What is a fasting insulin test?
Fasting insulin measures the level of insulin in your blood after fasting for 8-12 hours. Insulin is a hormone made by the beta cells of the pancreas. Its job is to move glucose from the blood into cells for energy. In insulin resistance, cells no longer respond well to insulin, so the pancreas compensates by making more and more insulin to keep blood sugar normal. High fasting insulin with normal blood sugar is the hallmark of early insulin resistance. The test is typically ordered alongside fasting glucose to calculate the HOMA-IR index.
Fasting insulin normal range
| Category | Fasting Insulin (microIU/mL or mU/L) | Interpretation |
|---|---|---|
| Optimal | < 5 | Excellent insulin sensitivity |
| Normal | 5 – 10 | Normal range |
| Borderline high | 10 – 15 | Early insulin resistance possible |
| High | > 15 | Likely insulin resistance |
| Very high | > 25 | Significant insulin resistance or insulinoma |
What is HOMA-IR?
HOMA-IR (Homeostasis Model Assessment of Insulin Resistance) is calculated from fasting insulin and fasting glucose:
HOMA-IR = (Fasting Insulin x Fasting Glucose) / 405 (glucose in mg/dL)
| HOMA-IR Value | Interpretation |
|---|---|
| < 1.0 | Optimal insulin sensitivity |
| 1.0 – 1.9 | Normal |
| 2.0 – 2.9 | Early insulin resistance |
| ≥ 3.0 | Significant insulin resistance |
What causes high fasting insulin?
- Overweight and obesity — especially excess belly fat
- Sedentary lifestyle
- High-carbohydrate diet (especially refined carbs and sugar)
- PCOS (polycystic ovary syndrome) — insulin resistance is a core feature
- Sleep deprivation and chronic stress (raises cortisol which antagonises insulin)
- Metabolic syndrome
- Family history of type 2 diabetes
- Certain medications (corticosteroids, antipsychotics, thiazide diuretics)
Why does this matter?
Insulin resistance is the foundation of type 2 diabetes, metabolic syndrome, PCOS, non-alcoholic fatty liver disease (NAFLD) and cardiovascular disease. Detecting it early through fasting insulin — when fasting glucose and HbA1c are still normal — allows for intervention through lifestyle change before damage occurs.
Questions to ask your doctor
- What is my HOMA-IR score?
- Does this mean I am at risk for diabetes?
- Should I have an OGTT (oral glucose tolerance test)?
- What diet and lifestyle changes reduce insulin resistance?
- If I have PCOS, is insulin resistance driving it?