Fasting Insulin: The Metabolic Marker Most Lab Panels Skip
Fasting glucose and A1c are lagging indicators. Fasting insulin often shifts years earlier — which makes it one of the most useful, and most overlooked, windows into your metabolic health.
When people get their blood sugar checked, they usually get a fasting glucose and maybe an A1c. Both are useful, but both are late to the story. By the time either one looks abnormal, the underlying problem has often been building for years. The marker that moves earlier — and rarely shows up on a standard panel — is fasting insulin.
What fasting insulin actually measures
Insulin is the hormone that moves glucose out of the blood and into cells. When cells start resisting it, the pancreas compensates by pumping out more insulin to keep blood sugar in the normal range. So for a long stretch, glucose looks fine — not because everything is fine, but because the pancreas is working overtime to make it look that way. A fasting insulin level captures that hidden effort. A high value points to insulin resistance even when glucose and A1c are still sitting in the "normal" column.
This was the core insight of Gerald Reaven's 1988 Banting Lecture, which argued that insulin resistance and the compensatory high insulin that follows sit upstream of a whole cluster of metabolic problems — what later became known as metabolic syndrome [1].
Why it shows up early
The clearest demonstration comes from the Whitehall II study, which tracked thousands of people for years before some of them developed type 2 diabetes. Working backward from diagnosis, researchers found that insulin sensitivity and insulin secretion were already on abnormal trajectories well before diagnosis, while fasting glucose stayed relatively flat until it rose steeply only in the last couple of years [2]. Earlier work following high-risk adults reached the same conclusion: rising insulin resistance and the hyperinsulinemia that compensates for it precede and predict the disease [3]. In other words, glucose is a lagging indicator; insulin is a leading one.
Reading it alongside glucose
Fasting insulin is even more informative when paired with fasting glucose. Combine the two and you can calculate HOMA-IR — the homeostasis model assessment of insulin resistance — a simple, validated estimate of how insulin-resistant someone is, derived from nothing more than those two fasting values [4]. It's an easy, inexpensive way to put a number on something a glucose reading alone will miss.
Why it's so often skipped
Mostly habit and cost-consciousness. Standard metabolic panels were built around glucose, and insurance-driven testing tends to follow the same template. Fasting insulin isn't expensive or exotic — it's just not part of the default order, so most people never see it. That's a missed opportunity, because catching insulin resistance early is exactly when lifestyle and, where appropriate, medical interventions have the most room to work.
We include fasting insulin and HOMA-IR in our metabolic profile for this reason: we'd rather see the problem while it's still a trend than wait for it to become a diagnosis.
See how our metabolic program works
References
- Reaven GM. Banting lecture 1988. Role of insulin resistance in human disease. Diabetes. 1988. PMID: 3056758
- Tabák AG, et al. Trajectories of glycaemia, insulin sensitivity, and insulin secretion before diagnosis of type 2 diabetes: an analysis from the Whitehall II study. Lancet. 2009. PMID: 19515410
- Weyer C, et al. The natural history of insulin secretory dysfunction and insulin resistance in the pathogenesis of type 2 diabetes mellitus. J Clin Invest. 1999. PMID: 10491414
- Matthews DR, et al. Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia. 1985. PMID: 3899825