InsulinmIU/L
Insulin-resistance sensitivity
Also known as: Fasting Insulin
Typical optimal range
2.6–24.9 mIU/L
Laboratories typically report a reference range of roughly 2.6–24.9 mIU/L, though many clinicians consider a fasting level well towards the lower end of that range more favourable; always interpret your result against your own lab's reference interval. These figures are indicative — the range printed on your own lab report is the definitive one for your results.
Interpretation of fasting insulin sits within broader metabolic and diabetes-risk assessment; NICE guideline NG28 covers type 2 diabetes prevention and risk identification, and NHS Health Check programmes use a similar cluster of metabolic markers for risk stratification.
What is Insulin?
Insulin is a hormone produced by the beta cells of the pancreas in response to rising blood glucose. It acts like a key, helping glucose enter cells to be used for energy, and it also plays a central role in fat storage and metabolism. Measuring fasting insulin — taken after an overnight fast — gives a snapshot of how hard the pancreas is working at rest, which can reveal early problems with insulin sensitivity long before blood glucose or HbA1c levels begin to shift.
What a high Insulin means
A raised fasting insulin suggests the pancreas is producing more insulin than would be expected to keep blood glucose stable — a pattern consistent with insulin resistance, though it is a signal to explore further rather than a diagnosis in itself.
- Insulin resistance. When muscle, liver, and fat cells respond less efficiently to insulin, the pancreas compensates by secreting more. This is strongly associated with excess visceral (abdominal) fat and is a core feature of metabolic syndrome and type 2 diabetes risk.
- High intake of refined carbohydrates and sugar. Diets rich in rapidly digested carbohydrates — white bread, sugary drinks, ultra-processed foods — produce repeated large glucose spikes, driving higher insulin output over time and gradually blunting cellular sensitivity.
- Physical inactivity. Skeletal muscle is the body's largest site of glucose disposal, and regular movement makes muscle cells considerably more responsive to insulin. A sedentary lifestyle is an independent contributor to raised fasting insulin levels.
Fasting insulin must genuinely be a fasting sample — even a small amount of food or a sugary drink before the test can substantially raise the result and make interpretation unreliable.
What a low Insulin means
A low fasting insulin is not usually a concern on its own and in isolation may simply reflect good insulin sensitivity; however, a low insulin paired with elevated fasting glucose warrants prompt GP review as it can indicate reduced insulin production.
- Type 1 diabetes or advanced beta-cell loss. In type 1 diabetes the immune system destroys insulin-producing beta cells, resulting in very low or undetectable insulin alongside high blood glucose. This combination requires urgent medical assessment.
Low insulin alone, without elevated glucose, is not generally considered clinically significant and does not require treatment.
Should I be worried?
A single fasting insulin result outside the expected range is useful information rather than cause for alarm — results can vary with test timing, recent diet, illness, and stress, so context matters enormously. If your level is persistently elevated, it is worth discussing with your GP, who may want to look at it alongside fasting glucose, HbA1c, a full lipid profile, blood pressure, and waist circumference to build a clearer picture of your metabolic health. Equally, if insulin is low and your glucose is raised, do mention both results together. Your GP is best placed to decide whether any follow-up or monitoring is needed.
What actually helps
Several well-evidenced lifestyle changes can meaningfully improve insulin sensitivity and help bring fasting insulin towards a healthier level.
- Reduce refined carbohydrates. Swapping rapidly digested carbohydrates — white bread, sugary drinks, pastries — for wholegrains, legumes, and vegetables reduces the frequency and size of glucose spikes, easing the demand on the pancreas over time.
- Resistance and aerobic exercise. Both resistance training and aerobic exercise increase glucose uptake into muscle cells and improve insulin sensitivity; current NHS physical activity guidelines recommend at least 150 minutes of moderate activity per week, with muscle-strengthening activities on two or more days.
- Time-restricted eating. There is growing evidence — though still emerging — that confining eating to a consistent window of around 8–10 hours per day may improve insulin sensitivity in some people, partly by extending the overnight fasting period; this is worth discussing with a GP if you have any underlying health conditions.
- Reduce visceral fat. Even modest weight loss of 5–10% of body weight, when someone is living with overweight, has been shown to significantly improve insulin sensitivity and lower fasting insulin; reducing visceral (abdominal) fat appears particularly beneficial.
There is good supporting evidence for the following foods and food patterns in improving insulin sensitivity.
Non-starchy vegetables
Low in rapidly digestible carbohydrate and high in fibre, they slow glucose absorption and support a lower post-meal insulin response.
Legumes (lentils, chickpeas, beans)
Their low glycaemic index and high fibre and protein content produce a modest, sustained glucose rise, associated with lower insulin demand compared with refined starchy foods.
Oily fish (salmon, mackerel, sardines)
Omega-3 fatty acids have been associated with modest improvements in insulin sensitivity and reductions in liver fat in several clinical studies.
Nuts and seeds
Regular nut consumption is associated with improved insulin sensitivity, likely due to their combination of unsaturated fats, fibre, magnesium, and low glycaemic load.
Wholegrains (oats, barley, wholegrain rye)
The soluble fibre in oats and barley (beta-glucan) slows gastric emptying and glucose absorption, resulting in a lower and more gradual insulin response.
Vinegar (e.g. apple cider vinegar with meals)
Small trials suggest acetic acid may modestly blunt post-meal glucose and insulin spikes when consumed with carbohydrate-containing meals, though evidence is preliminary and effect sizes are small.
Track your own Insulin
Upload a lab PDF and Better Days trends Insulin over time, flags it when it drifts, and suggests foods that help.
More in Metabolic
Sources & further reading
These are reputable UK health resources for further reading. This page is written to be consistent with their guidance but is not a substitute for them.
For information only — not medical advice, and not individually reviewed by a clinician. Reference ranges vary by lab and individual. Discuss any results with a qualified clinician before making changes.