Fasting insulin
Also called Fasting serum insulin, insulin in blood test
Fasting insulin is the amount of insulin in a blood sample taken after an overnight fast. Insulin is the hormone the pancreas releases to move glucose out of the blood and into cells; normally insulin and glucose rise and fall together.
When cells respond poorly to insulin, the pancreas compensates by making more of it, a state called hyperinsulinemia. For as long as that extra insulin keeps glucose in range, glucose and A1c can look ordinary, which is why fasting insulin is sometimes measured alongside them. It is not a diagnostic test for diabetes, which is diagnosed on glucose, and there is no routine standardized test for insulin resistance. What fasting insulin results look like and how they are read is covered in What a Normal Fasting Insulin Level Looks Like.
Why it matters after 45
The main drivers of insulin resistance, extra body fat (especially around the belly) and too little physical activity, tend to accumulate in midlife, and the risk of type 2 diabetes rises with age. People who notice weight settling around the middle despite unchanged habits often meet this marker for the first time, usually paired with fasting glucose so the two can be combined into HOMA-IR.
Where it comes up
- HOMA-IR: Turning Two Numbers Into a Picture of Insulin Resistancehow fasting insulin and glucose are combined
- Why what worked at 30 stops workingmidlife metabolic change behind weight gain
- Lab markers FAQhow fasting insulin and A1c answer different questions
- Comprehensive Lab Panelthe baseline panel that includes the metabolic markers
Related terms
References
- MedlinePlus. Insulin in Blood. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/insulin-in-blood/
- Cleveland Clinic. Insulin Resistance. https://my.clevelandclinic.org/health/diseases/22206-insulin-resistance
- National Institute of Diabetes and Digestive and Kidney Diseases. Insulin Resistance & Prediabetes. https://www.niddk.nih.gov/health-information/diabetes/overview/what-is-diabetes/prediabetes-insulin-resistance
Definitions here are educational and are not medical advice. They explain what a term means; the pages linked above cover what it means for you, and a clinician reviews your own results. Editorial & medical review policy · All glossary terms