Insulin and Insulin Resistance: What It Does, the Tests and the Indian Figures

Insulin is a hormone made by the beta cells of the pancreas. WHO describes it in one line: insulin is a hormone that regulates blood glucose. After a meal it signals muscle, liver and fat to take glucose out of the blood and use or store it. When those tissues respond weakly, the state is called insulin resistance, and the pancreas has to release more insulin to keep glucose normal. In India the consequences are common: ICMR-INDIAB found diabetes in 11.4 per cent and prediabetes in 15.3 per cent of adults. This entry is part of NNWA's handbook of hormones.

What does insulin do in the body?

It keeps blood glucose inside a narrow range and tells the body it has been fed. When glucose rises after a meal, the pancreas releases insulin; muscle and fat cells then take glucose in, and the liver stops releasing its own and stores some as glycogen. Insulin also promotes the storage of fat and the building of protein, which is why it is called a storage hormone. Between meals and overnight, insulin falls and the liver supplies glucose again.

Diabetes, in WHO's definition, occurs either when the pancreas does not produce enough insulin or when the body cannot effectively use the insulin it produces. Type 1 diabetes is the first kind: insulin production is deficient and daily insulin is required. Type 2 is mainly the second kind, and it is the one that insulin resistance leads towards.

What is insulin resistance?

It is a weaker response to a normal amount of insulin. Muscle takes up less glucose for a given signal, and the liver keeps releasing glucose when it should have stopped. For years the pancreas covers the gap by producing more, so blood glucose looks normal while insulin runs high. Only when the beta cells can no longer keep up does glucose rise, first into the prediabetes range and then into diabetes. That long silent phase is why a normal sugar report does not rule the problem out.

Resistance is linked with fat carried around the waist and inside the liver and muscle, with inactivity, short sleep and family history. It also runs through conditions such as polycystic ovary syndrome, covered in NNWA's guide to the PCOS diet in India, and it travels with raised blood pressure, for which the handbook has an entry on the DASH diet.

The Indian picture: ICMR-INDIAB and NFHS-5

ICMR-INDIAB is the largest measurement of metabolic disease in India. Between 2008 and 2020 it examined 113,043 adults aged 20 and over in urban and rural areas of 31 states, union territories and the National Capital Territory, using WHO criteria for diabetes and prediabetes. Every condition in the table except prediabetes was more frequent in towns than in villages, and the authors concluded that prevalence is considerably higher than earlier estimates.

Indians tend to meet this problem earlier and at a lower body weight than Europeans. A consensus statement published in the Journal of the Association of Physicians of India describes the pattern: excess body fat, abdominal adiposity and fat deposited in the liver and muscle, with limits of normal BMI that are narrower and lower than in white Caucasians. Waist is therefore a better home check than weight alone; the waist to hip ratio calculator and the diabetes risk calculator are places to start.

How does food change insulin?

Every meal that contains carbohydrate raises insulin; that is the hormone doing its job. What food changes is how high and how long. Refined grain and sugar are absorbed fast and call for a sharp release, while fibre, intact grains, pulses, vegetables and protein slow the rise. In a randomised crossover trial in Chennai, 15 overweight adults without diabetes ate the same meals for five days with white rice, brown rice, or brown rice with legumes. The glucose response was 19.8 per cent lower with brown rice, and the change in fasting insulin was 57 per cent lower.

That was a small, short trial, so read it as a direction and not a promise. It agrees with ICMR-NIN's food guide, My Plate for the Day, which builds a 2,000 kcal day from 250 g of cereals including millets, 85 g of pulses, 400 g of vegetables, 100 g of fruit, 35 g of nuts and seeds and 300 ml of milk or curd, and says the pattern prevents insulin resistance and maintains insulin sensitivity. It restricts sugar to 25 to 30 g a day.

The table shows why the usual advice is about swaps and proportions, not about removing rice or roti. Milled rice has a quarter of the fibre of atta or the millets, and pulses carry less carbohydrate with more fibre and protein. In practice: one katori of rice with a full katori of dal and a sabzi, not two katoris of rice with a little dal. The fibre table for Indian foods and the piece on the glycaemic index of Indian foods take this further.

How do sleep and activity change insulin sensitivity?

Both act quickly, within a day. In a clamp study, the reference method for measuring insulin action, nine healthy adults were tested after a normal night and after a night cut to four hours. After the short night the glucose infusion rate fell by about 25 per cent, with resistance seen in both the liver and the muscles. An earlier Lancet study restricted 11 young men to four hours in bed for six nights and found lower glucose tolerance and higher evening cortisol. ICMR-NIN advises six to eight hours of sleep a day.

Working muscle takes up glucose with less help from insulin, so activity has the opposite effect. A meta-analysis of seven crossover trials found that breaking up long sitting with light walking lowered glucose and insulin after meals compared with continued sitting, and that walking did better than simply standing. ICMR-NIN's guideline asks adults for a minimum of 30 to 45 minutes of brisk walking or moderate activity a day.

Which tests does a doctor order, and what do they measure?

Doctors diagnose prediabetes and diabetes on glucose tests, not on insulin: fasting plasma glucose, the oral glucose tolerance test and HbA1c, explained in NNWA's guide to prediabetes. Insulin itself is measured less often. The tests below may appear on a report when a doctor is looking into PCOS, unexplained weight gain, fatty liver or a strong family history. Their interpretation depends on the laboratory method and on the rest of the picture, so it belongs to the doctor who ordered them.

HOMA stands for homeostasis model assessment, described in 1985 by Matthews and colleagues. The commonly used formula multiplies fasting insulin in micro-units per millilitre by fasting glucose in millimoles per litre and divides by 22.5. The original paper found that the estimate tracked the clamp method closely across a group, but reported a coefficient of variation of 31 per cent, and said this low precision limits its use. A 2004 review by the same group judged it suited to cohort and population studies and asked that results be interpreted carefully.

A worked example: the HOMA-IR arithmetic

This is an illustrative case, not a real person, and the numbers are chosen to show the sum, not to suggest what is normal. A 34-year-old vegetarian IT analyst in Chennai, with a waist that has grown over five years of desk work and a mother with type 2 diabetes, is sent for fasting tests by her doctor. The report shows fasting glucose of 5.2 mmol/L and fasting insulin of 13 micro-units per millilitre.

What does the evidence say about changing course?

The strongest Indian evidence is the Indian Diabetes Prevention Programme. It randomised 531 adults with impaired glucose tolerance, who were younger and leaner than participants in the American, Finnish and Chinese prevention trials, to usual care, lifestyle advice, a medicine, or both. Over three years diabetes developed in 55.0 per cent of the control group and 39.3 per cent of the lifestyle group, a relative risk reduction of 28.5 per cent. About six people needed lifestyle advice to prevent one case.

Two cautions keep this honest. Even with advice, almost four in ten people in the lifestyle group went on to diabetes within three years, so changing habits lowers risk without removing it. And the sleep, walking and rice studies above are small experiments lasting a day to a week: good evidence that insulin action responds quickly, weaker evidence about what happens over years. Large trials, months long, are the evidence to trust most.

Common myths and mistakes

A second mistake is to chase the insulin number. Fasting insulin and HOMA-IR vary from day to day and from one laboratory to another, and no diet should be judged on a single reading. A third is to read insulin as the enemy and cut carbohydrate to almost nothing: insulin rising after a meal is normal physiology, and ICMR-NIN's plate still draws about 42 per cent of its energy from cereals and millets.

Insulin as a medicine: safety and when to see a doctor

Insulin is also a prescribed medicine. People with type 1 diabetes need it every day to live, and many people with type 2 diabetes and some women with diabetes in pregnancy are prescribed it. Nothing on this page is advice about doses, timing or type. If you use insulin or any glucose-lowering medicine, do not change the dose, skip it or stop it because of a diet, a fast or a home reading; speak to the prescriber first, including before Ramzan, Navratri or any long fast.

A change of diet can lower blood glucose within days, and in someone on insulin or certain tablets that can cause low blood sugar. This is the reason to tell your doctor before you cut carbohydrate, start an exercise plan or try to lose weight, so that treatment can be adjusted safely by the person responsible for it. Food and activity work alongside treatment and are not a replacement for it.

Is a high fasting insulin the same as diabetes?

No. Diabetes is defined by raised blood glucose, and a person can have high insulin for years with glucose still in the normal range. A high fasting insulin suggests the body is working harder to hold glucose steady, which is a reason to look at waist, sleep, activity and diet and to repeat glucose tests on the schedule a doctor sets. It is not a diagnosis on its own, and it does not mean diabetes is certain. NNWA's guide to a type 2 diabetes diet in India covers eating once a diagnosis has been made.

Learning this with NNWA

NNWA, a private skill academy in Kolkata, teaches online in English and in Hindi. Its courses, among them the Diabetes Nutrition course, teach people to coach healthy clients on food, activity and sleep and to refer medical cases to a doctor. None of them is a degree or a dietitian qualification.