Fasting Glucosemmol/L
Overnight glucose regulation
Also known as: Glucose (fasting)
Typical optimal range
3.9–5.5 mmol/L
A typical optimal fasting glucose sits between 3.9 and 5.5 mmol/L, though exact reference intervals vary between laboratories, so always interpret your result alongside the range provided on your report. These figures are indicative — the range printed on your own lab report is the definitive one for your results.
Diagnosis and management of type 2 diabetes and impaired glucose regulation in the UK follow NICE guideline NG28 (Type 2 diabetes in adults: management) and NICE guideline PH38 (preventing type 2 diabetes).
What is Fasting Glucose?
Fasting glucose is a measure of the sugar (glucose) circulating in your blood after at least eight hours without eating or drinking anything other than water. Glucose is the body's primary fuel, and its levels are tightly regulated by hormones — principally insulin, released by the pancreas. Measuring it in a fasted state removes the variable of a recent meal, giving a clearer picture of how well your body manages blood sugar overnight and between meals.
What a high Fasting Glucose means
A fasting glucose above the normal range suggests your body may be having difficulty keeping blood sugar in check overnight — it is a signal worth investigating further, not a diagnosis in itself. Levels in the impaired fasting glucose range (broadly 5.6–6.9 mmol/L) indicate a degree of insulin resistance, while a confirmed result of 7.0 mmol/L or above on a repeat test meets the diagnostic threshold for type 2 diabetes.
- Insulin resistance. When cells become less responsive to insulin — often driven by excess visceral fat, physical inactivity, or a diet high in refined carbohydrates — the pancreas compensates by producing more insulin, but blood glucose still rises higher than it should. This is the central mechanism behind pre-diabetes and type 2 diabetes.
- Poor sleep and chronic stress. Short or disrupted sleep raises cortisol and other counter-regulatory hormones that push glucose upward, while chronic psychological stress has a similar effect. Both are modifiable and often overlooked contributors to elevated fasting glucose.
- Diet and activity patterns. A diet high in ultra-processed foods and refined sugars, combined with low levels of physical activity, reduces the muscles' ability to take up glucose efficiently. Late-evening large carbohydrate meals in particular can leave glucose elevated into the following morning.
A single raised result does not confirm a diagnosis — illness, recent infection, certain medications (including steroids), and even a not-quite-complete fast can all push fasting glucose up temporarily. Your GP will typically want to confirm an abnormal result with a repeat test.
What a low Fasting Glucose means
A fasting glucose below approximately 3.9 mmol/L is less common and, if genuinely low rather than a sampling artefact, warrants attention — particularly if accompanied by symptoms such as dizziness, sweating, or feeling shaky.
- Over-medication in diabetes. The most common clinical cause of low fasting glucose is glucose-lowering medication (such as insulin or sulphonylureas) working more powerfully than intended, particularly if meals have been skipped or reduced. This should always be discussed promptly with the prescribing clinician.
- Adrenal insufficiency or other hormonal conditions. Conditions that reduce cortisol or other counter-regulatory hormones — such as Addison's disease — can impair the body's ability to maintain glucose between meals. This is uncommon but worth ruling out if low glucose is a persistent finding.
Isolated, mildly low readings in otherwise well individuals with no symptoms may reflect a prolonged fast or a sampling issue rather than a true clinical problem; your GP can help interpret the result in context.
Should I be worried?
A single fasting glucose result outside the optimal range is a useful prompt to have a conversation with your GP, but it is rarely cause for immediate alarm on its own. UK guidelines require at least two abnormal fasting glucose readings on separate occasions before a diagnosis of impaired fasting glucose or diabetes is made, unless you have clear symptoms. If your result is raised, your GP may wish to check HbA1c (a marker of average glucose over roughly three months), a fasting lipid profile, kidney function, and blood pressure — together these give a much fuller picture of metabolic risk. If your result is low and you are not on glucose-lowering medication, mention any symptoms you have noticed, as this will help your GP decide whether further investigation is needed.
What actually helps
Several well-evidenced lifestyle measures can support healthier fasting glucose levels over time.
- Zone-2 aerobic exercise. Regular moderate-intensity aerobic activity — such as brisk walking, cycling, or swimming at a pace where you can still hold a conversation — improves muscle glucose uptake and insulin sensitivity. NHS guidelines recommend at least 150 minutes of moderate activity per week.
- Reduce late-evening carbohydrates. Glucose metabolism follows a circadian rhythm and is less efficient in the evening. Shifting the bulk of your carbohydrate intake to earlier in the day and keeping evening meals lower in rapidly digested carbs can help fasting glucose the following morning.
- Prioritise sleep quality. Consistently poor or short sleep (under seven hours) is independently associated with raised fasting glucose and insulin resistance. Establishing a regular sleep schedule and reducing screen exposure before bed are practical starting points.
- Gradual, sustained weight management. Even modest weight loss of five to ten per cent of body weight in people with overweight or obesity is associated with meaningful improvements in fasting glucose and insulin sensitivity, as supported by NHS and NICE guidance on diabetes prevention.
Certain foods have good supporting evidence for helping to moderate fasting glucose and improve insulin sensitivity.
Non-starchy vegetables
Leafy greens, broccoli, courgette, and similar vegetables are low in rapidly absorbed carbohydrates and high in fibre, helping to slow glucose absorption and moderate post-meal and fasting glucose levels.
Wholegrains (e.g. oats, barley, wholegrain bread)
Wholegrains have a lower glycaemic index than refined grains and contain beta-glucan and other soluble fibres that blunt glucose and insulin responses — oats and barley in particular have strong trial evidence.
Legumes (lentils, chickpeas, beans)
High in both protein and soluble fibre, legumes produce a slow, sustained glucose release and have been shown in multiple studies to improve longer-term glycaemic markers.
Fatty fish (salmon, mackerel, sardines)
Rich in omega-3 fatty acids and high-quality protein, fatty fish supports insulin sensitivity and helps replace higher-glycaemic foods as a protein source; the NHS recommends two portions per week.
Nuts and seeds
Mixed nuts and seeds provide healthy unsaturated fats, protein, and fibre with minimal impact on blood glucose, and are associated with improved insulin sensitivity in prospective studies.
Extra-virgin olive oil
The primary fat source in a Mediterranean dietary pattern, extra-virgin olive oil is associated with reduced insulin resistance in observational and intervention studies, likely via its polyphenol and monounsaturated fat content.
Track your own Fasting Glucose
Upload a lab PDF and Better Days trends Fasting Glucose 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.