What it measures.
Alanine aminotransferase (ALT) is the most liver-specific enzyme in routine blood work. When hepatocytes are damaged, ALT leaks into the bloodstream. While standard lab reference ranges go up to 40-56 U/L, updated evidence shows optimal ALT is <30 U/L for men and <19 U/L for women — meaning millions of people with 'normal' ALT actually have early liver injury, most commonly from fatty liver disease (MASLD).
The activity of alanine aminotransferase enzyme in serum. ALT is found predominantly in the liver (with small amounts in kidney and muscle), making it more liver-specific than AST. Release into blood indicates hepatocyte membrane damage from any cause — inflammation, toxins, ischaemia, or metabolic stress.
Why it matters.
ALT is the gatekeeper of liver health. Mild persistent elevation (30-100 U/L) is the most common pattern and usually indicates MASLD — the most prevalent chronic liver disease affecting 25-30% of adults. ALT also detects viral hepatitis, drug-induced liver injury, autoimmune hepatitis, and celiac disease (cryptogenic elevation). Combined with AST, it calculates the critical AST:ALT ratio and feeds into the FIB-4 fibrosis score.
Physiology.
ALT catalyses the reversible transamination of alanine and alpha-ketoglutarate to pyruvate and glutamate — a key step in amino acid metabolism and gluconeogenesis. It is concentrated in hepatocyte cytoplasm. When cell membranes are disrupted (inflammation, oxidative stress, toxins), ALT leaks into the blood proportionally to the degree of damage.
Testing & preparation.
How to prepare
- No fasting strictly required
- Avoid strenuous exercise 24 hours before (can mildly elevate ALT)
- Avoid alcohol for 48-72 hours for accurate baseline
- Note all medications — statins, antibiotics, anticonvulsants, acetaminophen can affect ALT
- Herbal supplements (kava, comfrey, green tea extract at high doses) can elevate ALT
When to test
Part of any CMP or liver panel. When metabolic syndrome risk factors are present. Monitoring hepatotoxic medications. Evaluating fatigue, right upper quadrant pain, or jaundice. Screening in diabetes and obesity.
How often
Annually as part of routine metabolic panel. Every 3-6 months if elevated and under investigation or treatment. After starting hepatotoxic medications: baseline then 3-6 months.
Interpretation.
High alanine aminotransferase
Common causes:
- MASLD / fatty liver disease (most common cause of mild elevation in non-drinkers)
- Alcoholic liver disease
- Viral hepatitis (B and C)
- Drug-induced liver injury (acetaminophen, statins, antibiotics, herbal supplements)
- Autoimmune hepatitis
- Celiac disease (cryptogenic elevation that resolves on gluten-free diet)
- Haemochromatosis (iron overload)
- Wilson's disease (copper overload — rare)
- Heart failure (hepatic congestion)
- Strenuous exercise (mild, transient)
Implications:
- Mild elevation (1-3x ULN, ~30-100 U/L): most commonly MASLD; calculate FIB-4
- Moderate elevation (3-10x ULN, ~100-500 U/L): acute hepatitis, drug injury, autoimmune
- Severe elevation (>10x ULN, >500 U/L): acute viral hepatitis, acetaminophen toxicity, ischaemic hepatitis — urgent evaluation
- AST:ALT <1: typical of non-alcoholic liver disease
- AST:ALT >2:1: strongly suggests alcoholic liver disease or advanced cirrhosis
Low alanine aminotransferase
Common causes:
- Normal finding
- Vitamin B6 deficiency (ALT requires B6 as cofactor — can cause falsely low ALT)
- Very advanced cirrhosis (few functioning hepatocytes left to release ALT)
Implications:
- Low ALT is rarely clinically significant
- Very low ALT in advanced liver disease = poor prognostic sign (burnt-out liver)
Optimization.
Diet
- Mediterranean diet reduces ALT in MASLD independent of weight loss
- Reduce refined carbohydrates and fructose (major drivers of hepatic lipogenesis)
- Coffee consumption (2-3 cups/day) associated with lower ALT and reduced liver fibrosis risk
- Limit alcohol — even moderate intake elevates ALT in susceptible individuals
Lifestyle
- Weight loss: 7-10% body weight significantly reduces ALT in MASLD
- Regular exercise: reduces ALT independent of weight loss through improved insulin sensitivity
- Avoid excessive acetaminophen (>2g/day chronic; >4g/day acute risk)
- Review supplements — many herbal products are hepatotoxic
Supplements
- Vitamin E (800 IU/day) modestly reduces ALT in non-diabetic MASH (AASLD recommendation)
- Omega-3 fatty acids may reduce ALT in MASLD
- No supplement replaces weight loss and dietary change
FAQs.
My ALT is 42 U/L and my doctor says it's normal — should I be concerned?
Standard lab 'normal' (up to 40-56 U/L) is based on population averages that include people with undiagnosed liver disease. Updated guidance from the ACG recommends optimal ALT of <30 U/L for men and <19 U/L for women. An ALT of 42 with metabolic risk factors (overweight, insulin resistance) is likely early MASLD and warrants investigation — not reassurance.
Should I stop my statin because my ALT went up?
Almost certainly not. Mild ALT elevation (<3x ULN) on statins is common, benign, and does NOT require stopping the medication. Statins are actually beneficial in MASLD patients. Only stop if ALT exceeds 3x ULN with symptoms, or if there's a clear pattern of progressive liver injury. Discuss with your doctor — don't self-discontinue.
What is FIB-4 and how does it use ALT?
FIB-4 = (Age x AST) / (Platelets x √ALT). It estimates liver fibrosis from routine blood work. Score <1.3 = low fibrosis risk; 1.3-2.67 = indeterminate (get FibroScan); >2.67 = high risk (hepatology referral). If your ALT is elevated, your doctor should calculate FIB-4 before ordering expensive imaging.