What it measures
Fasting insulin measures the amount of insulin in your blood after an overnight fast (typically 8-12 hours, no food). It reflects the basal insulin production required to maintain stable glucose.
Why it's an early marker
Insulin resistance develops in stages:
- Tissues become less responsive to insulin (early)
- Pancreas compensates by producing more insulin (fasting insulin rises)
- Glucose stays normal because of compensatory hyperinsulinemia
- Years later, beta cells can't keep up; glucose begins rising
- Eventually fasting glucose and HbA1c become abnormal
Fasting insulin captures stage 2-3, when insulin resistance is present but glucose still looks normal. HbA1c only captures stage 4-5.
Optimal ranges
- <5 mIU/mL: Optimal
- 5-7: Good
- 7-10: Borderline
- 10-15: Insulin resistance
- >15: Significant insulin resistance
"Reference ranges" on lab reports are wide (often up to 25 mIU/mL) but optimal is much tighter.
vs glucose and HbA1c
| Marker | Catches insulin resistance at stage |
|---|---|
| Fasting insulin | 2-3 (early) |
| Fasting glucose | 4-5 (later) |
| HbA1c | 4-5 (later) |
| OGTT (oral glucose tolerance test) | 3-4 (intermediate) |
Interpretation
Use fasting insulin with fasting glucose to calculate HOMA-IR (covered in next article). Both markers together are more informative than either alone.
A fasting insulin of 12 with fasting glucose of 90 indicates compensated insulin resistance, hyperinsulinemia keeping glucose normal. This is high-risk metabolic state.
How to lower it
- Reduce refined carbohydrates and added sugars
- Lose visceral fat
- Exercise (especially resistance training and zone 2)
- Reduce alcohol
- Address sleep apnea
- GLP-1 therapy (substantial reduction)
- Metformin (modest reduction)
The clinical pearl: Fasting insulin is the single most underused metabolic test. Many "metabolically healthy" patients have elevated fasting insulin and are several years into compensatory hyperinsulinemia. Catching it early enables intervention before downstream consequences.
Bottom line
Fasting insulin reveals insulin resistance years before glucose-based markers. Standard panels often don't include it. Optimal under 7 mIU/mL. Tracking fasting insulin captures metabolic improvement faster than HbA1c.