Intermittent Fasting and Diabetes: Risks and Safety

Intermittent fasting and diabetes is a topic that requires careful consideration, as fasting can meaningfully affect blood glucose levels, insulin sensitivity and medication safety. For people living with type 1 or type 2 diabetes in the UK, the decision to try intermittent fasting carries real clinical implications, from hypoglycaemia risk to interactions with insulin and SGLT-2 inhibitors. This article explains how fasting affects blood sugar, what NHS and NICE guidance says, which medications need reviewing, and how to approach fasting safely with support from your diabetes care team.

What is intermittent fasting and how does it affect blood sugar?

Intermittent fasting lowers circulating glucose and can improve insulin sensitivity by depleting glycogen stores, but it is not endorsed by NICE or the NHS as a standard diabetes intervention and carries particular risks for those on insulin or with type 1 diabetes.

Intermittent fasting can lower blood glucose levels and improve insulin sensitivity in people with type 2 diabetes, though its effects vary considerably between individuals. Intermittent fasting (IF) is an eating pattern that cycles between defined periods of eating and voluntary fasting, rather than focusing on which foods are consumed. Common approaches include the 5:2 method (eating normally for five days and restricting calories significantly on two non-consecutive days) and time-restricted eating, such as limiting food intake to an eight-hour window each day.

During a fasting period, the body depletes its glycogen stores and begins drawing on fat for energy, a process that reduces circulating glucose and can lower insulin demand. For people with type 2 diabetes, this mechanism may be beneficial: a number of clinical studies have reported reductions in HbA1c (a measure of average blood glucose over approximately three months) and improvements in fasting glucose following structured IF regimens. However, the quality and size of this evidence base varies, and IF is not currently endorsed as a standard dietary intervention by NICE (NG28: Type 2 diabetes in adults: management) or NHS dietary guidance.^[1][2]

The picture is more complex for people with type 1 diabetes or those on insulin and certain oral medications. In these groups, fasting without careful medication adjustment can trigger hypoglycaemia (dangerously low blood sugar). People with type 1 diabetes also face a heightened risk of diabetic ketoacidosis (DKA) during fasting, even without SGLT-2 inhibitors, because reduced carbohydrate intake can accelerate ketone production; this risk means that anyone with type 1 diabetes should only consider fasting under close specialist supervision.^[11] Conversely, hyperglycaemia can occur if compensatory eating follows a fast. The physiological response to fasting also differs depending on the duration of diabetes, current glycaemic control, kidney function and individual metabolic factors.

Intermittent fasting is not suitable for everyone. People who are pregnant, planning a pregnancy or breastfeeding, those under 18, people who are frail or have a low BMI, and anyone with an active or recent history of an eating disorder should not attempt fasting without explicit specialist advice, as the risks in these groups outweigh any potential benefit.

Intermittent fasting is not a medically standardised treatment for diabetes in the UK, and any decision to try it should be made in close consultation with a GP, diabetes specialist nurse or dietitian.

How does fasting interact with diabetes medications like insulin?

Fasting can cause hypoglycaemia in people taking insulin or sulphonylureas, and increases euglycaemic DKA risk in those on SGLT-2 inhibitors; medication doses and timing must be reviewed by a diabetes team before starting any fasting regimen.

Intermittent fasting can significantly alter the way diabetes medications behave in the body, and dose adjustments are often necessary to prevent hypoglycaemia or hyperglycaemia. This interaction is particularly important for people taking insulin, sulphonylureas (such as gliclazide or glibenclamide), or SGLT-2 inhibitors (such as dapagliflozin or empagliflozin).

Insulin and sulphonylureas both lower blood glucose actively, meaning that if food intake is reduced or delayed during a fast, these medicines can push glucose too low. Sulphonylureas are especially associated with hypoglycaemia risk during fasting because they stimulate insulin release regardless of whether carbohydrates have been consumed. For people on insulin, basal insulin should never be stopped without specialist advice; mealtime or bolus doses typically require review when fasting, as the usual carbohydrate load that drives the dose calculation is absent.

SGLT-2 inhibitors carry a separate concern: prolonged fasting combined with these medicines can increase the risk of euglycaemic diabetic ketoacidosis (DKA), a serious condition where ketones accumulate even when blood glucose appears normal or only mildly elevated. The MHRA has issued specific safety guidance on this risk. People taking SGLT-2 inhibitors should monitor for ketones during illness or prolonged periods of reduced food intake, and should seek urgent medical assessment if blood ketones are elevated (generally 1.5 mmol/L or above) or if they develop nausea, vomiting, abdominal pain or fatigue even with near-normal glucose readings. SGLT-2 inhibitors should be paused during acute illness, periods of significantly reduced food intake, and before surgery, in line with MHRA and NHS sick-day guidance; discuss this plan with your diabetes team before starting any fasting regimen.^[10]

Metformin does not directly cause hypoglycaemia and is generally considered lower risk during fasting periods, though gastrointestinal side effects may be more pronounced if taken without food. DPP-4 inhibitors (such as sitagliptin or alogliptin) also carry a low intrinsic hypoglycaemia risk and may require fewer adjustments, though a medication review is still advisable. GLP-1 receptor agonists (such as semaglutide, available as Ozempic or Wegovy) and dual GIP/GLP-1 agonists (such as tirzepatide, available as Mounjaro, which is MHRA-authorised both for type 2 diabetes and for chronic weight management) already reduce appetite and slow gastric emptying, so combining them with fasting requires careful monitoring.^[12][13]

Medication timing, dose and type may all need to be reviewed before starting any fasting regimen. Never adjust insulin or other diabetes medication doses without explicit guidance from your diabetes care team, as incorrect changes can be life-threatening.

What do NHS and NICE guidelines say about fasting with diabetes?

Neither NICE nor the NHS currently recommends intermittent fasting as a standard dietary intervention for diabetes; structured low-calorie programmes such as the NHS Type 2 Diabetes Path to Remission Programme have a stronger evidence base.

Neither NICE nor the NHS currently recommends intermittent fasting as a standard dietary intervention for diabetes management, though both acknowledge the importance of personalised dietary support. NICE guidelines on type 2 diabetes (NG28) and obesity (NG187) emphasise that dietary approaches should be tailored to the individual, culturally sensitive and sustainable, and that no single eating pattern is universally superior.^[1][2] Structured low-calorie diets and total diet replacement programmes have stronger evidence bases within NHS frameworks, particularly for type 2 diabetes remission, as reflected in the NHS Type 2 Diabetes Path to Remission Programme.^[16]

NICE does not recommend intermittent fasting specifically, and there is currently no NICE technology appraisal or clinical guideline that positions IF as a first-line or recommended strategy for glycaemic management. This does not mean fasting is prohibited, but it does mean that people with diabetes who wish to try it are doing so outside a formally endorsed pathway and should therefore receive close clinical supervision.

The NHS Diabetes Prevention Programme and structured education courses such as DESMOND and DAFNE focus on carbohydrate awareness, physical activity and behaviour change rather than fasting schedules.^[1][2] People with diabetes are encouraged to access these programmes through their GP or diabetes team. Integrated Care Boards (ICBs) commission local diabetes services, and the availability of specialist dietary support may vary by region.

For those seeking an online consultation to explore weight management options alongside their diabetes care, Bolt Pharmacy is a GPhC-registered UK online pharmacy offering prescriber-led consultations, with clinical review and pharmacist support as part of the process.

Risks to watch for and when to seek medical advice

The most serious risk is hypoglycaemia (blood glucose below 4.0 mmol/L), but euglycaemic DKA, hyperglycaemia, dehydration and disordered eating are also concerns; seek urgent care if ketones reach 1.5 mmol/L or above.

The most serious risk of intermittent fasting in people with diabetes is hypoglycaemia, which can occur rapidly and sometimes without warning, particularly in those taking insulin or sulphonylureas. In UK clinical practice, hypoglycaemia is generally defined as a blood glucose reading below 4.0 mmol/L. Symptoms include shakiness, sweating, confusion, palpitations and, in severe cases, loss of consciousness. If you experience these symptoms, follow your agreed hypoglycaemia action plan: the standard first step is to take 15 to 20 g of fast-acting carbohydrate (such as glucose tablets, a small glass of fruit juice or sugary drink), then recheck your blood glucose after 10 to 15 minutes and repeat if still below 4.0 mmol/L. If the person is unconscious or cannot swallow safely, call 999 for emergency assistance immediately.

Keep fast-acting carbohydrate accessible at all times during fasting periods, and agree specific glucose thresholds and stopping rules with your diabetes team before you begin.

Other risks to monitor include:

• Hyperglycaemia following a fast, particularly if compensatory overeating occurs or if medication doses have been reduced too aggressively

• Euglycaemic DKA, especially in people taking SGLT-2 inhibitors, which may present with nausea, vomiting, abdominal pain and fatigue even when blood glucose readings appear near-normal; check blood ketones if these symptoms occur and seek urgent care if ketones are 1.5 mmol/L or above

• Dehydration, which can worsen kidney function and affect the clearance of some diabetes medications

• Disordered eating patterns, as restrictive eating schedules are not appropriate for people with an active or recent history of eating disorders

Contact your GP if you notice persistent unexplained changes in blood glucose readings, increased frequency of hypoglycaemic episodes, or difficulty managing your diabetes during fasting periods. Use NHS 111 for urgent advice outside of GP hours if you are concerned but not in immediate danger.

Suspected adverse effects from diabetes medications should be reported to the MHRA via the Yellow Card scheme (yellowcard.^[20][21]mhra.gov.uk), which accepts reports from both patients and healthcare professionals.

Worsening of pre-existing diabetic eye disease, particularly pre-proliferative or proliferative retinopathy, has been associated with rapid improvements in glycaemic control.^[17][18] If your HbA1c is falling quickly as a result of dietary changes, inform your diabetes eye screening team or ophthalmologist so that your retinal screening schedule can be reviewed accordingly.

How to start intermittent fasting safely with your diabetes team's support

Before changing your eating pattern, discuss your HbA1c, medications and any complications with your GP or diabetes team, agree glucose targets and stopping rules, and never adjust insulin or other diabetes medication independently.

Starting intermittent fasting safely with diabetes requires a structured conversation with your diabetes care team before making any changes to your eating pattern or medication regimen. Your GP, diabetes specialist nurse or dietitian can review your current HbA1c, medication list, kidney function and any complications to assess whether fasting is appropriate for you and, if so, which approach carries the least risk.

Certain groups should not attempt intermittent fasting without explicit specialist supervision: people with type 1 diabetes, those who are pregnant, planning a pregnancy or breastfeeding, anyone under 18, people who are frail or have a low BMI, and anyone with an active or recent history of an eating disorder. For these individuals, the risks are substantially higher and safer alternatives should be explored with a specialist.

Practical steps to discuss with your team include:

• Blood glucose monitoring: more frequent self-monitoring (or use of a continuous glucose monitor or flash glucose monitor where available) is usually recommended during fasting periods, particularly for those on insulin or sulphonylureas, to detect hypoglycaemia or hyperglycaemia early; agree specific glucose targets, action thresholds and stopping rules before you begin

• Hypo preparedness: always keep fast-acting carbohydrate (such as glucose tablets or a sugary drink) readily available during fasting periods

• Medication review: your team may adjust the timing, dose or type of your diabetes medication before you begin fasting; never make these changes independently

• SGLT-2 inhibitor safety: if you take an SGLT-2 inhibitor, agree a ketone monitoring plan and sick-day rules with your team, including when to pause the medication and when to seek urgent care

• Choosing a suitable fasting method: time-restricted eating (for example, eating within a 10 to 12 hour window) may carry a lower risk than prolonged fasting days for people on complex medication regimens

• Hydration: maintaining adequate fluid intake during fasting periods is important, particularly for those on SGLT-2 inhibitors or with reduced kidney function

• Breaking the fast sensibly: reintroducing food gradually and avoiding large carbohydrate loads can help prevent post-fast glucose spikes

If you are managing your weight alongside diabetes, your team may also discuss structured low-calorie programmes available through the NHS, such as the NHS Type 2 Diabetes Path to Remission Programme, which has a stronger evidence base than intermittent fasting for achieving diabetes remission. Any dietary change that affects glycaemic control should be treated as a clinical intervention, not a lifestyle experiment, and ongoing review appointments are an essential part of doing this safely. Keep a record of your glucose readings and any symptoms to share at each review.

Scientific References

1. Type 2 diabetes in adults: management (NG28).
2. Dietary advice and interventions – Type 2 diabetes in adults (NG28).
3. February 2026 exceptional surveillance of type 2 diabetes in adults: management (NG28).
4. Overweight and obesity management (NG246).
5. Intermittent fasting to treat diabetes: time to update clinical guidelines.
6. Beneficial effects of time-restricted eating on cardiometabolic factors and anthropometric measures in adults with type 2 diabetes: an umbrella meta-analysis.
7. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials.
8. Intermittent fasting versus continuous energy-restricted diet for patients with type 2 diabetes mellitus and metabolic syndrome for glycemic control: a systematic review and meta-analysis.
9. A Prospective Trial of Risk Factors for Sulfonylurea-Induced Hypoglycemia. JAMA.
10. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
11. Diabetic ketoacidosis.
12. MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss.
13. Tirzepatide | Drugs | BNF – NICE.
14. Hypoglycaemia | Treatment summaries – BNF – NICE.
15. Low blood sugar (hypoglycaemia).
16. Early findings from the NHS Type 2 Diabetes Path to Remission Programme. The Lancet Diabetes & Endocrinology.
17. NG242 Diabetic retinopathy: management and monitoring – evidence review C: effectiveness of intensive treatments to lower blood glucose levels.
18. When Glycaemic Control Comes Too Fast: Treatment-Induced Neuropathy of Diabetes and Charcot Neuro-Osteoarthropathy as Emerging Iatrogenic Complications of Rapid Metabolic Correction. Diabetes/Metabolism Research and Reviews.
19. Early Worsening of Diabetic Retinopathy Following Initiation of Hybrid Closed-Loop/Automated Insulin Delivery Systems in Type 1 Diabetes: A Systematic Review. Diabetes, Obesity & Metabolism.
20. Yellow Card: please help to reverse the decline in reporting of suspected adverse drug reactions.
21. Adverse reactions to drugs | Medicines guidance – BNF – NICE.

Frequently Asked Questions