What Increases Blood Glucose: Causes Explained

What increases blood glucose is a question relevant to millions of people in the UK, whether managing diabetes, pre-diabetes, or simply trying to understand their health. Blood glucose rises in response to a wide range of factors beyond food alone, including stress, illness, poor sleep, certain medicines, and underlying health conditions. Understanding these triggers helps people make informed decisions about diet, lifestyle, and when to seek medical advice. This article explains the key causes of raised blood sugar, which foods and drinks have the greatest effect, and how to recognise when levels may be dangerously high.

What causes blood glucose levels to rise?

Blood glucose rises when glucose enters the bloodstream faster than insulin can clear it, driven by carbohydrate digestion, liver glucose release, stress hormones, and impaired insulin action in diabetes.

Blood glucose levels rise whenever the body absorbs more glucose than its cells can take up and use, most commonly after eating carbohydrates, but also in response to hormones, illness, and certain medicines. Glucose is the body's primary fuel source, and after a meal the pancreas releases insulin to move glucose from the bloodstream into cells. When this process is disrupted, or when glucose enters the blood faster than insulin can clear it, blood sugar climbs.

Several overlapping mechanisms drive this rise. Carbohydrate digestion breaks down starches and sugars into glucose, which passes rapidly into the bloodstream through the small intestine. The liver also releases stored glucose between meals and overnight through two processes: glycogenolysis (breaking down stored glycogen) and gluconeogenesis (producing new glucose from non-carbohydrate sources such as amino acids). Both contribute to fasting blood glucose levels, which is why they are not always low even after a long overnight fast. Some people also notice higher readings first thing in the morning due to the dawn phenomenon, in which a natural early-morning surge in cortisol and growth hormone stimulates the liver to release glucose before waking.^[1]

Hormones such as glucagon, cortisol, adrenaline, and growth hormone all signal the liver to release more glucose, particularly during stress or illness.

In people with type 2 diabetes or insulin resistance, the body's cells respond poorly to insulin, so more glucose remains in circulation even after a normal meal. In type 1 diabetes, the pancreas produces little or no insulin, meaning glucose cannot enter cells at all without injected insulin. Understanding these mechanisms helps explain why blood glucose is influenced by far more than food alone.

Which foods and drinks push blood sugar up the most?

High-GI foods such as white bread, white rice, sugary drinks, and refined baked goods cause the sharpest blood glucose rises; fibre, protein, and fat slow absorption and blunt the response.

Foods and drinks that are high in rapidly digested carbohydrates cause the sharpest rises in blood glucose, particularly those with a high glycaemic index (GI). The glycaemic index ranks carbohydrate-containing foods by how quickly they raise blood sugar compared with pure glucose. High-GI foods include white bread, white rice, most breakfast cereals, sugary drinks, sweets, and baked goods made with refined flour.

Sugary drinks deserve particular attention because they deliver large amounts of glucose or fructose with no fibre to slow absorption. A standard 330 ml can of regular cola contains approximately 35 g of sugar (figures vary by brand; check the nutrition label).^[5] This can cause a noticeable rise in blood glucose within around 30 to 60 minutes of consumption, though the exact timing varies between individuals.^[5] Fruit juices, energy drinks, and sweetened coffees carry a similar risk, even when marketed as natural or healthy.

Foods that raise blood glucose more gradually include those rich in fibre, protein, or fat, because these nutrients slow gastric emptying and reduce the rate at which glucose enters the bloodstream. Examples include:

• Legumes (lentils, chickpeas, kidney beans)

• Wholegrains (oats, barley, wholegrain bread)

• Non-starchy vegetables (broccoli, spinach, courgette)

• Nuts and seeds

Portion size matters as much as food type. Even lower-GI foods can raise blood glucose significantly if eaten in large quantities, a concept captured by the glycaemic load (GL), which accounts for both the quality and quantity of carbohydrate consumed. NICE guidance on type 2 diabetes in adults (NG28) supports a diet rich in fibre and low in refined carbohydrates as part of a structured approach to blood glucose control.^[2][3] Diabetes UK provides accessible patient-facing guidance on GI, GL, and carbohydrate choices.

Can stress, illness, and poor sleep raise your blood sugar?

Yes, all three raise blood glucose by triggering cortisol and adrenaline release, which stimulate the liver to produce more glucose and reduce insulin sensitivity.

Yes, stress, illness, and poor sleep can all raise blood glucose, even in people without diabetes, because each triggers the release of counter-regulatory hormones that signal the liver to produce more glucose. During physical or psychological stress, the adrenal glands release cortisol and adrenaline. These hormones evolved to provide rapid energy in emergencies, but in modern life they can cause persistent, unwanted rises in blood sugar.

Acute illness, particularly infections, is a well-recognised cause of elevated blood glucose. Inflammatory cytokines released during infection increase insulin resistance, and the stress response further stimulates hepatic glucose output.^[4] People with diabetes are advised to monitor their blood glucose more frequently when unwell and to follow sick-day rules. These typically involve maintaining hydration and continuing diabetes medication, but some medicines may need to be paused during acute illness. In particular, UK guidance (including NHS and Diabetes UK sick-day rules) advises that SGLT2 inhibitors (such as dapagliflozin, empagliflozin, and canagliflozin) should generally be stopped during acute illness or dehydration because of the risk of diabetic ketoacidosis, and that metformin should be paused if there is a risk of dehydration or reduced kidney perfusion.^[16] People at risk of ketoacidosis should also check their ketone levels when unwell. Always follow the specific advice given by your diabetes team, and contact your GP or NHS 111 if readings remain persistently high, you are unable to keep fluids down, or you are unsure whether to continue any medicine.

Sleep deprivation has a measurable effect on glucose metabolism. Research has shown that even one or two nights of poor sleep can reduce insulin sensitivity and increase fasting glucose levels.^[6][7] Chronic sleep restriction is associated with higher HbA1c values in people with type 2 diabetes.^[8] The mechanisms include elevated evening cortisol, increased appetite for high-carbohydrate foods, and reduced physical activity the following day.

Emotional stress, including anxiety, bereavement, and work pressure, can produce similar hormonal effects. Keeping a diary of blood glucose readings alongside notes on sleep quality and stress levels can help identify personal patterns and support conversations with a healthcare professional.

What medicines and health conditions affect blood glucose?

Corticosteroids, thiazide diuretics, antipsychotics, and immunosuppressants are among the medicines that raise blood glucose; conditions such as PCOS, Cushing's syndrome, and pancreatitis also impair glucose regulation.

Several commonly prescribed medicines raise blood glucose as a recognised side effect, and people with diabetes or pre-diabetes should be aware of them. Corticosteroids such as prednisolone are among the most significant: they increase hepatic glucose production and reduce insulin sensitivity, sometimes causing steroid-induced diabetes in people who were previously normoglycaemic.^[15] The effect is usually dose-dependent and may resolve when the steroid is stopped, though monitoring is essential throughout treatment.

Other medicines associated with raised blood glucose include:

• Thiazide diuretics (for example, bendroflumethiazide), which can impair insulin secretion at higher doses^[11][12]

• Beta-blockers (for example, propranolol), which may mask some symptoms of hypoglycaemia; effects on blood glucose vary between agents and are generally modest, so discuss any concerns with your prescriber

• Antipsychotics, particularly olanzapine and clozapine, which are associated with insulin resistance and weight gain; other atypical antipsychotics may also carry this risk to varying degrees^[9][10]

• Immunosuppressants such as tacrolimus and ciclosporin, used after organ transplantation

• Some hormonal contraceptives: modern combined hormonal contraceptives generally have minimal impact on glycaemic control at current doses, but anyone with diabetes or impaired glucose tolerance should discuss their contraceptive choice with their prescriber or a sexual health specialist; FSRH guidance provides detailed advice

This list is not exhaustive. Other medicines, including some protease inhibitors and high-dose niacin, can also affect blood glucose. Anyone starting a new medicine should ask their prescriber or pharmacist whether it may affect blood glucose, particularly if they already have a diagnosis of diabetes or impaired fasting glucose. The BNF is a useful reference for healthcare professionals checking for drug-induced hyperglycaemia.

Several health conditions also raise blood glucose independently of diet or lifestyle. Polycystic ovary syndrome (PCOS) is strongly associated with insulin resistance. Cushing's syndrome, caused by excess cortisol, raises glucose through the same mechanism as steroid medicines. Acromegaly (excess growth hormone) and phaeochromocytoma (a rare adrenal tumour) both stimulate hepatic glucose output. Pancreatitis and pancreatic cancer can impair insulin secretion by damaging beta cells. People with these conditions should discuss with their specialist or GP whether testing for diabetes or impaired glucose regulation is appropriate as part of their ongoing care.

How do you know if your blood glucose is too high?

Classic symptoms of hyperglycaemia include increased thirst, frequent urination, fatigue, and blurred vision; DKA and HHS are emergencies requiring immediate 999 or A&E attendance.

High blood glucose (hyperglycaemia) often produces recognisable symptoms, though mild or moderate elevations can sometimes occur without any noticeable signs, particularly in people with long-standing type 2 diabetes who have adapted to higher levels over time. The classic symptoms of hyperglycaemia include increased thirst, passing urine more frequently (especially at night), fatigue, blurred vision, headaches, and difficulty concentrating.

In people with type 1 diabetes, persistently high glucose can progress to diabetic ketoacidosis (DKA), a serious and potentially life-threatening condition. Symptoms of DKA include nausea, vomiting, abdominal pain, deep or rapid breathing, and a fruity smell on the breath. Anyone with these symptoms should call 999 or go to A&E immediately. People with type 2 diabetes can develop a different emergency called hyperosmolar hyperglycaemic state (HHS). Symptoms of HHS include extreme thirst, passing large amounts of urine, progressive drowsiness or confusion, and in severe cases seizures. HHS also requires urgent hospital treatment: call 999 or go to A&E if you suspect it.

For people who monitor their blood glucose at home, individual targets should be agreed with your diabetes team or GP, as these vary depending on the type of diabetes, treatment, and personal circumstances. Diabetes UK provides patient-facing guidance on typical blood glucose ranges. As a general reference, WHO and UK diagnostic criteria define a fasting plasma glucose of 7.0 mmol/l or above, or a plasma glucose of 11.1 mmol/l or above two hours after a glucose load, as consistent with diabetes; however, these are laboratory plasma values used for diagnosis and are not the same as self-monitoring targets agreed with your healthcare team. HbA1c, a blood test measuring average glucose over approximately three months, is the standard diagnostic and monitoring tool used in NHS practice.^[2][4]

If you experience persistent symptoms of high blood glucose, or if home readings are consistently above your agreed target, contact your GP for review. For urgent advice when you are unsure whether to seek immediate care, NHS 111 is available 24 hours a day. Suspected adverse effects of any medicine affecting blood glucose can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Scientific References

1. Nocturnal spikes of growth hormone secretion cause the dawn phenomenon in type 1 (insulin-dependent) diabetes mellitus by decreasing hepatic (and extrahepatic) insulin sensitivity.
2. Type 2 diabetes in adults: management (NG28).
3. Dietary advice and interventions | Type 2 diabetes in adults (NG28).
4. High blood sugar (hyperglycaemia).
5. Effects of high and low sucrose-containing beverages on blood glucose.
6. A single night of partial sleep deprivation induces insulin resistance in multiple metabolic pathways in healthy subjects.
7. A single night of partial sleep loss impairs fasting insulin sensitivity.
8. Habitual Short Sleep Duration, Diet, and Development of Type 2 Diabetes.
9. Comparative effects of 18 antipsychotics on metabolic syndrome elements.
10. Pharmacological Management of Glucose Dysregulation in Patients Treated with Second-Generation Antipsychotics.
11. Effects of low dose versus conventional dose thiazide diuretics on metabolic outcomes.
12. Thiazides Attenuate Insulin Secretion Through Inhibition of Mitochondrial Carbonic Anhydrase 5b in β-Islet Cells in Mice.
13. Post-transplant diabetes mellitus. The role of immunosuppression.
14. Immunomodulation in post-transplant diabetes mellitus: Challenges and management.
15. Glucocorticoid-induced diabetes mellitus: mechanisms, risk factors, and clinical pathways with insights from autoimmune rheumatic diseases.
16. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
17. Is fasting glucose sufficient to define diabetes?.

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