Fasting Insulin

MetabolicMeasured in µIU/mL

What it measures

Circulating insulin concentration after an overnight fast.

Why it matters

It is arguably the single most underused early marker of metabolic dysfunction in routine practice, and it is rarely ordered by conventional primary care.

Where the claimed ranges actually sit

Measured in µIU/mL. Each row is one claim about where your result should fall.

What a laboratory reportsWhat a seller markets as “optimal”

Laboratory reference range2–25 µIU/mL

Laboratory-specific reference intervals; no harmonized standard exists

2–6 µIU/mLC2–6 µIU/mL

Longevity platforms, functional medicine, and metabolic programs

›View as table

Claim	Range	Source
Laboratory reference range	2–25 µIU/mL	Laboratory-specific reference intervals; no harmonized standard exists
2–6 µIU/mL	2–6 µIU/mL	Longevity platforms, functional medicine, and metabolic programs

Where the sources disagree

This marker is a genuine case where the consumer longevity industry is ahead of routine clinical practice: fasting insulin really does detect problems earlier, and standard care really does under-order it. But the assay standardization problem is severe and almost never disclosed. Comparing your fasting insulin across two different platforms that use different labs is not a valid comparison, and no consumer dashboard will tell you that.

The marketed ranges, examined

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

2–6 µIU/mL

Promoted by longevity platforms, functional medicine, and metabolic programs

CConfounded

The strongest case for it

Fasting insulin rises years before glucose does, so it genuinely detects insulin resistance earlier than any glucose-based marker. The direction of the claim is right.

What it leaves out

Insulin immunoassays are poorly standardized between manufacturers; the same sample can return materially different values at different labs. A tight target of 2–6 implies a precision the assay does not have, and it is frequently used to justify supplement protocols.

Before you act on your result

Assay standardization between laboratories is poor; do not compare values across platforms.
Requires a true fast.
Not meaningful in anyone taking exogenous insulin.

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

More in Metabolic

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HbA1c

Fasting Glucose

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Insulin resistance index

Uric Acid

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