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 you get your blood sugar checked, you usually get a fasting glucose and maybe an A1c. Both are useful, but both also show up late. By the time either one looks abnormal, the problem underneath has often been building for years. There's a marker that moves earlier and almost never makes it onto a standard panel, fasting insulin.
What fasting insulin measures
Insulin is the hormone that moves glucose out of your blood and into your cells. When cells start resisting it, your pancreas compensates by pumping out more insulin to keep blood sugar in the normal range. So for a long stretch, your glucose looks fine, not because everything's fine, but because the pancreas is working overtime to make it look that way. Fasting insulin catches that hidden effort. A high value flags insulin resistance even while your glucose and A1c still sit in the normal column.
Gerald Reaven made this case back in his 1988 Banting Lecture, arguing that insulin resistance and the high insulin that compensates for it sit upstream of the whole cluster we now call metabolic syndrome [1].
Why it moves early
The clearest proof comes from the Whitehall II study. Tabák and colleagues tracked thousands of people for years before some of them developed type 2 diabetes, then worked backward from diagnosis. Insulin sensitivity and insulin secretion drifted onto abnormal paths well ahead of time, while fasting glucose stayed nearly flat until it shot up only in the last two or three years [2]. Earlier work by Weyer and colleagues, following high-risk adults, found the same thing, that rising insulin resistance and the hyperinsulinemia that papers over it come first and predict the disease [3]. Glucose is a lagging indicator, while insulin is a leading one.
Reading it with glucose
Fasting insulin tells you even more when you pair it with fasting glucose. Put the two together and you can calculate HOMA-IR, a simple, validated estimate of insulin resistance that needs nothing but those two fasting numbers, described by Matthews and colleagues in 1985 [4]. It's a cheap, easy way to put a number on something a glucose reading alone misses entirely.
Why it gets skipped
The reasons come down to habit and cost. Standard panels were built around glucose, and insurance-driven testing follows the template. Fasting insulin isn't expensive or exotic. It isn't part of the default order, so most people never see it. That's a real missed opportunity, because insulin resistance is most fixable early, when diet, exercise, and, where it fits, medication have the most room to work.
We put fasting insulin and HOMA-IR in our metabolic profile for exactly this reason. We'd rather catch 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