Fasting insulin: what it adds, and what it can't
"Your blood sugar is fine" is usually true and occasionally incomplete. Insulin resistance can be building for years while the numbers your provider actually ordered still read normal.
Why glucose and A1C can look fine anyway
The standard tests measure the end result, not the effort it took to get there. As insulin sensitivity falls, the pancreas compensates by making more insulin — and as long as that compensation holds, glucose stays in range. Only when it stops keeping up does glucose drift.
The tests that define prediabetes reflect that later stage: an A1C of 5.7-6.4%, or fasting glucose of 100-125 mg/dL. Useful, and by design not early.
What fasting insulin adds, and the catch nobody mentions
Measured alongside fasting glucose, insulin lets you calculate HOMA-IR, an estimate of insulin resistance — in principle flagging the compensating phase before glucose moves.
Here is the part that gets left out of the wellness version. NIDDK states plainly that health care professionals may not test for insulin resistance, and that the test for it is primarily used only for research studies. There is no agreed clinical cutoff. You will see confident numbers quoted online; they aren't coming from a standards body, and different labs and practitioners use different ones.
So treat it as a directional signal, not a diagnosis. A result is more meaningful compared against your own previous result than against someone's internet threshold.
Where it's most relevant
PMOS — polyendocrine metabolic ovarian syndrome, renamed from polycystic ovary syndrome by global consensus in 2026 because the old name implied ovarian cysts and obscured the endocrine and metabolic features. It affects roughly one in eight women, and insulin resistance sits near the centre of it.
If you have PMOS, or perimenopausal metabolic changes, or a strong family history of type 2 diabetes, fasting insulin is a reasonable thing to ask about. On supplements in PMOS, the evidence is real but bounded: a review of nutritional and complementary approaches found inositol improved ovulation rate and menstrual cycle frequency, while evidence for pregnancy, miscarriage and live birth rates is lacking.
What's reasonable to do
Ask for it if you have a specific reason, and expect A1C and fasting glucose to remain the tests that actually diagnose anything. Then treat the answer the way it deserves: as a prompt to protect insulin sensitivity — muscle, sleep, protein, fibre — rather than a number to chase.
