Glucose intolerance is a term used to describe a state in which the body cannot process blood sugar normally — sitting on a metabolic spectrum between healthy glucose levels and type 2 diabetes. In UK clinical practice, this is most commonly referred to as non-diabetic hyperglycaemia (NDH) and encompasses conditions such as impaired fasting glucose and impaired glucose tolerance, collectively known as prediabetes. Affecting an estimated 7 million adults in England, glucose intolerance is largely asymptomatic, making routine screening essential. Understanding what it means, how it is diagnosed, and what can be done about it is a vital first step towards preventing progression to type 2 diabetes.

What Is Glucose Intolerance and How Is It Defined?

Glucose intolerance, termed non-diabetic hyperglycaemia (NDH) in UK practice, describes blood glucose above normal but below the diabetes threshold — defined as HbA1c 42–47 mmol/mol or fasting plasma glucose 5.5–6.9 mmol/L under NHS DPP criteria.

Glucose intolerance is an umbrella term describing a range of conditions in which the body is unable to process blood sugar (glucose) normally. It sits on a spectrum between healthy glucose metabolism and type 2 diabetes, and is most commonly associated with two specific clinical states: impaired fasting glucose (IFG) and impaired glucose tolerance (IGT). These are often referred to collectively as prediabetes.

In UK clinical practice, the preferred term is non-diabetic hyperglycaemia (NDH), which is used by NHS England, the Office for Health Inequalities and Disparities (OHID), and the NHS Diabetes Prevention Programme (NHS DPP). NDH encompasses anyone whose blood glucose is above the normal range but below the threshold for a diabetes diagnosis.

In a healthy individual, the pancreas releases insulin in response to rising blood glucose after eating. Insulin acts as a key, allowing glucose to enter cells and be used for energy. In glucose intolerance, this process becomes less efficient — either because the pancreas does not produce enough insulin, or because the body's cells do not respond to it properly (insulin resistance). As a result, blood glucose rises higher than normal but does not yet reach the threshold for a formal diagnosis of type 2 diabetes.

The World Health Organisation (WHO) defines the clinical thresholds for IFG and IGT as follows:

• Impaired fasting glucose (IFG): fasting plasma glucose of 6.1–6.9 mmol/L

• Impaired glucose tolerance (IGT): plasma glucose of 7.8–11.0 mmol/L two hours after a 75 g oral glucose tolerance test (OGTT)

For NHS DPP eligibility and risk management in UK primary care, NDH is identified using:

• HbA1c of 42–47 mmol/mol, or

• Fasting plasma glucose of 5.5–6.9 mmol/L

These NDH thresholds are set out in the NHS DPP service specification published by NHS England and OHID. It is important to note that the OGTT is not routinely used to screen non-pregnant adults in UK primary care; it is reserved for specific clinical indications, including pregnancy.

HbA1c testing may be unreliable in certain circumstances — for example, during pregnancy, in people with haemoglobinopathies (such as sickle cell disease or thalassaemia), or in those with iron-deficiency or haemolytic anaemia. In these situations, fasting plasma glucose or an OGTT may be preferred.

Glucose intolerance or NDH is not a disease in itself, but a significant metabolic warning sign that warrants clinical attention and proactive management.

Common Causes and Risk Factors in the UK

The main risk factors are excess weight, physical inactivity, unhealthy diet, family history of type 2 diabetes, and ethnicity — with South Asian, Black African, and Black Caribbean adults at higher risk at lower BMI thresholds.

Glucose intolerance develops when a combination of genetic predisposition and lifestyle or environmental factors disrupts normal insulin function. According to Diabetes UK and OHID modelling, an estimated 7 million adults in England are living with non-diabetic hyperglycaemia, though many are unaware of their condition (figures vary by source and year; refer to the latest Diabetes UK statistics for current estimates). Understanding the underlying causes is essential for both prevention and early intervention.

Key risk factors include:

• Overweight and obesity: Excess body fat, particularly visceral (abdominal) fat, is strongly associated with insulin resistance. For most adults, a BMI above 25 kg/m² or a waist circumference greater than 94 cm in men and 80 cm in women significantly increases risk. However, for people of South Asian, Chinese, or Black African or Caribbean descent, lower thresholds apply — a BMI of 23 kg/m² or above is used to define increased risk, with waist circumference thresholds of approximately 90 cm for South Asian men and 80 cm for South Asian women, reflecting higher metabolic risk at lower body weights.

• Physical inactivity: Sedentary behaviour reduces the body's sensitivity to insulin and impairs glucose uptake by muscle tissue.

• Unhealthy diet: Diets high in refined carbohydrates, added sugars, and saturated fats contribute to weight gain and metabolic dysfunction.

• Family history: Having a first-degree relative with type 2 diabetes increases individual risk, reflecting a heritable component to insulin resistance.

• Ethnicity: People of South Asian, Black African, or Black Caribbean descent are at higher risk and may develop glucose intolerance at a lower BMI threshold (see above).

• Age: Risk increases with age, particularly after 40, though rising obesity rates mean younger adults are increasingly affected. Targeted testing is recommended from age 25 in high-risk ethnic groups.

• Gestational diabetes: Women who develop diabetes during pregnancy face a significantly elevated lifetime risk of glucose intolerance and type 2 diabetes.

• Polycystic ovary syndrome (PCOS): This hormonal condition is closely linked to insulin resistance in women.

• Certain medicines: Long-term use of systemic corticosteroids, some antipsychotics, higher-dose thiazide diuretics, and non-selective beta-blockers can impair glucose metabolism. The absolute risk from any individual medicine is generally small, and the benefits of treatment often outweigh this risk. Do not stop any prescribed medicine without first speaking to your GP or pharmacist.

Risk assessment in UK primary care uses validated tools including the Diabetes UK Know Your Risk score and QDiabetes, both of which are used to identify individuals who may benefit from early intervention and referral to the NHS DPP.

Recognising the Symptoms and Getting a Diagnosis

Glucose intolerance is frequently asymptomatic; when symptoms occur they include mild fatigue, increased thirst, and frequent urination. Diagnosis is confirmed via HbA1c or fasting plasma glucose testing in UK primary care.

One of the most clinically significant challenges with glucose intolerance is that it is frequently asymptomatic. Many people live with non-diabetic hyperglycaemia for years without experiencing any noticeable symptoms, which is why routine screening and risk assessment are so important.

When symptoms do occur, they tend to be subtle and non-specific. These may include:

• Mild fatigue or low energy, particularly after meals

• Increased thirst or a dry mouth

• Needing to urinate more frequently

• Difficulty concentrating

These symptoms overlap considerably with other common conditions and should not be used alone to diagnose glucose intolerance. If you are concerned, speak to your GP, who can arrange appropriate blood tests.

Important — when to seek urgent help: If you or someone you know experiences marked thirst and frequent urination alongside unexplained weight loss, recurrent infections (such as thrush or urinary tract infections), severe fatigue, vomiting, abdominal pain, or signs of dehydration or drowsiness, contact your GP or call NHS 111 the same day. If someone is acutely unwell, call 999 or go to your nearest emergency department. These symptoms may indicate a more serious degree of hyperglycaemia requiring prompt assessment.

Diagnosis in the UK is typically made through one or more of the following tests:

• Fasting plasma glucose test: A blood sample taken after an overnight fast of at least 8 hours

• HbA1c (glycated haemoglobin) test: Reflects average blood glucose levels over the preceding 2–3 months; a result of 42–47 mmol/mol indicates non-diabetic hyperglycaemia (NDH/prediabetes)

• Oral glucose tolerance test (OGTT): A two-hour test measuring glucose response to a standard glucose drink; used in pregnancy screening (as per NICE NG3) and in specific clinical situations where HbA1c is unreliable

For a diagnosis of type 2 diabetes, NICE (NG28) requires that an abnormal result is confirmed with a second test on a separate day in an asymptomatic individual. For NDH, a confirmatory test is not required before referral to the NHS DPP, though a repeat test may be appropriate if the result is borderline or discordant between two measures.

NICE guidance recommends that adults identified as high risk through tools such as the NHS Health Check (offered to adults aged 40–74 in England) should be offered blood glucose testing. People from higher-risk ethnic groups may be offered targeted testing from age 25. Early diagnosis creates a valuable window for intervention before progression to type 2 diabetes occurs.

NHS-Recommended Treatments and Lifestyle Changes

Lifestyle modification is the primary treatment, with the NHS Diabetes Prevention Programme offering structured support; metformin may be considered off-label in high-risk individuals following individual clinical assessment.

The cornerstone of managing glucose intolerance is lifestyle modification, and the evidence base supporting this approach is robust. The US Diabetes Prevention Programme (DPP) trial demonstrated that structured lifestyle intervention reduced progression to type 2 diabetes by approximately 58%. This finding is supported by European evidence, including the Finnish Diabetes Prevention Study (DPS), as well as NICE evidence reviews and evaluations of the NHS Diabetes Prevention Programme (NHS DPP). The NHS DPP offers a free, evidence-based behavioural programme to eligible adults in England identified with non-diabetic hyperglycaemia.

Dietary changes are central to management. In line with NICE and NHS guidance, a healthy eating approach should:

• Reduce overall calorie intake to support gradual, sustained weight loss (a 5–10% reduction in body weight can meaningfully improve insulin sensitivity)

• Limit refined carbohydrates, added sugars, and sugar-sweetened drinks

• Emphasise a Mediterranean-style pattern: wholegrains, vegetables, legumes, lean proteins, healthy fats, and high-fibre foods

• Reduce consumption of ultra-processed foods

Physical activity is equally important. In line with the UK Chief Medical Officers' Physical Activity Guidelines (2019), adults are advised to aim for at least 150 minutes of moderate-intensity aerobic activity per week, alongside muscle-strengthening activities on two or more days. Even modest increases in daily movement — such as brisk walking — have been shown to improve insulin sensitivity.

Pharmacological treatment is not routinely recommended for glucose intolerance in the UK. However, metformin may be considered in specific circumstances, in line with NICE prevention guidance (PH38/NG215) — for example, in individuals at very high risk of progression who are unable to achieve lifestyle goals, or women with a history of gestational diabetes. It is important to note that metformin for diabetes prevention is an off-label use in the UK; this decision should always be made by a clinician following individual assessment.

Metformin works by reducing hepatic glucose production and improving peripheral insulin sensitivity. It is generally well tolerated, though gastrointestinal side effects (nausea, diarrhoea) are common initially and usually improve with time. Metformin requires monitoring of renal function before and during use, and long-term use is associated with reduced vitamin B12 absorption, which may require periodic monitoring. Any decision to start metformin should be discussed with your GP or specialist.

If you experience any suspected side effects from metformin or any other medicine, you can report these to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Regular monitoring is also recommended. In line with NICE prevention guidance, individuals with non-diabetic hyperglycaemia should have their HbA1c or fasting glucose reviewed at least annually to detect any progression promptly.

Long-Term Outlook and Reducing Your Risk of Progression

Glucose intolerance is potentially reversible with sustained lifestyle changes; without intervention, around 5–10% of people progress to type 2 diabetes annually, and cardiovascular risk is also elevated.

The long-term outlook for individuals with glucose intolerance is genuinely encouraging, provided that appropriate action is taken. Unlike type 2 diabetes, glucose intolerance is potentially reversible. With sustained lifestyle changes, a significant proportion of people can return their blood glucose levels to the normal range and substantially reduce their risk of developing type 2 diabetes.

However, without intervention, the risk of progression is real. Meta-analyses published in journals such as the BMJ and Diabetologia suggest that approximately 5–10% of people with prediabetes progress to type 2 diabetes each year, and over a 10-year period, up to half may develop the condition (rates vary depending on baseline risk factors and the definition used). Beyond diabetes, glucose intolerance is also associated with an increased risk of cardiovascular disease, including heart attack and stroke, even before diabetes develops. This underscores the importance of addressing not just blood glucose, but the broader cardiometabolic risk profile — including blood pressure, cholesterol, and smoking status. Your GP can assess your cardiovascular risk using tools such as QRISK and, where appropriate, discuss management of blood pressure and lipids in line with NICE guidance (NG136 and NG238).

Women who have had gestational diabetes are at particularly elevated risk and should be offered a fasting plasma glucose test at their 6–13 week postnatal check, and annual HbA1c or fasting glucose thereafter, as recommended by NICE (NG3).

Key strategies to reduce the risk of progression include:

• Achieving and maintaining a healthy body weight

• Staying physically active on a consistent basis, in line with UK CMO guidelines

• Following a balanced, high-fibre, reduced-energy diet with limited sugar-sweetened drinks and refined carbohydrates

• Avoiding smoking and limiting alcohol intake

• Attending all recommended follow-up appointments and blood tests

• Engaging with structured support programmes such as the NHS DPP

It is also worth noting that mental health and stress can influence glucose metabolism. Chronic stress raises cortisol levels, which in turn can elevate blood glucose. Addressing psychological wellbeing — through mindfulness, adequate sleep, and social support — forms a meaningful part of holistic management.

If you have been told your blood glucose is in the non-diabetic hyperglycaemia or prediabetic range, contact your GP to discuss your results, understand your individual risk, and explore the support available to you. Early, informed action remains the most powerful tool in preventing progression to type 2 diabetes.

Frequently Asked Questions