Ketones and Intermittent Fasting: A UK Guide

Ketones and intermittent fasting are closely linked: when you fast, your body shifts from burning glucose to burning fat, producing ketone molecules as an alternative fuel. This metabolic switch is a normal physiological response, but understanding how ketones are produced, what raised levels feel like, and when they become a safety concern is essential before starting any fasting regimen. This article explains the science behind ketogenesis, how different fasting patterns affect ketone levels, how to measure them at home in the UK, and when to seek medical advice, including important guidance for people with diabetes or those taking certain medicines.

What are ketones and why does your body produce them?

Ketones are produced by the liver when glucose is insufficient, converting fatty acids into acetoacetate, beta-hydroxybutyrate and acetone as an alternative fuel. This process is a normal metabolic adaptation, distinct from the dangerous condition of diabetic ketoacidosis.

Ketones are water-soluble molecules produced by the liver when the body breaks down fat for energy in the absence of sufficient glucose. Under normal circumstances, glucose derived from dietary carbohydrates is the body's preferred fuel. When glucose availability falls, whether through fasting, carbohydrate restriction or prolonged exercise, the liver converts fatty acids into three ketone bodies: acetoacetate, beta-hydroxybutyrate and acetone.^[17][18] These molecules are then transported in the bloodstream and used as an alternative energy source by the brain, heart and muscles.

This process, known as ketogenesis, is a normal and well-regulated metabolic adaptation. It is not inherently dangerous in healthy individuals and represents an evolutionary mechanism that allowed humans to survive periods of food scarcity. The brain, which cannot directly use fatty acids for fuel, relies heavily on ketones during prolonged fasting.

It is important to distinguish between the mild to moderate ketosis that occurs during fasting and the pathological condition known as diabetic ketoacidosis (DKA). DKA occurs almost exclusively in people with type 1 diabetes or, less commonly, type 2 diabetes, and involves dangerously high ketone levels alongside high blood glucose.^[1] Nutritional ketosis from fasting involves far lower ketone concentrations and is accompanied by normal or low blood glucose levels. A separate and less well-known variant, euglycaemic DKA, can occur with near-normal blood glucose levels, most notably in people taking SGLT2 inhibitors; this is discussed further in the safety section below.^[2][3] Understanding these distinctions is essential before interpreting any home ketone readings.

How does intermittent fasting raise ketone levels?

Intermittent fasting depletes liver glycogen within 12 to 16 hours, causing insulin to fall and the liver to accelerate fat breakdown and ketone production. Ketone levels typically peak between 18 and 24 hours of fasting.

Intermittent fasting raises ketone levels by depleting liver glycogen stores, which prompts the body to switch from glucose to fat as its primary fuel source. During a fasting window, insulin levels fall and glucagon rises, signalling the liver to begin breaking down stored glycogen. Once glycogen is largely exhausted, typically after around 12 to 16 hours of fasting in most adults, the liver accelerates fatty acid oxidation and ketone production increases measurably. Ketone levels may continue to rise and often peak somewhere between 18 and 24 hours of fasting, though this varies considerably between individuals depending on metabolic rate, physical activity and prior diet. These timelines are approximate and are drawn from physiological research into fasting metabolism; individual responses differ widely.

Common intermittent fasting patterns in the UK include the 16:8 method (16 hours fasting, 8 hours eating), the 5:2 approach (two days of significant calorie restriction per week) and alternate-day fasting. The degree of ketosis achieved depends on the duration of the fast, individual metabolic rate, physical activity levels and the composition of meals consumed during eating windows. Higher carbohydrate intake during eating periods tends to blunt ketone production.

Regular fasting can also improve the body's efficiency at entering ketosis over time. This is sometimes described as metabolic flexibility, meaning the body's ability to switch more readily between glucose and fat as fuel sources. It is worth noting that the evidence base for specific health outcomes associated with intermittent fasting-induced ketosis is still developing. At the time of writing, NICE has not published specific clinical guidance on intermittent fasting as an intervention for the general population, though this may be subject to change; checking the NICE website directly is advisable for the most current position.^[7] The British Dietetic Association (BDA) has published a Food Fact Sheet on intermittent fasting that provides a useful UK patient-facing overview.^[8][9]

What do raised ketone levels actually feel like?

Raised ketones can cause initial symptoms including headaches, fatigue, nausea and bad breath, often called 'keto flu', which usually resolve within a few days to a week. Some people also report improved mental clarity and reduced hunger once adapted.

Raised ketone levels during intermittent fasting can produce a range of physical sensations, some of which people find beneficial and others uncomfortable, particularly in the early stages. Many individuals report increased mental clarity and sustained energy once they have adapted to fasting, which is thought to reflect the brain's use of beta-hydroxybutyrate as fuel. Some people also report reduced hunger between meals; this has been attributed in part to the effects of ketones and falling insulin levels on appetite signalling, though the evidence in humans is mixed and this should be regarded as a reported observation rather than an established effect.

However, the transition into ketosis can initially cause symptoms sometimes referred to informally as the "keto flu". These may include:

• Headaches and difficulty concentrating

• Fatigue or low energy, particularly in the first few days

• Irritability or mood changes

• Nausea or mild digestive discomfort

• Bad breath, caused by acetone being exhaled through the lungs

• Muscle cramps, often related to electrolyte shifts

These symptoms are generally transient and tend to resolve within a few days to a week as the body adapts. Staying well hydrated and ensuring adequate electrolyte intake (sodium, potassium and magnesium) can help manage them. However, people with kidney disease, heart failure or those taking medicines that affect electrolyte balance (such as diuretics or certain blood pressure medications) should not increase their electrolyte intake without first seeking medical advice, as doing so may be harmful in these groups.

Not everyone experiences these symptoms, and individual responses to fasting vary considerably. If symptoms are severe or persistent, this warrants a conversation with a GP rather than continuing the fasting regimen without review.

Are high ketones during fasting safe for everyone?

Mild to moderate ketosis is safe for most healthy adults, but people with type 1 diabetes, those on insulin, sulfonylureas or SGLT2 inhibitors, and pregnant women face meaningful risks and should seek medical advice first.

Mild to moderate ketosis from intermittent fasting is considered safe for most healthy adults, but there are important groups for whom fasting and raised ketones carry meaningful risks. People with type 1 diabetes are at risk of diabetic ketoacidosis even during short fasting periods, particularly if insulin doses are not carefully adjusted. Anyone with type 1 diabetes considering intermittent fasting should only do so under close specialist supervision, in line with guidance from Diabetes UK and clinical diabetes teams.^[1]

People with type 2 diabetes, especially those taking insulin or sulfonylureas such as gliclazide, face a risk of hypoglycaemia during fasting windows.^[13][14] SGLT2 inhibitors (such as dapagliflozin or empagliflozin) can independently raise ketone levels and may increase the risk of euglycaemic DKA, a condition where ketones reach dangerous levels even without high blood glucose.^[2][3] The MHRA has issued a Drug Safety Update on this risk, and people taking SGLT2 inhibitors are generally advised to follow sick-day rules, which may include temporarily stopping the medicine during periods of reduced food intake or acute illness; this should be discussed with a GP or diabetes team before making any changes.^[2][3] If someone taking an SGLT2 inhibitor feels unwell during fasting, they should seek prompt medical advice even if their blood glucose appears normal.

Additional groups who should seek medical advice before attempting intermittent fasting include:

• Pregnant or breastfeeding women, for whom fasting is not recommended

• Children and adolescents, whose nutritional needs differ from adults

• People with a history of eating disorders, as fasting may exacerbate disordered eating patterns

• Those with kidney or liver disease, where ketone metabolism may be impaired

• People who are underweight or malnourished

For otherwise healthy adults without these risk factors, intermittent fasting and the associated mild ketosis are generally well tolerated. Nevertheless, it is always sensible to discuss any significant dietary change with a GP or registered dietitian, particularly if you have an existing health condition or take regular medication.

How can you measure your ketone levels at home in the UK?

Blood ketone meters measuring beta-hydroxybutyrate are the most accurate home option, with readings of 0.5 to 3.0 mmol/L indicating nutritional ketosis. Urine strips and breath analysers are alternatives but are less reliable.

Ketone levels can be measured at home in the UK using three main methods, each with different levels of accuracy, convenience and cost. The most accessible option is urine ketone testing strips, which are widely available in UK pharmacies without a prescription. These strips detect acetoacetate in the urine and provide a rough indication of ketosis, typically displayed as a colour change. They are inexpensive but less reliable as a person becomes more adapted to fasting, because the body becomes more efficient at using ketones rather than excreting them.

Blood ketone meters offer greater accuracy and measure beta-hydroxybutyrate directly from a fingertip blood sample, in a similar way to blood glucose monitoring. Devices and compatible test strips are available from UK pharmacies and online retailers. Blood ketone readings are generally interpreted as follows, based on guidance from Diabetes UK and clinical diabetes resources:

• Below 0.5 mmol/L: not in ketosis

• 0.5 to 3.0 mmol/L: nutritional ketosis, typical of fasting

• Above 3.0 mmol/L: warrants caution and medical review, particularly in people with diabetes

People with diabetes should follow the ketone testing thresholds and actions set out in their individual care plan or by their diabetes team, as these may differ from the general ranges above.

Breath ketone analysers measure exhaled acetone and are a non-invasive alternative, though they tend to be more expensive and their accuracy can vary between devices. Importantly, breath meters are not suitable for diagnosing or excluding DKA. If you feel unwell, particularly if you have diabetes or take an SGLT2 inhibitor, you should seek urgent medical advice regardless of what a breath or urine reading shows; a blood ketone test and clinical assessment are needed in that situation.

For most people using intermittent fasting for general health or weight management, routine ketone monitoring is not necessary. Testing is more relevant for those with diabetes or other metabolic conditions who need to ensure their ketone levels remain within a safe range. If you are unsure which method is appropriate for your circumstances, a pharmacist or GP can offer guidance.

When should you speak to a GP about ketones and fasting?

Seek urgent medical advice if you experience vomiting, severe abdominal pain, rapid breathing or confusion during fasting, especially if you have diabetes or take an SGLT2 inhibitor. Also consult a GP before starting fasting if you take prescription medication or have a relevant medical history.

You should speak to a GP about ketones and fasting if you experience symptoms that could indicate dangerously elevated ketone levels, particularly if you have diabetes or another metabolic condition. Symptoms that require prompt medical attention include persistent vomiting, severe abdominal pain, rapid or laboured breathing, confusion or difficulty staying alert, and a fruity or acetone-like smell on the breath that is accompanied by feeling unwell. If these occur, contact NHS 111 for urgent advice or attend A&E if symptoms are severe or deteriorating rapidly.

People taking SGLT2 inhibitors should be aware that DKA can occur with near-normal blood glucose levels. If you take one of these medicines and feel unwell during or after fasting, seek medical advice promptly even if your blood sugar appears normal.

You should also speak to your GP before starting intermittent fasting if you take any prescription medication, especially insulin, sulfonylureas or SGLT2 inhibitors, as dose adjustments may be needed. A GP review is equally important if you have a history of kidney disease, liver disease, an eating disorder, or if you are pregnant or breastfeeding.

More generally, a GP or registered dietitian is the right first point of contact if:

• Home ketone readings are consistently above 3.0 mmol/L during fasting

• Symptoms of the transition to ketosis are not resolving after one to two weeks

• You are losing weight unexpectedly or more rapidly than intended

• You feel unwell in a way that does not fit the typical early fasting experience

Healthcare professionals can also help you assess whether intermittent fasting is appropriate for your individual circumstances and refer you to a dietitian for personalised nutritional support. Suspected adverse effects from any medicine taken alongside fasting can be reported to the MHRA via the Yellow Card scheme at yellowcard.^[19][20]mhra.gov.uk. Approaching fasting with informed, professional support is the safest way to explore its potential benefits.

Scientific References

1. Diabetic ketoacidosis.
2. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
3. SGLT2 inhibitors: monitor ketones in blood during treatment interruption for surgical procedures or acute serious medical illness.
4. SGLT2 inhibitors – referral.
5. Empagliflozin 10 mg film-coated tablets – safety alert (SGLT2 inhibitors: advice on the risk of diabetic ketoacidosis).
6. Dapagliflozin – drug monograph.
7. Overweight and obesity management (NG246).
8. All Food Fact Sheets – British Dietetic Association.
9. Fad diets – British Dietetic Association.
10. Breath acetone is a reliable indicator of ketosis in adults (Musa-Veloso et al.).
11. Measuring breath acetone for monitoring fat loss: Review (Obesity, Silver Spring).
12. Update on Measuring Ketones (Journal of Diabetes Science and Technology).
13. Is Time-Restricted Eating Safe in the Treatment of Type 2 Diabetes? – A Review of Intervention Studies (Nutrients).
14. Impact of Ramadan Diurnal Intermittent Fasting on Hypoglycemic Events in Patients With Type 2 Diabetes (Frontiers in Endocrinology).
15. Intermittent fasting and 'metabolic switch': Effects on metabolic syndrome, prediabetes and type 2 diabetes (Diabetes, Obesity & Metabolism).
16. The catabolic triad: fasting, major cardiac surgery and SGLT2 inhibitors leading to perioperative euglycaemic ketoacidosis (Interactive Cardiovascular and Thoracic Surgery).
17. Endocrine regulation of the hepatic fasting response: cues, cooperation and consequences (Nature Reviews Endocrinology).
18. The Multifaceted Influence of Beta-Hydroxybutyrate on Autophagy, Mitochondrial Metabolism, and Epigenetic Regulation (Journal of Cellular Biochemistry).
19. The Yellow Card scheme: guidance for healthcare professionals, patients and the public.
20. Adverse reactions to drugs – BNF medicines guidance.

Frequently Asked Questions