Fasting insulin and HOMA-IR: catching insulin resistance early

You train seriously, you eat with intent, and your fasting glucose comes back at a clean number every single time. Yet you feel a dip after lunch, or notice your midsection holding fat a little faster than it used to. Here is the blind spot: glucose is a late signal. The value that shifts years earlier rarely appears on a standard blood test. That value is your fasting insulin.

Why fasting insulin rises years before glucose

Insulin resistance develops gradually. Your cells respond less and less to insulin, and your pancreas compensates by simply producing more of it. Thanks to that extra insulin, your blood sugar stays right where it should be. This is called compensatory hyperinsulinaemia: your glucose looks normal, but only because your system is working hard in the background to keep it there.

The result is a gap in the timeline. Your fasting insulin can climb for years before your fasting glucose or your HbA1c move a millimetre. The Dutch Diabetes Fonds describes insulin resistance explicitly as a process that runs unnoticed for a long time and often only becomes visible once blood sugar starts rising. If you steer by glucose alone, you miss exactly the window where course-correcting is easiest.

My take: for anyone working on performance and longevity, fasting insulin is one of the most underrated values you can measure. Not because a single number decides your future, but because it gives you feedback years earlier than the markers your doctor checks by default.

HOMA-IR: your insulin sensitivity in one number

HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) combines your fasting glucose and fasting insulin into a single index. The formula is simple:

• HOMA-IR = (fasting glucose in mmol/l × fasting insulin in mIU/l) / 22.5

A worked example. Say your fasting glucose is 5.2 mmol/l and your fasting insulin is 9 mIU/l. You calculate: 5.2 × 9 = 46.8, divided by 22.5 = a HOMA-IR of roughly 2.1. Your glucose looks perfectly fine, but your HOMA-IR already sits in the zone that asks for attention. That is exactly the kind of signal you would never have seen from glucose alone.

Interpreting HOMA-IR by band

Treat these cut-offs as guidance, not a diagnosis. Reference values differ by laboratory and by population, and HOMA-IR is a model, not an exact measurement. The trend across several measurements says more than one isolated number.

Why your GP rarely tests insulin

Ask your GP for a fasting insulin and there is a good chance you walk away empty-handed. That is not stubbornness. Guidelines such as those from the Dutch NHG, and the patient information on Thuisarts, focus on detecting and treating diabetes, and for that purpose glucose and HbA1c are the validated markers. Fasting insulin and HOMA-IR are not routinely used in regular care to diagnose diabetes. That makes sense for that goal, but it leaves a gap for anyone who wants to know where they stand much earlier.

This is the difference between ruling out disease and optimising your health. If you want to see early insulin resistance coming instead of waiting for your glucose to rise, having your fasting insulin measured yourself is a logical step.

Caveats: how to avoid a false picture

• One measurement varies. Insulin fluctuates from day to day. Do not pin any conclusion on a single outlier; look at the pattern across several measurements.
• Being truly fasted matters. At least 8 to 12 hours without food, no coffee with milk, and ideally a calm evening beforehand. A big night out will still be influencing your insulin the next morning.
• Assays differ. Different labs measure insulin with different methods, so absolute values can vary. Compare measurements from the same lab where you can.

How to improve your insulin sensitivity

The good news is that insulin sensitivity responds well to what you do. The following levers can help lower your HOMA-IR, though the effect varies from person to person:

• Strength training. Muscle tissue soaks up glucose, and building muscle expands your storage capacity. The WHO advises adults to do muscle-strengthening activity at least twice a week, which fits neatly with better insulin sensitivity.
• Losing fat around the waist. Visceral fat in particular is linked to insulin resistance. Even a modest reduction can nudge your values in the right direction.
• Sleep. A few nights of poor sleep can measurably blunt your insulin sensitivity. Consistent, sufficient sleep is not a luxury but a lever.
• Fibre. A fibre-rich diet can flatten your post-meal glucose and insulin response.

None of these points is a guarantee or a replacement for medical advice. Treat them as direction, and test the effect by measuring again after a few months.

Pair it with HbA1c and your lipids

One marker rarely tells the whole story. Fasting insulin gets stronger in combination. Put your HOMA-IR next to your HbA1c for the long-term picture of your blood sugar, and look at your triglycerides and HDL. A high triglyceride-to-HDL ratio often points in the same direction as a raised HOMA-IR and is a handy, cheap second check on your metabolic health.

If you want to dig deeper into which values actually reveal your metabolic health, read our guide on measuring metabolic health. You can also read on about HbA1c in non-diabetics, our piece on fasted training, and what changes on a keto diet. If you want to fold insulin into a broader picture right away, take a look at our lipid panel.

In short: fasting glucose tells you how things are right now, fasting insulin and HOMA-IR show you which way things are moving. For anyone who wants to think ahead about their metabolic health, that is the difference between reacting and steering.

Further reading

• Which blood values reveal insulin resistance