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Insulin Resistance: What It Is, How It's Tested, and How to Improve It

Insulin resistance starts years before diabetes. Former NIH scientist Dr. Sayantan Banerjee explains fasting insulin, HOMA-IR, triglyceride-to-HDL ratio, and how to improve your results.

PREDIABETES REMISSION

Dr. Sayantan Banerjee

10/31/20268 min read

Insulin resistance is the engine behind most cases of prediabetes and type 2 diabetes. It usually begins years, sometimes more than a decade, before blood sugar rises enough to be noticed on a routine test. It is also linked to fatty liver, high blood pressure, high triglycerides, PCOS and heart disease.

Because of this, many people want to know whether they have it, and how to find out. The answer is a little more complicated than it might seem. There is no single standard test for insulin resistance in everyday practice. But there are several useful clues and measures. This article explains what insulin resistance is, the tests you may hear about, how to interpret them, why doctors don't always order them, and what improves insulin resistance.

What insulin resistance means

Insulin is a hormone made by the beta cells of the pancreas. After you eat, it rises and does several jobs. It helps muscle and fat cells take up glucose from the blood. It tells the liver to stop releasing glucose and to store some as glycogen. And it tells fat tissue to store energy rather than release it.

In insulin resistance, these tissues respond less strongly to insulin. Muscles take up less glucose. The liver keeps releasing glucose even when it isn't needed. Fat tissue releases more fatty acids into the blood.

At first, the pancreas compensates by making more insulin. Blood glucose stays normal, but insulin levels are high. This stage can last for years. Eventually, if insulin resistance continues and the beta cells can't keep up, glucose begins to rise, first after meals, then in the fasting state. That is when prediabetes appears on blood tests.

What causes it

The main drivers are:

excess fat in the liver and around the organs, often linked to weight gain around the waist
physical inactivity, which reduces how much glucose muscles take up
low muscle mass
poor or short sleep
chronic stress
genetics and family history
ageing
certain medicines, such as steroids and some antipsychotics
hormonal conditions, such as PCOS and Cushing's syndrome

Ethnicity also matters. People of South Asian, East Asian, Black African, African-Caribbean and Hispanic heritage tend to develop insulin resistance at lower body weights.

Clues you can check yourself

You don't need a specialist test to suspect insulin resistance. Common clues include:

a waist more than half your height
dark, velvety skin on the neck, armpits or groin (acanthosis nigricans)
a family history of type 2 diabetes
high blood pressure
PCOS, or a history of gestational diabetes
fatty liver on a scan, or raised liver enzymes

If several of these apply, it is very likely that some degree of insulin resistance is present.

Tests your doctor might use

A1C and fasting glucose. These are the routine tests for prediabetes and diabetes. They don't measure insulin resistance directly, but a rising result usually means insulin resistance has been present for some time.

Lipid profile. High triglycerides and low HDL cholesterol are a common pattern in insulin resistance. Some researchers use the ratio of triglycerides to HDL as a rough marker. In US units, a ratio above about 3 (both in mg/dL) has been linked to insulin resistance in some populations. In UK units (mmol/L), the equivalent figure is lower, roughly above 1.3. These cut-offs vary between ethnic groups and aren't used for diagnosis, but they can be a helpful pointer.

Liver tests. Raised ALT can suggest fatty liver, which is closely linked to insulin resistance. Normal results don't rule it out.

Fasting insulin. A blood test for insulin after an overnight fast. Higher values suggest the pancreas is working harder to keep glucose normal. However, insulin tests aren't fully standardised between laboratories, there is no universally agreed normal range, and results vary from day to day. That is a major reason doctors don't order them routinely.

HOMA-IR. This is a calculation using fasting glucose and fasting insulin. The formula is fasting insulin (in µU/mL) multiplied by fasting glucose (in mg/dL), divided by 405. If glucose is in mmol/L, divide by 22.5 instead. Higher values suggest more insulin resistance. Many studies use cut-offs somewhere between about 2 and 3, but there isn't a single agreed threshold, and values differ by population and laboratory.

Triglyceride-glucose (TyG) index. A calculation using fasting triglycerides and glucose, used mainly in research. It doesn't need an insulin test.

Oral glucose tolerance test. You drink a measured amount of glucose, and glucose is checked over two hours, sometimes along with insulin. It is very good at detecting prediabetes, especially raised glucose after meals.

The hyperinsulinaemic-euglycaemic clamp. This is the research gold standard. It involves infusing insulin and glucose for several hours to measure how much glucose the body takes up. It is far too complex for routine care.

Why doctors don't routinely test insulin

Many people are frustrated when their doctor declines to test fasting insulin or HOMA-IR. There are reasonable reasons.

The results are variable and not standardised, so a single number can mislead. There is no agreed target, so it isn't clear what treatment decision a result would change. And for most people, the practical advice is the same whether insulin resistance is confirmed or just suspected: lose weight if needed, be more active, eat well and sleep well.

That said, in some situations, such as PCOS, unexplained acanthosis nigricans, or research settings, specialists may use insulin measurements. If you have your results done privately, interpret them with caution, and discuss them with your doctor.

Example: putting the pieces together

Imagine someone with an A1C of 5.9% (41 mmol/mol), a waist of 39 inches (99 cm) at a height of 5 ft 8 in (173 cm), triglycerides of 190 mg/dL (2.1 mmol/L) and HDL of 38 mg/dL (1.0 mmol/L). This is a hypothetical example. Their waist-to-height ratio is about 0.57, their triglyceride-to-HDL ratio is about 5 in US units, and their A1C is in the US prediabetes range. Even without an insulin test, this pattern strongly suggests insulin resistance.

How to improve insulin resistance

The good news is that insulin resistance responds well to change, often within weeks.

Lose weight if you need to. Weight loss reduces fat in the liver, which quickly improves the liver's response to insulin. Even 5% of body weight helps. In the US Diabetes Prevention Program, a 7% weight loss target was central to a 58% reduction in diabetes risk.

Move your muscles. A single session of exercise improves insulin sensitivity in muscle for up to a day or two. Regular aerobic exercise, such as brisk walking or cycling, and strength training both help. Walking for 10 to 15 minutes after meals is especially effective for glucose.

Build muscle. More muscle means more capacity to take up glucose. Aim for strength exercise at least twice a week.

Eat fewer refined carbohydrates and sugary drinks. These cause larger glucose and insulin spikes. Build meals around vegetables, protein, whole grains, beans and healthy fats.

Eat more fibre. Fibre slows glucose absorption and supports gut hormones involved in glucose control.

Sleep at least seven hours. Even a few nights of short sleep reduce insulin sensitivity in laboratory studies.

Manage stress. Chronic stress raises cortisol, which opposes insulin.

Limit alcohol. Heavy drinking adds to liver fat.

Stop smoking. Smoking worsens insulin resistance.

Review medicines. If you take a medicine that may worsen insulin resistance, ask your doctor whether there are alternatives. Don't stop any medicine on your own.

Medicines that improve insulin resistance

In some situations, medicines help. Metformin reduces glucose release from the liver and is sometimes used in prediabetes and PCOS. Pioglitazone improves insulin sensitivity but has side effects such as weight gain and fluid retention. GLP-1 medicines improve insulin resistance largely through weight loss. These are prescribed for specific conditions, and your doctor can advise whether any is appropriate.

How quickly can it improve?

Faster than many people expect. Studies of very low calorie diets have shown improvements in liver insulin sensitivity within days to weeks, as liver fat falls. Exercise improves muscle insulin sensitivity from the first session. Over three to six months of sustained change, many people see their fasting glucose, A1C, triglycerides and waist all improve.

Insulin resistance in PCOS and pregnancy

PCOS. Insulin resistance is present in many women with polycystic ovary syndrome, including some who aren't overweight. High insulin levels can increase androgen (male hormone) production by the ovaries, which contributes to irregular periods, excess hair growth and acne. Women with PCOS have a higher risk of prediabetes and type 2 diabetes, and NICE and international guidelines recommend regular glucose testing. Weight loss of 5% to 10%, where needed, often improves both insulin resistance and PCOS symptoms.

Pregnancy. Insulin resistance naturally increases during pregnancy, especially in the second half, to make sure the baby gets enough glucose. In some women, the pancreas can't keep up, and gestational diabetes develops. This usually resolves after birth, but it reveals an underlying tendency to insulin resistance, and the long-term risk of type 2 diabetes is much higher. Follow-up testing after pregnancy is important.

Can you be insulin resistant and slim?

Yes. Some people with a normal BMI have significant insulin resistance. This is more common in people who carry fat around the waist and organs, people with low muscle mass, people who are inactive, and people of South Asian and East Asian heritage. It is sometimes described as metabolically unhealthy normal weight.

For these people, the main levers are physical activity, especially strength training to build muscle, reducing refined carbohydrates and sugary drinks, improving sleep, and keeping waist size down.

Myths about insulin resistance

Myth: You need a special insulin resistance diet. There is no single diet that works best for everyone. Patterns that reduce refined carbohydrates, increase fibre and help with weight loss all improve insulin resistance.

Myth: Fruit causes insulin resistance. Whole fruit is linked with lower diabetes risk in large studies. Fruit juice and sugary drinks are the problem.

Myth: Supplements can fix it. No supplement has matched the evidence for weight loss, activity and sleep.

Myth: Eating every two to three hours keeps insulin balanced. There is no good evidence that frequent small meals improve insulin resistance. Some people do better with fewer meals. Choose the pattern that suits your hunger and routine.

How my book can help

The Prediabetes Exit Strategy explains insulin resistance in plain language, including why it starts years before diabetes, and sets out practical changes to eating, movement, sleep and daily routine to reverse it. More on the book page.

Frequently asked questions

What is the best test for insulin resistance? There isn't one perfect routine test. A1C, fasting glucose, lipids, waist size and sometimes fasting insulin or HOMA-IR are used together.

What is a normal HOMA-IR? There is no single agreed normal value. Many studies treat values above about 2 to 3 as suggesting insulin resistance.

Can you have insulin resistance with normal blood sugar? Yes. In the early stages, the pancreas makes extra insulin, so glucose stays normal.

Can insulin resistance be reversed? Yes, in many people. Weight loss, exercise, better sleep and diet changes all improve it.

Does insulin resistance cause weight gain? It is linked with weight gain, especially around the waist, and weight gain worsens insulin resistance.

Should I pay for a private insulin test? It isn't usually necessary. If you do, discuss the result with your doctor rather than interpreting it alone.

How long does it take to improve insulin resistance? Muscle insulin sensitivity improves from the first exercise session. Liver insulin sensitivity can improve within weeks of weight loss.

Is intermittent fasting good for insulin resistance? It can help if it leads to weight loss. In trials, results are similar to other calorie-reduction approaches.

Can insulin resistance cause tiredness? It may contribute to energy dips after meals, but tiredness has many causes. Check with your doctor if it persists.