Insulin Resistance: The Engine Under the Bonnet

Insulin resistance is the most consequential thing about your metabolism that you will probably never be told you have. It doesn't appear on a routine blood test. It produces no symptom for years. And yet it sits underneath type 2 diabetes, much of high blood pressure, a good deal of polycystic ovary syndrome, and a meaningful slice of cardiovascular risk — the engine under the bonnet, turning over quietly while the dashboard stays dark.

It's worth lifting the bonnet, because once you understand what's happening, a lot of otherwise-confusing metabolic medicine falls into place.

What insulin resistance actually is

Insulin is a signal. After you eat, your pancreas releases it, and it travels to your muscles, liver and fat tissue carrying a simple instruction: take glucose out of the blood. In a metabolically healthy person, the tissues respond promptly to a small amount of insulin, glucose moves out of the circulation, and the system settles.

In insulin resistance, the tissues have gone partially deaf to the signal. The same instruction now needs to be shouted to get the same response. The pancreas obliges by producing more insulin — sometimes a great deal more — and for a long time this compensation works. Blood glucose stays normal. The cost is hidden: chronically high insulin levels, doing a normal job inefficiently.

A useful analogy is a thermostat in a draughty house. The temperature on the wall reads fine — but only because the boiler is running flat out to keep it there. Glance at the thermometer and all is well. Look at the gas bill and the strain becomes obvious. Insulin resistance is the gas bill of metabolism, and standard tests mostly read the thermometer.

Why it builds for years before anything shows

This is the part that catches people out. Because the pancreas compensates so effectively, insulin resistance can advance for a decade or more while every routine result stays normal. Fasting glucose: normal. HbA1c: normal. The person feels well. Nothing flags.

What's often accumulating underneath is fat in the wrong places. Not the visible fat under the skin, but visceral fat packed around the abdominal organs, and — more importantly — ectopic fat lodged inside the liver and muscle where it doesn't belong. Fat in these locations appears to interfere directly with insulin signalling. This is why waist size often tells you more than weight, and why some people of apparently normal weight are insulin-resistant while some larger people are not: it's less about how much fat you carry than where your body stores it.

The reason it eventually "shows" is that the pancreas can't shout forever. When insulin production starts to flag against rising resistance, glucose begins to climb — first into the prediabetes range, then, for some, into diabetes. By then the underlying process has usually been running for years. The diagnosis is late news of an old story.

Why it's nearly invisible on standard tests

Routine blood panels measure glucose, not insulin. So in the long compensated phase — where insulin is high but glucose is still controlled — the standard tests are reassuring precisely when the underlying problem is building.

There are research measures of insulin resistance (fasting insulin, calculated indices, formal clamp studies), but these aren't part of routine UK practice, and interpreting them outside a research setting is genuinely tricky. In day-to-day medicine, clinicians infer insulin resistance from its company rather than measuring it directly: a larger waist, raised blood pressure, raised triglycerides with low HDL cholesterol, fatty liver on a scan, a prediabetic HbA1c, PCOS. None of these is insulin resistance itself; together they paint its portrait. The honest position is that it's usually deduced, not detected — which is one more reason not to chase a single number.

What genuinely improves it

Here is the encouraging half, because insulin resistance is among the more modifiable things in metabolic medicine — and the levers are unglamorous, free, and well-supported.

Physical activity is the standout. Muscle that contracts pulls glucose out of the blood through a pathway that partly bypasses the faulty insulin signal — which is why even a single session of exercise improves insulin sensitivity for a day or two, and why regular activity, both aerobic and resistance training, improves it durably. More muscle also means more glucose-disposal capacity at rest. Movement is, in effect, a direct treatment.

Losing excess fat — especially from the liver — can substantially reverse insulin resistance, which is the mechanism thought to underlie diabetes remission with significant weight loss. Even modest losses help, because the liver and pancreas seem to shed their fat relatively early in the process.

Sleep and stress matter more than people expect. Short or poor sleep measurably worsens insulin sensitivity within days, and chronic stress nudges it the wrong way through hormonal routes. Fixing sleep isn't a wellness flourish here; it's metabolically active.

Diet quality plays a role — more fibre, less ultra-processed food and fewer sugary drinks — largely, though not entirely, through its effect on weight and liver fat.

What does not improve insulin resistance is the growing aisle of products marketed to "balance blood sugar" or "fix insulin resistance" — berberine sachets, cinnamon capsules, "metabolism" supplements. The evidence for these ranges from thin to absent, and none rivals the boring levers above. There is no insulin-resistance supplement. There is muscle, weight, sleep and food.

Practical takeaways

• Insulin resistance means tissues respond poorly to insulin, so the pancreas overproduces it — glucose stays normal for years while the strain stays hidden.
• It's driven heavily by fat stored in the liver and around the organs, which is why waist size can matter more than weight, and why slim people aren't automatically exempt.
• Standard tests measure glucose, not insulin, so the condition is usually inferred from its company — waist, blood pressure, triglycerides, fatty liver — rather than measured directly.
• The genuine levers are activity (especially building and using muscle), losing excess liver fat, decent sleep and better diet quality — not "blood sugar" supplements.

What this doesn't mean

It doesn't mean insulin resistance is a formal diagnosis you can demand a single test for, nor that having it guarantees diabetes — many people improve it well before glucose ever rises. It also doesn't mean weight is the only factor; genetics and ethnicity strongly influence who develops it and at what body size.

When to seek medical advice

If you have risk factors — a larger waist, raised blood pressure, prediabetes, PCOS, fatty liver, or a strong family history of type 2 diabetes — it's worth discussing your overall metabolic picture with your GP, who can arrange appropriate checks. Seek prompt advice for symptoms of diabetes such as marked thirst, frequent urination or unexplained weight loss. Decisions about any medication belong with your own clinician.

A closing thought

Most of metabolic medicine is, at bottom, a long argument with insulin resistance — and it's an argument you can engage years before any diagnosis, with tools that cost nothing and improve almost everything else at the same time. The engine under the bonnet is quiet, but it's not sealed. You can reach it.

Further reading and sources

• Diabetes UK — information on insulin resistance and type 2 diabetes risk
• NICE NG28 — Type 2 diabetes in adults: management
• American Diabetes Association — Standards of Care in Diabetes
• Peer-reviewed reviews on ectopic fat, exercise and insulin sensitivity
• NHS — information on metabolic risk and physical activity