By the time your A1c crosses 5.7, insulin resistance may have been present for 5–10 years (Sagesaka et al., 2018). A1c can measure the consequences. Fasting insulin can measure 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 can be a highly actionable number for anyone who wants to catch metabolic drift early enough to reverse it.
What fasting insulin can tell you
Insulin is the hormone that tells your cells to absorb glucose. When cells start ignoring the signal - insulin resistance - your pancreas can compensate by making more insulin (Mari et al., 2005). Glucose can stay normal (Dankner et al., 2012). A1c can stay normal. But your fasting insulin can climb, years before anything else moves.
- Fasting insulin < 5 µIU/mL: may mean optimal sensitivity.
- 5–8: may be acceptable, monitor.
- 8–12: may mean early resistance - intervene.
- > 12: may mean resistance, which can precede A1c drift.
The HOMA-IR shortcut
Multiply fasting insulin × fasting glucose, divide by 405 (Seo et al., 2020). Under 1.5 may be optimal; over 2.0 may point to resistance.
Why women miss this
Female insulin resistance can show up first as PCOS, weight redistributing to the midsection, or perimenopausal weight gain. Estrogen can be insulin-sensitising, so when it drops in perimenopause, insulin resistance can surface 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.
References
- Sagesaka, H., et al. (2018). Type 2 Diabetes: When Does It Start? Journal of the Endocrine Society, 2(5), 476-484. PMID 29732459
- Mari, A., et al. (2005). Assessment of insulin secretion in relation to insulin resistance. Current Opinion in Clinical Nutrition and Metabolic Care, 8(5), 529-533. PMID 16079624
- Dankner, R., et al. (2012). Basal state hyperinsulinemia in healthy normoglycemic adults heralds dysglycemia after more than two decades of follow up. Diabetes/Metabolism Research and Reviews, 28(7), 618-624. PMID 22865584
- Seo, Y. G., et al. (2020). Fat-to-muscle ratio as a predictor of insulin resistance and metabolic syndrome in Korean adults. Journal of Cachexia, Sarcopenia and Muscle, 11(3), 710-725. PMID 32030917
