Intermittent Fasting and Hypoglycaemia: Key Facts

Intermittent fasting and hypoglycaemia are closely linked for certain groups, particularly those taking glucose-lowering medicines or living with diabetes. While healthy adults generally maintain stable blood glucose during short fasts through glycogen breakdown and gluconeogenesis, the picture changes significantly when medicines, underlying conditions, or impaired counter-regulatory responses are involved. This article explains who is most at risk, how to recognise the signs of low blood sugar during a fast, and what to consider if you take diabetes medication, in line with NHS, NICE, and MHRA guidance.

Can intermittent fasting cause low blood sugar?

Intermittent fasting can cause hypoglycaemia in people taking glucose-lowering medicines or with impaired counter-regulatory responses, though healthy adults without diabetes rarely develop clinically significant low blood sugar during short fasts.

Intermittent fasting can cause low blood sugar (hypoglycaemia) in certain individuals, particularly those taking glucose-lowering medicines or those with underlying metabolic conditions. In healthy adults without diabetes, the body has robust mechanisms to maintain blood glucose within a normal range during periods of fasting, primarily through glycogen breakdown in the liver and, over longer fasts, through gluconeogenesis. For most people without diabetes, these processes are sufficient to prevent clinically significant hypoglycaemia during short fasts.

However, when fasting is combined with medicines that actively lower blood glucose, or when an individual has impaired counter-regulatory responses, the risk increases considerably. In UK clinical practice, hypoglycaemia is generally defined as a blood glucose level below 4.0 mmol/L, consistent with NHS guidance on low blood sugar. Symptoms can vary between individuals, and the threshold at which they appear differs from person to person.

Alcohol can also increase the risk of hypoglycaemia during fasting by impairing hepatic glucose output, and this risk is relevant even for people who do not take glucose-lowering medicines.

It is also worth noting that some people confuse the mild lightheadedness or fatigue that can accompany caloric restriction with true hypoglycaemia. Genuine hypoglycaemia involves a measurable drop in blood glucose and typically produces a recognisable cluster of symptoms. Understanding this distinction is important before attributing any fasting-related discomfort to low blood sugar, and anyone uncertain about their symptoms should speak to their GP for appropriate assessment.

Who is most at risk of hypoglycaemia when fasting?

People with diabetes taking insulin or sulphonylureas are at highest risk; older adults, those on beta-blockers, people with adrenal insufficiency, chronic liver or kidney disease, and pregnant women also require medical advice before fasting.

People with type 1 or type 2 diabetes who are taking insulin or certain oral glucose-lowering medicines are at the highest risk of hypoglycaemia during intermittent fasting. Sulphonylureas (such as gliclazide and glibenclamide) and insulin stimulate insulin secretion or action independently of food intake, meaning that if a meal is skipped or delayed, blood glucose can fall to dangerous levels.^[3][4] By contrast, metformin, DPP-4 inhibitors (such as sitagliptin), and thiazolidinediones carry a low intrinsic risk of hypoglycaemia when used alone, and people taking only these medicines are at considerably lower risk.^[4][8] NICE guidance on diabetes management highlights the importance of reviewing medicines before any significant dietary change.^[2]

People with a history of severe hypoglycaemia or hypoglycaemia unawareness should seek specialist advice before considering any fasting protocol, as their ability to recognise and respond to falling blood glucose may be impaired.

Beyond those with diagnosed diabetes, other groups warrant caution:

• Older adults, whose counter-regulatory hormonal responses (particularly glucagon and adrenaline release) may be blunted

• People taking beta-blockers, as these medicines can mask the adrenergic warning symptoms of hypoglycaemia (such as shakiness and palpitations), making early recognition more difficult^[6][7]

• People with adrenal insufficiency or hypopituitarism, where cortisol and growth hormone responses to falling glucose are impaired

• People with chronic liver disease or significant chronic kidney disease, in whom glucose production and medicine clearance may be altered, increasing hypoglycaemia risk

• People who drink alcohol regularly or heavily, as alcohol impairs the liver's ability to release glucose during a fast

• Individuals with a history of bariatric surgery, who may experience reactive hypoglycaemia after eating and altered glucose dynamics during fasting

• Those with low body weight or very limited glycogen stores, who have less metabolic reserve during prolonged fasting

• Pregnant women, for whom fasting is generally not recommended due to altered glucose metabolism and the nutritional needs of the developing foetus; breastfeeding women should also seek medical advice before fasting, as caloric restriction may affect milk supply and maternal glucose balance

People in any of these groups should seek medical advice from their GP before starting any form of intermittent fasting. A structured review of current medicines, monitoring requirements and individual risk factors is essential before making significant changes to eating patterns.

How to recognise the signs of low blood sugar during a fast

Early hypoglycaemia causes shakiness, sweating, palpitations, and difficulty concentrating; if untreated, it progresses to confusion, slurred speech, seizures, or loss of consciousness, which is a medical emergency requiring 999.

The early warning signs of hypoglycaemia during a fast include shakiness, sweating, palpitations, hunger, pallor, and difficulty concentrating. These symptoms are driven by the release of adrenaline and other counter-regulatory hormones as the body attempts to raise blood glucose. They typically appear when blood glucose falls below approximately 4.0 mmol/L, though individual thresholds vary, and people who have experienced frequent hypoglycaemia may have reduced awareness of these early signals.

If early symptoms are not addressed, blood glucose can fall further, leading to neuroglycopenic symptoms, which reflect the brain's reduced access to glucose. These include:

• Confusion or difficulty thinking clearly

• Slurred speech

• Visual disturbances

• Unusual behaviour or mood changes

• Seizures or loss of consciousness in severe cases

Anyone experiencing mild to moderate symptoms during a fast should break the fast immediately by consuming approximately 15 to 20 g of fast-acting carbohydrate. Examples include 3 to 4 glucose tablets, or 150 to 200 ml of fruit juice or a sugary (non-diet) drink. Blood glucose should be rechecked after 10 to 15 minutes; if it remains below 4.0 mmol/L, the same amount of fast-acting carbohydrate should be taken again. Once blood glucose has returned to normal, a longer-acting carbohydrate snack (such as a biscuit or slice of bread) should be eaten to help maintain levels. This approach is consistent with NHS and Diabetes UK guidance on treating mild hypoglycaemia.

For severe hypoglycaemia, where a person is unconscious or unable to swallow safely, do not attempt to give food or drink by mouth. If a glucagon kit is available and the person caring for them has been trained to use it, this may be administered. Place the person in the recovery position and call 999 immediately. This is a medical emergency.

People who use a continuous glucose monitor (CGM) should be aware that sensor readings can lag behind actual blood glucose levels, particularly during rapid changes.^[11][12] If symptoms do not match the CGM reading, a capillary finger-prick test should be used to confirm the result where it is safe to do so. Anyone experiencing recurrent episodes or uncertainty about their symptoms should contact their GP.

Is intermittent fasting safe if you take diabetes medication?

Intermittent fasting may be possible for some people with diabetes but requires medical supervision and often medicine adjustment, particularly for those taking insulin, sulphonylureas, or SGLT-2 inhibitors, before any fasting protocol begins.

Intermittent fasting can be undertaken by some people with diabetes, but it requires careful medical supervision and, in many cases, adjustment of medicines before fasting begins. The safety profile depends heavily on which medicines are being taken.

Metformin, DPP-4 inhibitors and thiazolidinediones carry a low intrinsic risk of hypoglycaemia when used alone.^[4][8] GLP-1 receptor agonists (such as semaglutide, available as Ozempic or Wegovy) also carry a lower intrinsic risk of hypoglycaemia compared with sulphonylureas or insulin, because their glucose-lowering effects are largely glucose-dependent. SGLT-2 inhibitors (such as dapagliflozin and empagliflozin) similarly have a low intrinsic risk of hypoglycaemia, but carry a separate and important risk of euglycaemic diabetic ketoacidosis (DKA) during prolonged fasting or significant caloric restriction, which the MHRA has highlighted in safety communications. Euglycaemic DKA can occur even when blood glucose appears normal or only mildly elevated. Warning signs include nausea, vomiting, abdominal pain, rapid or deep breathing, and feeling generally unwell. Anyone taking an SGLT-2 inhibitor who develops these symptoms during or after a fast should stop fasting, check for ketones if a testing kit is available, and seek urgent medical attention. The MHRA advises that SGLT-2 inhibitors should be used with caution in situations of reduced food intake.

For people taking insulin or sulphonylureas, fasting without medicine adjustment significantly increases hypoglycaemia risk. A GP or diabetes specialist should review the treatment regimen before any fasting protocol is started. Dose timing, dose reduction, and enhanced blood glucose monitoring are all strategies that may be considered, but these decisions must be made on an individual basis by a qualified clinician.

People with type 1 diabetes face a higher risk of both hypoglycaemia and DKA during fasting and should only undertake a fasting protocol with the guidance of their specialist diabetes team and with enhanced monitoring in place.^[14][15]

People with type 2 diabetes who are exploring weight management options, including dietary approaches such as intermittent fasting, may also be eligible for pharmacological support. Bolt Pharmacy is a GPhC-registered online pharmacy in Great Britain offering online weight-management consultations, with prescriber review and pharmacist support, where a range of options may be considered if clinically appropriate.

Anyone taking diabetes medication who experiences recurrent hypoglycaemia, or who is unsure whether intermittent fasting is appropriate for their situation, should contact their GP in the first instance. Suspected adverse effects from any medicine should be reported via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk. Fasting is not a substitute for prescribed treatment, and any dietary change should complement, not replace, an agreed diabetes management plan.

Scientific References

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11. Lag Time Remains with Newer Real-Time Continuous Glucose Monitoring Technology During Aerobic Exercise in Adults Living with Type 1 Diabetes.
12. The nuts and bolts of achieving end points with real-time continuous glucose monitoring.
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