Insulin

What is insulin?

Insulin is the hormone your pancreas regulates your blood sugar with. As soon as glucose flows into your blood after a meal, the beta cells release insulin, and that hormone opens your muscle, liver and fat cells to that glucose. At the same time it stops your liver making sugar of its own and sets your fat tissue to storage. Without insulin, glucose keeps circulating in your blood and arrives nowhere. Something else is created along the way. Your beta cells first build one long protein and cut a piece out of it just before release; the remaining part is insulin, the piece cut out is called C-peptide. They are always released in equal amounts, which is why C-peptide has a page of its own. What a fasting insulin shows is different from what glucose shows. Glucose says how much sugar is in your blood. Insulin says how much effort your body puts in to keep it there. Two people with the same fasting glucose can need very different amounts of insulin for it, and that difference is exactly what insulin resistance is: cells that listen less well to the hormone, and a pancreas that works harder to reach the same result. Your report usually shows insulin in mU/l; some labs report in pmol/L, and the two cannot simply be converted into each other. Normal fasting is, in mU/l, up to about 17 mU/l, and the lower you sit within that range, the more sensitive your metabolism.

Why is insulin relevant?

Insulin is the earliest signal in the blood sugar series. Insulin resistance builds up over years, and in all those years your fasting glucose stays normal, because your pancreas makes up the difference. What you do see in that phase is a rising fasting insulin. The number that moves first is therefore not the sugar, but the hormone that keeps the sugar in check. Knowing only your glucose, you see it once the pancreas gives up; knowing your insulin, you see it years earlier. That is the reason to measure it, and also the reason to act on it. A high insulin is not only a forewarning. The hormone itself sets your fat tissue to storage and holds back fat burning, which makes losing weight harder with a high insulin than with a low one. It goes together with a higher blood pressure, higher triglycerides and a lower HDL, the pattern known together as the metabolic syndrome. Anyone who brings insulin down usually sees those other values move with it. What insulin is not: the test diabetes is diagnosed with. That is what fasting glucose and HbA1c are for, and rightly so, because insulin says something about the effort and not about the damage. For anyone who wants to stay healthy it is interesting the other way round: the number that shows where your metabolism stands, long before anything needs diagnosing. Together with fasting glucose, insulin forms HOMA-IR, a calculation that estimates insulin resistance in one number: fasting glucose in mmol/L times fasting insulin in mU/l, divided by 22.5. You rarely get that number back ready-made, but with both values from the same draw you work it out yourself. Around 1 belongs to a sensitive metabolism; well above 2 points to insulin resistance.

Insulin reference ranges

There is no national threshold for insulin; the range comes from the labs themselves and differs per lab and per unit. Dutch labs report in mU/l, sometimes written as mE/l, which is the same unit, or in pmol/L. The two cannot simply be converted into each other, because the conversion factor differs per assay; that is why the lab reporting in pmol/L appears in the table above in its own unit. Compare your value with your own lab's range, and read it beside the fasting glucose from the same draw.

Insulin high or low: what it means

A high fasting insulin with a normal glucose is the classic picture of insulin resistance, and the causes are the familiar ones: belly fat above all, little exercise, little muscle mass, plenty of sugar and fast carbohydrates, lack of sleep and lasting stress. It is one of the most sensitive values to what you did in the days before. Then there is the draw itself: anyone not fasting measures the insulin peak after the last meal, and that easily sits several times above the fasting value. Medication that stimulates release, such as certain diabetes medicines, lifts the number too. A low fasting insulin with a normal glucose is favourable: your cells listen well and your pancreas has little to do. That is the ordinary outcome in anyone who is lean and fit and exercises a lot. Low only becomes a different story beside a high glucose. Then the pancreas no longer makes enough, as in long-standing type 2 diabetes where the beta cells become exhausted, or in type 1 and LADA where the immune system breaks them down. The combination is everything: low with a normal sugar is fit, low with a high sugar is a pancreas that can no longer keep up. Insulin responds quickly to lifestyle, faster than HbA1c and usually faster than glucose too. Losing weight around the middle is the biggest lever, and it does not take much to improve sensitivity. Strength training and daily movement, because working muscles take up glucose without insulin and trained muscles stay more sensitive. Less sugar, soft drink, juice and white carbohydrates, so your pancreas has fewer peaks to absorb, and more fibre, protein and healthy fats that slow the uptake. Seven to eight hours of sleep, because one short night already makes you measurably more resistant the next morning. In most people that is the whole solution; medication that improves sensitivity is there for anyone who has diabetes or is close to it. In practice insulin is a fragile test. The tube has to be processed or frozen within an hour, which is why you cannot have it drawn at every location and why the result often takes longer. Always read the value beside the fasting glucose from the same draw, beside triglycerides and HDL, and as a trend. Anyone who cannot have insulin measured gets a long way in the same direction with the TyG index, a calculation from triglycerides and glucose.

How does an insulin blood test work?

Referral

Not needed. You can have insulin measured without going through your GP.

Fasting

Yes: at least 8 hours without food and preferably 12, water only. Anyone who has eaten measures the insulin peak of that meal and not the starting point.

Where you can go

Not everywhere. Insulin is a fragile hormone that has to be processed or frozen within an hour of the draw, so only locations that can do that take it, often a hospital outpatient clinic. Check before you schedule an appointment.

The draw

One tube of blood from a vein in your arm. The blood is spun quickly and the serum sent to the lab chilled or frozen.

The result

A single number, usually in mU/l and at some labs in pmol/L. Count on one to two weeks, because the test does not run daily at every location.

Repeating

Insulin responds within weeks to losing weight, moving and less sugar. Measure again after three months, fasting and the same way, together with the glucose.

Where to draw

You can draw at 250+ locations near you, no referral needed. See the locations

Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.