What is insulin resistance, and do you have it?

The science, in full

Insulin is an anabolic hormone released by the beta cells of your pancreas, mostly in response to a rise in blood glucose after a meal. Its job is to move fuel out of the bloodstream and into storage. It signals your skeletal muscle and fat cells to pull glucose from the blood, it tells your liver to store glucose as glycogen and to stop producing its own, and it restrains the release of fatty acids from your fat tissue. When this system works well, your blood glucose settles back to baseline within a couple of hours and stays remarkably stable across the day.

Insulin resistance is a state in which target tissues respond less to a given amount of insulin. At the level of the cell, insulin binds its receptor and normally triggers a signalling cascade that moves glucose transporters, chiefly GLUT4, to the cell surface so glucose can enter. In resistance this signalling is blunted, so each unit of insulin produces less glucose uptake. The body compensates by secreting more insulin, and this compensatory hyperinsulinaemia is the key reason blood glucose can remain normal for years while the underlying defect grows.

The resistance is not uniform across the body. Skeletal muscle disposes of the majority of glucose after a meal and is widely regarded as the primary site of the earliest defect, so muscle that takes up glucose poorly is central to the picture. In the liver, insulin fails to suppress glucose production properly, which nudges fasting glucose upward. In fat tissue, insulin no longer restrains the breakdown of stored fat, so more free fatty acids spill into the circulation, and these in turn worsen resistance in muscle and liver.

A leading explanation for why cells stop listening is ectopic fat, meaning fat stored where it does not belong. When the capacity of your subcutaneous fat is exceeded, lipid accumulates inside muscle cells and inside the liver. Intracellular lipid intermediates such as diacylglycerol interfere directly with the insulin signalling pathway, dampening the receptor's downstream message. Physical inactivity, excess energy intake, visceral fat that releases inflammatory signals, and inherited genetic susceptibility all feed into this process, which is why it clusters in families and with a sedentary modern life.

Measuring insulin resistance directly is not simple. The research gold standard is the hyperinsulinaemic-euglycaemic clamp, which is precise but too laborious for everyday care. In practice, clinicians and studies often use surrogate markers derived from fasting blood, most commonly HOMA-IR, calculated from fasting glucose and fasting insulin. A raised fasting insulin, a high triglyceride to HDL ratio, and an enlarged waist all point in the same direction. These proxies are useful signals rather than a single definitive test, and interpreting them belongs with your own doctor.

Left unaddressed, insulin resistance tends to follow a slow trajectory. For years the pancreas keeps pace by making more insulin, but beta cell function is finite. When secretion can no longer overcome the resistance, glucose begins to rise, first into the prediabetes range and later into type 2 diabetes. Long cohort studies show this drift can unfold over a decade or more, with fasting glucose and insulin often changing subtly well before any threshold is crossed. This is why the condition is described as a silent early stage.

The most encouraging evidence is that this trajectory can be bent. In the Diabetes Prevention Program and the Finnish Diabetes Prevention Study, both large randomised trials in people with impaired glucose tolerance, a structured lifestyle programme of modest weight loss, dietary change and regular activity reduced progression to type 2 diabetes by roughly 58 per cent, outperforming medication in the American trial. These are among the strongest results in preventive medicine and show that insulin sensitivity is genuinely modifiable.

Weight loss can go further than prevention. In the DiRECT trial, a primary care programme built around substantial weight loss put a large share of people with recent type 2 diabetes into remission, with normal blood sugar off medication in close to half at one year and durable results in many at two years. The proposed mechanism, supported by imaging work on the twin cycle and personal fat threshold, is that losing fat from inside the liver and pancreas restores their insulin response. Exercise adds an independent lever, since muscle contraction moves glucose transporters to the surface even without insulin, and both a single session and regular training improve insulin sensitivity for hours to days.

It is important to hold this hope alongside honest limits. The amount of internal fat a given person can carry before their glucose rises, sometimes called the personal fat threshold, varies widely, and genetic makeup shapes both susceptibility and response. Two people with similar habits can therefore end up in very different places, and not everyone reaches full remission. None of this reflects a failure of willpower. If you want to understand your own numbers, confirm a diagnosis, or weigh up medication, that assessment belongs with your own doctor, who can see your full picture.

Putting this into practice. You cannot feel insulin resistance directly, so watch the clues instead: a growing waistline, an afternoon dip and craving for sweets after meals, and trouble losing weight. The habits that make your cells sensitive again are refreshingly ordinary. Go easier on fast sugars and refined carbohydrates, move your body most days, and build some strength in your muscles, which soak up glucose after meals. Protect your sleep too, since short nights nudge things the wrong way. Measure your waist now and then as a simple check. If several signals ring true, do not try to diagnose yourself, but ask your own doctor for a blood test to know where you really stand.