Insulin is a hormone made by the beta cells of the pancreas that lowers blood sugar by moving glucose out of the bloodstream into muscle, liver and fat cells. It is released whenever blood glucose rises, mainly after a meal, and it switches the body from burning stored fuel to storing it. A fasting insulin test reads how much of the hormone circulates after an overnight fast — in practice, how hard the pancreas works to keep glucose normal. It is an early signal, because it often drifts upward while fasting glucose still looks fine.
What it measures
Fasting insulin comes from a single blood draw after an overnight fast. Labs report it in microunits per milliliter (written µIU/mL, the same unit as mIU/L) or in pmol/L, using an immunoassay. Those assays are not fully standardized across manufacturers, so one sample can return different numbers in different labs. Read the result against the range printed on your own report, and track changes in one lab.
Insulin alone says little. It is read next to Blood glucose (fasting) from the same draw, because the same insulin level means one thing when glucose is low-normal and another when glucose is already high. The HOMA-IR index combines them: fasting insulin multiplied by fasting glucose, divided by a constant that depends on the glucose units.
Typical values
There is no universal cut-off for fasting insulin. Published ranges differ by assay and lab, and to a degree by age, sex and body composition, so a value flagged high on one report can sit inside the range on another. A reference range also describes the population tested, not an optimum.
| Measure | Usual units | What to keep in mind |
|---|---|---|
| Fasting insulin | µIU/mL (mIU/L) or pmol/L | Assay-dependent; compare with your lab’s stated range and your previous results. |
| HOMA-IR | Index, no units | Calculated from fasting insulin and glucose; the “high” threshold varies between populations and studies. |
| Insulin during a glucose tolerance test | Same units as fasting insulin | Rises and falls over hours with no single agreed normal curve; mainly a specialist tool. |
Fasting insulin is not on a routine panel in most countries. It is ordered when insulin resistance, polycystic ovary syndrome or unexplained low blood sugar is suspected.
Why it matters for longevity
When cells stop responding well to the hormone — the state described under Insulin resistance — the pancreas compensates by secreting more. Insulin rises first; glucose stays normal until that compensation runs out. In observational cohorts, higher fasting insulin and insulin resistance are associated with type 2 diabetes, cardiovascular disease, fatty liver, several cancers and higher all-cause mortality. That is association: fasting insulin marks a metabolic state rather than proving the hormone does the damage. In animals the link is more direct, since reduced insulin and IGF-1 signaling extends lifespan in worms, flies and mice — one of the most reproducible lifespan levers in model organisms, and one not shown to transfer to people. For the clinical detail, see Insulin.
What changes it
- Losing excess fat, especially around the abdomen and liver. The largest and best-evidenced lever for most people.
- Training. Aerobic work and resistance training both improve insulin sensitivity, and a single session raises glucose uptake by muscle for hours. A short walk after meals blunts the post-meal rise.
- What you eat. Cutting sugary drinks and refined starch lowers the insulin demand of a meal; fiber, protein and intact whole grains flatten it.
- Sleep. Short or fragmented sleep reduces insulin sensitivity in controlled studies, within days.
- Medication. Metformin, GLP-1 receptor agonists and SGLT2 inhibitors reduce the demand on beta cells, but are prescribed for defined indications, not as longevity tools.
- Things you do not control. Genetics, aging, pregnancy, and conditions or drugs that raise insulin, such as polycystic ovary syndrome or corticosteroids.
- Test conditions. Eating before the draw, or an unusually short night, shifts the number; follow the fasting instruction, and repeat an odd result before acting on it.
Related reading
- How to Improve Metabolic Health: A Practical Guide
- Which Blood Tests Reflect Aging and How to Read Them
- Intermittent Fasting and Longevity: The Evidence
- Caloric Restriction and Lifespan
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.