ALT
Alanine aminotransferase — a liver enzyme that leaks into the blood when liver cells are damaged.
Optimal Range
< 25 U/L (optimal) · < 40 U/L (lab range)
Risk-Stratified Targets
| Population / Context | Target |
|---|---|
| Optimal (longevity) | < 25 U/L |
| Normal lab range | < 40 U/L (men) · < 31 U/L (women) |
| Mildly elevatedMost commonly NAFLD; investigate metabolic causes | 40–80 U/L |
| Moderately elevatedConsider hepatitis, medication toxicity, or alcohol | 80–200 U/L |
| Severely elevatedUrgent evaluation — acute hepatitis, drug toxicity, or biliary obstruction | > 200 U/L |
Why It Matters
ALT is a sensitive marker for liver inflammation and metabolic dysfunction. Even mildly elevated levels are associated with non-alcoholic fatty liver disease (NAFLD), which affects up to 30% of adults and accelerates metabolic aging.
Understanding ALT
ALT (alanine aminotransferase) is an enzyme found predominantly in liver cells (hepatocytes). When liver cells are damaged or inflamed, ALT leaks into the bloodstream, making it one of the most sensitive markers of liver health. It is routinely included in standard blood panels but is often interpreted too loosely — a level of 35 U/L may fall within the 'normal' lab range but is not optimal.
The most common cause of mildly elevated ALT in the modern world is non-alcoholic fatty liver disease (NAFLD), now estimated to affect 25–30% of the global adult population. NAFLD is driven by insulin resistance, excess carbohydrate intake, and visceral obesity — the same metabolic dysfunction that underlies type 2 diabetes and cardiovascular disease. Left unchecked, NAFLD can progress to non-alcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and liver cancer.
In longevity medicine, the target is an ALT below 25 U/L — a level that suggests minimal liver inflammation and good metabolic health. Even modest elevations (25–40 U/L) warrant investigation of metabolic causes, alcohol intake, medication effects (statins, acetaminophen, NSAIDs), and viral hepatitis. ALT responds readily to lifestyle interventions: weight loss, reduced sugar and alcohol intake, and regular exercise can normalize ALT within weeks to months.