Why 'normal' A1c is hiding your insulin resistance

By the time your A1c crosses 5.7, insulin resistance has typically been present for 5–10 years. A1c measures the consequences. Fasting insulin measures the cause.

Most standard metabolic panels skip fasting insulin entirely. It's rarely ordered in primary care, rarely included in corporate wellness screens, and almost never reimbursed outside of a diabetes workup. And yet it's the single most actionable number for anyone who wants to catch metabolic drift early enough to reverse it.

What fasting insulin actually tells you

Insulin is the hormone that tells your cells to absorb glucose. When cells start ignoring the signal - insulin resistance - your pancreas compensates by making more insulin. Glucose stays normal. A1c stays normal. But your fasting insulin climbs, years before anything else moves.

• Fasting insulin < 5 µIU/mL: optimal sensitivity.
• 5–8: acceptable, monitor.
• 8–12: early resistance - intervene.
• > 12: clear resistance, usually preceding A1c drift.

The HOMA-IR shortcut
Multiply fasting insulin × fasting glucose, divide by 405. Under 1.5 is optimal; over 2.0 signals resistance.

Why women miss this

Female insulin resistance often shows up first as PCOS, weight redistributing to the midsection, or perimenopausal weight gain. Estrogen is insulin-sensitising, so when it drops in perimenopause, insulin resistance often surfaces for the first time - even at a stable weight.

If your cycles are irregular, you've had gestational diabetes, or you have a family history of type-2 diabetes, fasting insulin belongs on your annual labs. So does a 2-hour oral glucose tolerance test with paired insulin if the fasting number is borderline.