Blood Sugar and Blood Glucose: UK Guide

Blood sugar and blood glucose refer to the same thing: the concentration of glucose circulating in your bloodstream at any given moment. Keeping this level within a healthy range is fundamental to metabolic health, and disruptions in either direction can have serious consequences. Whether you have been diagnosed with diabetes, are in the non-diabetic hyperglycaemia (prediabetes) range, or simply want to understand your results, this guide explains what blood glucose is, what normal levels look like in the UK, what causes fluctuations, how testing works, and when to seek medical advice.

What is blood glucose and why does it matter for your health?

Blood glucose is the concentration of glucose in the bloodstream, serving as the body's main energy source; persistently high levels cause diabetes complications including heart disease, kidney disease and sight loss, while very low levels can cause seizures or loss of consciousness.

Blood glucose is the concentration of sugar (glucose) circulating in your bloodstream at any given moment, and it is one of the most important markers of metabolic health. Glucose comes primarily from the carbohydrates you eat and drink. Once absorbed through the gut wall, it enters the bloodstream and is transported to cells throughout the body, where it serves as the main fuel for energy production.

The hormone insulin, produced by the beta cells of the pancreas, acts like a key that unlocks cells so glucose can enter them. When this system works correctly, blood glucose rises modestly after a meal and then returns to a stable baseline within a couple of hours. When it does not work correctly, glucose can accumulate in the blood to harmful levels.

Persistently high blood glucose (hyperglycaemia) is the hallmark of diabetes mellitus. Over time, elevated glucose damages blood vessels and nerves, increasing the risk of serious complications including heart disease, stroke, kidney disease, sight loss and foot problems (NHS; Diabetes UK). Persistently low blood glucose (hypoglycaemia) is also dangerous and can cause confusion, seizures or loss of consciousness if severe.

Understanding blood glucose is therefore relevant not only to people already diagnosed with diabetes but also to anyone at risk of developing it, including those with non-diabetic hyperglycaemia (NDH, also referred to as prediabetes), polycystic ovary syndrome, obesity or a strong family history of type 2 diabetes.

What are normal blood sugar levels in the UK?

Normal fasting blood glucose is 4.0 to 5.4 mmol/L; a fasting level of 7.0 mmol/L or above on two occasions, or an HbA1c of 48 mmol/mol or above, is diagnostic of diabetes in UK clinical practice.

In the UK, normal fasting blood glucose for an adult without diabetes is generally between 4.0 and 5.4 mmol/L, according to NHS and Diabetes UK guidance. These figures refer to a measurement taken after at least eight hours without food or caloric drink. For diagnosis, blood glucose must be measured using a laboratory plasma sample rather than a home glucometer or continuous glucose monitor, and in most cases two abnormal results are required unless classic symptoms of diabetes are present.

The table below summarises the key diagnostic thresholds used in UK clinical practice, aligned with NICE and WHO guidance:

• Normal fasting glucose: 4.0 to 5.4 mmol/L

• Impaired fasting glucose (non-diabetic hyperglycaemia / prediabetes range): 6.1 to 6.9 mmol/L

• Diabetes (fasting): 7.0 mmol/L or above on two separate occasions, or once if symptoms are present

• Two-hour post-glucose-load (OGTT): below 7.8 mmol/L is normal; 7.8 to 11.0 mmol/L indicates impaired glucose tolerance; 11.1 mmol/L or above indicates diabetes

• HbA1c (average glucose over approximately two to three months): below 42 mmol/mol is normal; 42 to 47 mmol/mol suggests non-diabetic hyperglycaemia (NDH / prediabetes); 48 mmol/mol or above is diagnostic of diabetes^[6][2]

HbA1c is not suitable for diagnosis in all situations. It should not be used during pregnancy, in children or young people where type 1 diabetes is suspected, or in people with haemoglobinopathies, anaemia, high red-cell turnover or certain forms of chronic kidney disease.^[5][2] In these circumstances, an OGTT or fasting plasma glucose is preferred (NICE NG28; NICE NG3).

For people already living with type 1 or type 2 diabetes, NICE recommends individual target ranges agreed with their clinical team. A commonly used target for self-monitored fasting glucose in people with diabetes is 5.0 to 7.0 mmol/L, though this varies depending on treatment, age and other factors (NICE NG17; NICE NG28).

Blood glucose fluctuates naturally throughout the day in response to meals, physical activity, stress and sleep, so a single reading should always be interpreted in context rather than in isolation.

What causes blood sugar levels to rise or fall?

Blood glucose rises after eating carbohydrates and is also increased by illness, corticosteroids, inactivity and insufficient insulin; it falls when glucose is used faster than replaced, or when insulin or sulphonylureas are taken in excess relative to food intake.

Blood glucose rises after eating carbohydrates, because the digestive system breaks down starches and sugars into glucose, which is then absorbed into the bloodstream. The speed and magnitude of this rise depend on the type and quantity of carbohydrate consumed, as well as the presence of fibre, fat and protein in the same meal, all of which slow absorption.

Several other factors can push blood glucose higher:

• Illness and infection: stress hormones such as cortisol and adrenaline raise glucose as part of the body's fight-or-flight response

• Certain medicines: corticosteroids, some antipsychotics and thiazide diuretics are well-recognised causes of raised blood glucose (NHS; BNF)^[9][8]

• Inactivity: muscles use glucose during exercise, so a sedentary lifestyle reduces glucose uptake

• Hormonal changes: puberty, pregnancy and the menopause can all affect insulin sensitivity

• Insufficient insulin: in type 1 diabetes, the pancreas produces little or no insulin; in type 2 diabetes, cells become resistant to its effects

Blood glucose falls when glucose is used up faster than it is replaced, or when insulin or glucose-lowering medicines are present in excess relative to food intake. Common triggers for hypoglycaemia include:

• Skipping or delaying meals

• Unusually intense or prolonged physical activity

• Taking too high a dose of insulin or certain diabetes medicines (particularly sulphonylureas)

• Drinking alcohol, especially on an empty stomach; this risk is greater in people taking insulin or sulphonylureas, as alcohol can suppress the liver's release of glucose (Diabetes UK)

Understanding these triggers helps people with diabetes and those at risk to anticipate and manage fluctuations more effectively.

How do doctors test and monitor your blood glucose?

UK doctors use fasting plasma glucose, the oral glucose tolerance test (OGTT) and HbA1c to diagnose and monitor blood glucose; HbA1c is the preferred diagnostic test for type 2 diabetes in most adults but is not suitable in pregnancy or haemoglobinopathies.

Doctors in the UK use several different tests to measure blood glucose, each suited to a different clinical purpose. The most commonly used tests are the fasting plasma glucose test, the oral glucose tolerance test (OGTT) and the HbA1c blood test.

Fasting plasma glucose requires the patient to fast for at least eight hours before a blood sample is taken from a vein. It gives a snapshot of glucose at a single point in time and is used both for diagnosis and routine monitoring.

The OGTT involves a fasting blood sample, followed by the patient drinking a standardised glucose solution (75 g), with a further blood sample taken two hours later (NHS; WHO).^[2][5] It is particularly useful for diagnosing gestational diabetes and impaired glucose tolerance, and is the preferred diagnostic method during pregnancy (NICE NG3).

HbA1c measures the proportion of haemoglobin in red blood cells that has glucose attached to it. Because red blood cells live for approximately 120 days, HbA1c reflects average blood glucose over the preceding two to three months.^[2] NICE recommends HbA1c as the preferred diagnostic test for type 2 diabetes in most adults, and it is also used to monitor long-term glucose control in people already diagnosed (NICE NG28).^[2][3] However, HbA1c is not appropriate for diagnosis in pregnancy, in children or young people where type 1 diabetes is suspected, or in people with haemoglobinopathies, anaemia, high red-cell turnover or certain forms of chronic kidney disease; in these situations, plasma glucose testing is used instead.^[5][2]

For people managing diabetes at home, self-monitoring of blood glucose (SMBG) using a finger-prick test and a glucometer is widely used. Increasingly, continuous glucose monitors (CGMs) are available on the NHS for eligible patients, particularly those with type 1 diabetes, providing real-time glucose readings without repeated finger pricks. Eligibility and access criteria are set out in NICE guidance (NICE NG17; NICE NG28), and local implementation follows national recommendations.

How can you keep your blood sugar levels in a healthy range?

A lower glycaemic index diet, at least 150 minutes of moderate aerobic activity per week and weight management are the key lifestyle measures; medication such as metformin or an SGLT-2 inhibitor may be needed for diagnosed diabetes, guided by NICE NG28.

Maintaining blood glucose within a healthy range involves a combination of dietary choices, physical activity, weight management and, where necessary, medication. For people without diabetes, these lifestyle measures can substantially reduce the risk of developing type 2 diabetes, particularly if blood glucose is already in the non-diabetic hyperglycaemia (NDH / prediabetes) range.

Diet plays a central role. Choosing foods with a lower glycaemic index (such as wholegrains, legumes, vegetables and most fruits), reducing portion sizes of starchy carbohydrates, limiting added sugars and ultra-processed foods, and eating regular balanced meals all help to moderate post-meal glucose spikes. The NHS Eatwell Guide provides a practical framework for a balanced diet.

Physical activity improves insulin sensitivity and helps muscles absorb glucose more efficiently. The UK Chief Medical Officers' Physical Activity Guidelines recommend that adults aim for at least 150 minutes of moderate-intensity aerobic activity per week, alongside muscle-strengthening activities on two or more days.^[11][12] Research evidence also suggests that a short walk after meals can help to reduce post-meal glucose rises.^[15]

Weight management is particularly important in type 2 diabetes and non-diabetic hyperglycaemia, as excess body fat, especially around the abdomen, is strongly associated with insulin resistance. Structured weight-management programmes, including those supported by NHS services, can lead to meaningful improvements in blood glucose control.

For people with diagnosed diabetes, medication may be needed alongside lifestyle measures. NICE NG28 recommends an individualised approach to first-line pharmacotherapy for type 2 diabetes. For many people who can tolerate it, metformin remains a typical first choice. However, an SGLT-2 inhibitor is recommended as first-line treatment for people with established or high risk of atherosclerotic cardiovascular disease, chronic kidney disease or heart failure, regardless of HbA1c. Other classes, including GLP-1 receptor agonists, may be added depending on individual circumstances. Treatment decisions should always be made in discussion with a clinician.

Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, where clinically appropriate options may be discussed.

When should you see a GP about your blood sugar readings?

See a GP if home readings are consistently outside the normal range, if you have symptoms such as increased thirst, frequent urination or blurred vision, or if you feel unwell alongside a high reading; attend A&E immediately if unconsciousness or seizure occurs.

You should see your GP if a home blood glucose reading is consistently outside the normal range, if you have symptoms that suggest high or low blood glucose, or if you have risk factors for diabetes that have not yet been formally assessed. Early detection of non-diabetic hyperglycaemia (NDH / prediabetes) or type 2 diabetes allows treatment to begin before complications develop.

Symptoms that warrant a GP appointment include:

• Increased thirst and frequent urination, particularly at night

• Unexplained tiredness or fatigue

• Blurred vision

• Slow-healing cuts or wounds

• Recurrent infections, such as thrush or urinary tract infections

• Unintentional weight loss (more typical of type 1 diabetes)

If you are already diagnosed with diabetes and experience a blood glucose reading that is very high and does not come down with your usual management, or if you feel unwell alongside a high reading, contact your GP or diabetes team promptly. People with type 1 diabetes, those who are unwell, and people taking SGLT-2 inhibitors should also check their blood or urine ketones when glucose is elevated, as raised ketones can indicate diabetic ketoacidosis (DKA) even when glucose is not extremely high.^[13][14] If you are unable to reach your GP or diabetes team, NHS 111 can provide urgent advice.

Warning signs of DKA include nausea or vomiting, abdominal pain, rapid or deep breathing, a fruity smell on the breath, dehydration and confusion. If any of these occur alongside high glucose or positive ketones, seek urgent medical assessment.

Seek emergency care via 999 or attend A&E immediately if you or someone else loses consciousness, has a seizure, or cannot be roused, as these may indicate severe hypoglycaemia or DKA, both of which are medical emergencies.

If you experience an unexpected or serious side effect from a glucose-lowering medicine, you or your healthcare professional can report it to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk. Routine NHS diabetes reviews, including HbA1c measurement, blood pressure and foot checks, are recommended at least annually by NICE and are an important opportunity to discuss any concerns about your readings with your clinical team.^[7]

Scientific References

1. High blood sugar (hyperglycaemia).
2. Type 2 diabetes in adults: management (NG28) – Blood glucose management chapter.
3. Type 2 diabetes in adults: management (NG28) – Recommendations.
4. Type 2 diabetes in adults: management (NG28) – Initial medicines.
5. Diabetes in pregnancy: management from preconception (NG3) – Recommendations.
6. Type 2 diabetes: prevention in people at high risk (PH38) – Recommendations.
7. 9 key care processes – Type 2 diabetes in adults (QS209).
8. Hypoglycaemia – Treatment summaries, BNF / NICE.
9. Type 2 diabetes – Treatment summaries, BNF / NICE.
10. The threshold for diagnosing impaired fasting glucose.
11. Physical activity guidelines: UK Chief Medical Officers' report.
12. Physical activity guidelines for adults aged 19 to 64.
13. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
14. SGLT2 inhibitors: monitor ketones in blood during treatment interruption for surgical procedures or acute serious medical illness.
15. Effects of short-bout accumulated exercise on postprandial metabolism in adults: A systematic review and meta-analysis. Complementary Therapies in Medicine.

Frequently Asked Questions