyme found in the cells lining the bile ducts (as well as in bone). Elevated ALP often indicates cholestasis—a blockage or sluggish flow of bile, which can be caused by gallstones, tumors, or certain medications.
- GGT (Gamma-Glutamyl Transferase): GGT is highly sensitive to oxidative stress in the liver. It is the first enzyme to elevate in response to heavy alcohol consumption or exposure to hepatotoxic drugs. Crucially, GGT is also an excellent early marker for Non-Alcoholic Fatty Liver Disease (NAFLD) and insulin resistance. If ALP is elevated, a simultaneously elevated GGT confirms that the issue is originating in the liver/biliary system, rather than the bones.
Bilirubin and True "Function"
Interestingly, the enzymes discussed above indicate liver damage, not necessarily liver function. To assess how well the liver is actually performing its jobs, we look at other markers:
- Bilirubin: This is a yellow pigment produced during the normal breakdown of old red blood cells. The liver is responsible for processing bilirubin and excreting it into the bile. If the liver is failing, or if the bile ducts are blocked, bilirubin builds up in the blood, eventually causing jaundice (yellowing of the skin and eyes).
- Albumin: The liver synthesizes albumin, the most abundant protein in the blood, which keeps fluid from leaking out of blood vessels. Low albumin levels indicate chronic, long-term liver dysfunction (cirrhosis), as the liver has lost its ability to manufacture essential proteins.
The most common cause of abnormal LFTs in the UK today is not alcohol, but Non-Alcoholic Fatty Liver Disease (NAFLD), driven by obesity and insulin resistance. The empowering clinical reality is that the liver is remarkably resilient. If caught early through routine blood testing, fatty liver disease and early-stage inflammation are highly reversible through targeted dietary changes, weight loss, and metabolic management.
Check Your Liver Health
Because liver disease is silent in its early stages, routine blood testing is the only way to detect problems early enough to reverse them:
- Liver Function Blood Test Kit: A complete LFT panel measuring ALT, AST, ALP, GGT, bilirubin, and albumin — the standard clinical liver screen.
Medical References
- Newsome, P. N., et al. (2018). Guidelines on the management of abnormal liver blood tests. Gut, 67(1), 6-19.
- Giannini, E. G., Testa, R., & Savarino, V. (2005). Liver enzyme alteration: a guide for clinicians. Canadian Medical Association Journal, 172(4), 367-379.
- National Institute for Health and Care Excellence (NICE). (2016). Non-alcoholic fatty liver disease (NAFLD): assessment and management. NICE guideline [NG49].