Alanine Aminotransferase

Also written ALT

Organ functionMeasured in U/L

What it measures

A liver enzyme released into circulation when hepatocytes are damaged. It is relatively liver-specific.

Why it matters

It is the most accessible screen for metabolic fatty liver disease, now the most common liver disease in the developed world and frequently silent.

Where the claimed ranges actually sit

Measured in U/L. Each row is one claim about where your result should fall.

What a laboratory reportsWhat a seller markets as “optimal”

Laboratory reference range< 40 U/L

Laboratory reference intervals, with an active shift toward lower sex-specific cutoffs

Under 25 U/L (men) or 20 U/L (women)B< 25 U/L

Hepatology researchers and metabolic health platforms

›View as table

Claim	Range	Source
Laboratory reference range	< 40 U/L	Laboratory reference intervals, with an active shift toward lower sex-specific cutoffs
Under 25 U/L (men) or 20 U/L (women)	< 25 U/L	Hepatology researchers and metabolic health platforms

Where the sources disagree

This is a case where the 'optimal' range is probably closer to correct than the conventional one, and where the mainstream is actively moving. The reference population problem is real and well documented: if you build a normal range from a population in which a third have fatty liver, your normal range accommodates fatty liver. We grade this B rather than A because the outcome data supporting the specific lower cutoffs is still accumulating.

The marketed ranges, examined

Each claim gets its strongest possible case before it gets its criticism. Several of these are substantially correct.

Under 25 U/L (men) or 20 U/L (women)

Promoted by hepatology researchers and metabolic health platforms

BAssociation only

The strongest case for it

Historical upper limits were set using reference populations that included many people with undiagnosed fatty liver disease, which inflated the normal range. Lower sex-specific limits detect metabolic liver disease considerably earlier, and this position has genuine academic support.

What it leaves out

The lower thresholds substantially increase the number of people flagged abnormal, and most will have mild metabolic fatty liver for which the intervention is weight and metabolic management, not the supplements often sold alongside the finding.

Before you act on your result

Strenuous exercise in the preceding days raises ALT and AST substantially.
Muscle injury raises AST more than ALT; check the ratio before assuming liver origin.
A normal ALT does not exclude significant liver disease, including cirrhosis.

Last reviewed July 28, 2026How we grade evidence and attribute claims →

More in Organ function

Gamma-Glutamyl Transferase

GGT

Estimated Glomerular Filtration Rate

eGFR

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