Intermittent Fasting and Ketosis: Key Facts

Intermittent fasting and ketosis are closely linked metabolic processes that have attracted considerable interest in the context of weight management and metabolic health. When the body is deprived of food for an extended period, glucose stores become depleted and the liver begins producing ketone bodies from fatty acids, shifting the body into a fat-burning state known as nutritional ketosis. Understanding how these two approaches interact, how quickly ketosis develops during a fast, and who should exercise caution is essential before making any significant dietary change.

What is the link between intermittent fasting and ketosis?

Intermittent fasting accelerates depletion of liver glycogen, prompting the liver to produce ketone bodies from fatty acids, inducing nutritional ketosis. This is a normal metabolic adaptation and is distinct from the dangerous condition diabetic ketoacidosis.

Intermittent fasting and ketosis are linked because prolonged periods without food deplete the body's glucose stores, prompting a metabolic switch to fat-burning that produces ketone bodies. Ketosis is a physiological state in which the liver converts fatty acids into ketones, which then serve as an alternative fuel source for the brain and other tissues.^[3][4] Intermittent fasting, whether through time-restricted eating, alternate-day fasting or extended fasting windows, accelerates the depletion of liver glycogen, making ketosis more likely to occur than it would during a standard eating pattern.

It is important to understand that ketosis in this context is nutritional ketosis, a normal metabolic adaptation, and is distinct from diabetic ketoacidosis (DKA), which is a dangerous medical emergency. The two conditions share a name but differ fundamentally in their cause, severity and clinical significance. Nutritional ketosis involves a modest, controlled rise in blood ketone levels, whereas DKA involves a severe, uncontrolled accumulation that disrupts blood pH and requires emergency treatment.^[1][2] Warning signs of DKA include abdominal pain, vomiting, deep or rapid breathing, excessive thirst, confusion and a fruity smell on the breath.^[1][2] Anyone with diabetes who develops these symptoms, or who measures high blood ketones, should call 999 or attend an accident and emergency department immediately and should follow their diabetes sick-day rules.

The relationship between fasting and ketosis has attracted growing research interest, particularly in the context of weight management, metabolic health and type 2 diabetes. However, the evidence base is still developing, and neither intermittent fasting nor ketosis is a one-size-fits-all approach. Individual responses vary considerably depending on factors such as baseline metabolic health, activity level, dietary composition and the specific fasting protocol followed.

How does your body shift into ketosis during a fast?

Fasting lowers blood glucose and insulin, triggering glycogen breakdown and then fat mobilisation; the liver converts fatty acids via beta-oxidation into three ketone bodies: beta-hydroxybutyrate, acetoacetate and acetone.

The body shifts into ketosis during a fast through a stepwise depletion of carbohydrate stores followed by an increase in fat mobilisation and hepatic ketone production. When food intake stops, blood glucose falls and insulin levels decline. The pancreas responds by increasing glucagon secretion, which signals the liver to break down stored glycogen into glucose to maintain blood sugar levels. Once glycogen stores are substantially depleted, which may occur at varying points depending on the individual, their prior diet and activity level, the liver begins to draw on fatty acids released from adipose tissue.

These fatty acids undergo a process called beta-oxidation in the liver, generating acetyl-CoA. When acetyl-CoA production exceeds the capacity of the citric acid cycle, the liver channels the excess into ketone body synthesis, producing three main ketones:

• Beta-hydroxybutyrate, the predominant circulating ketone

• Acetoacetate, which can be converted to beta-hydroxybutyrate or spontaneously decarboxylated

• Acetone, a minor by-product exhaled in the breath

Low insulin levels are central to this process because insulin normally suppresses both lipolysis and ketogenesis.^[3][4] As insulin falls during fasting, both pathways are disinhibited, allowing fat stores to be mobilised and ketones to accumulate in the blood. The brain, which ordinarily relies almost exclusively on glucose, gradually increases its uptake of beta-hydroxybutyrate, reducing its dependence on hepatic glucose output. This capacity for metabolic flexibility is considered by many researchers to be an adaptation to periods of food scarcity, though the precise evolutionary basis remains a subject of ongoing study.

How long does intermittent fasting take to trigger ketosis?

Timing varies by individual, but mild ketosis may develop towards the end of a 16-hour fast, while extended fasts of 24 hours or more most reliably produce measurable blood ketone levels.

The timing of the transition into nutritional ketosis varies considerably between individuals and depends on habitual carbohydrate intake, physical activity levels, body composition and individual metabolic rate. Someone who regularly follows a low-carbohydrate diet may enter ketosis more quickly because their glycogen stores are already partially depleted, whereas someone accustomed to a high-carbohydrate diet may take longer. As a general guide, mild ketosis may begin to develop after an extended overnight or daytime fast, but the precise point differs from person to person.

Common intermittent fasting protocols and their likely relationship to ketosis include:

• 16:8 fasting (16 hours fasting, 8 hours eating): may produce mild, transient ketosis towards the end of the fasting window, particularly overnight

• 5:2 fasting (two very low-calorie days per week): may induce brief ketosis on restricted days, depending on calorie and carbohydrate intake

• Extended fasts of 24 hours or more: more reliably produce measurable ketosis, with blood ketone levels rising progressively

Blood ketone levels can be measured using a finger-prick ketone meter, which provides the most accurate assessment. Urine ketone strips are widely available but become less reliable once the body adapts to using ketones efficiently, as urinary excretion decreases.^[3] Breath acetone monitors offer a non-invasive alternative, though they are less precise. People with diabetes who feel unwell during a fast should use a blood ketone meter rather than urine strips, and should follow their diabetes sick-day rules.

Brief, mild ketosis during an overnight fast is a normal physiological occurrence for many people and does not necessarily indicate a significant metabolic shift. Sustained nutritional ketosis, as sought by those following a ketogenic diet alongside fasting, typically requires more deliberate dietary restriction of carbohydrates. Fasting beyond 24 to 36 hours carries additional risks, including refeeding complications when eating resumes, and should not be undertaken without clinical supervision.^[6][7] Anyone considering extended fasting should discuss this with their GP or a registered dietitian beforehand.

Is combining intermittent fasting with a ketogenic diet safe?

Combining the two approaches is generally well tolerated in healthy adults but carries risks including electrolyte imbalance, raised LDL cholesterol, gallstones and, in those on SGLT2 inhibitors, euglycaemic diabetic ketoacidosis.

Combining intermittent fasting with a ketogenic diet is generally considered safe for healthy adults, and the two approaches are often used together because they share a common mechanism of lowering insulin and promoting fat oxidation. A ketogenic diet restricts carbohydrate intake substantially, keeping glycogen stores low, meaning that even a relatively short fasting window can sustain or deepen ketosis. This combination may accelerate the transition into ketosis compared with either approach used alone.

However, the combination is not without potential drawbacks, and these should be considered carefully:

• Electrolyte imbalance: both fasting and ketogenic eating increase urinary excretion of sodium, potassium and magnesium, which can cause symptoms such as muscle cramps, fatigue and headaches, sometimes called the "keto flu"^[3]

• Caloric restriction: the appetite-suppressing effects of ketosis combined with a restricted eating window may lead to unintentional under-eating, which can affect energy levels and, over time, lean muscle mass

• Fibre and micronutrient intake: restricting carbohydrates can reduce dietary fibre and certain micronutrients; adequate fluid intake, a variety of non-starchy vegetables and, for longer-term use, dietitian input are advisable

• Sustainability: the combined approach is highly restrictive and may be difficult to maintain long-term, potentially leading to cycles of adherence and abandonment

• Lipid profile changes: some individuals experience rises in LDL cholesterol on a ketogenic diet, and this warrants monitoring, particularly in those with existing cardiovascular risk factors^[14][15]

• Gallstone risk: rapid weight loss, which can occur with this combination, is associated with an increased risk of gallstones.^[12][13] Anyone who develops right upper abdominal pain, fever or yellowing of the skin or eyes should seek prompt medical attention

• SGLT2 inhibitors: people taking SGLT2 inhibitor medicines (such as dapagliflozin or empagliflozin) face an increased risk of euglycaemic diabetic ketoacidosis when following a low-carbohydrate or fasting regimen. This can occur even when blood glucose appears normal. Anyone taking an SGLT2 inhibitor should discuss any planned dietary changes with their diabetes care team before starting

For most healthy adults without underlying medical conditions, short-term use of this combined approach appears to be well tolerated. Those considering it for longer periods, or who have any pre-existing health conditions, should discuss the approach with their GP before starting. Adequate hydration and attention to micronutrient intake are important practical considerations.

Who should be cautious about fasting and ketosis in the UK?

People with type 1 or type 2 diabetes, those on SGLT2 inhibitors or sulfonylureas, pregnant women, children, and those with eating disorders, kidney or liver disease should seek GP advice before fasting.

Several groups of people in the UK should exercise particular caution before attempting intermittent fasting or pursuing nutritional ketosis, and some should avoid these approaches entirely without close medical supervision. People living with type 1 diabetes face a significant risk of hypoglycaemia and, in some circumstances, diabetic ketoacidosis during fasting, and should not attempt extended fasting or ketogenic eating without specialist guidance from their diabetes care team. Those with type 2 diabetes who are taking insulin or sulfonylureas (such as gliclazide) are also at risk of hypoglycaemia and require medication review before changing their dietary pattern substantially. Insulin or sulfonylurea doses should never be stopped or adjusted without clinical advice.

Other groups who should seek GP advice before starting include:

• People taking SGLT2 inhibitors (such as dapagliflozin or empagliflozin), who face an increased risk of euglycaemic diabetic ketoacidosis with low-carbohydrate or fasting approaches, even when blood glucose appears normal

• People with a history of eating disorders, as fasting protocols may reinforce restrictive behaviours

• Those with kidney disease, as high protein intake sometimes associated with ketogenic diets can increase the burden on the kidneys

• People with liver conditions, since the liver plays a central role in ketone production and may be affected by rapid metabolic changes

• Individuals taking certain medications, including anticoagulants, antihypertensives and diuretics, where dietary changes can alter drug efficacy or safety

• People with a history of gallstones or gout, as fasting can precipitate flares of gout and may worsen gallstone-related symptoms

• Those who are underweight (BMI below 18.5) or at risk of malnutrition or frailty, for whom caloric restriction carries additional risks

• Pregnant and breastfeeding women, for whom caloric restriction and ketosis are not recommended due to the nutritional demands of pregnancy and lactation

• Children and adolescents, who have different nutritional requirements and should not follow adult fasting protocols

Older adults should also be cautious, as fasting may increase the risk of muscle loss (sarcopenia) and falls.^[16] Anyone who experiences dizziness, palpitations, confusion or persistent nausea during a fast should stop and contact their GP or call NHS 111 for advice. If DKA is suspected, including symptoms such as abdominal pain, vomiting, deep or rapid breathing, confusion or high blood ketones, call 999 or attend an accident and emergency department immediately.

What do NHS and NICE guidelines say about low-carb fasting approaches?

NICE and the NHS do not specifically endorse intermittent fasting or ketogenic diets for the general population, though NICE guidance on type 2 diabetes acknowledges low-carbohydrate options within individualised care plans.

NICE and the NHS have not issued specific guidance endorsing intermittent fasting or ketogenic diets as standard treatments for the general population, though both bodies acknowledge the growing evidence base for low-carbohydrate approaches in certain clinical contexts. NICE guidance on type 2 diabetes (NG28) recognises that healthcare professionals may discuss a range of dietary approaches with patients, including low-calorie and low-carbohydrate options, as part of an individualised care plan.^[11] NHS England has supported structured low-calorie diet programmes for type 2 diabetes remission through the Type 2 Diabetes Path to Remission Programme. However, these programmes are delivered under medical supervision and are distinct from self-directed fasting or ketogenic eating.

For weight management more broadly, NICE has published guidance that focuses on behavioural, dietary and pharmacological interventions and does not specifically recommend intermittent fasting or ketosis as first-line strategies. The NHS Eatwell Guide, which underpins general dietary advice in England, promotes a balanced diet that includes carbohydrates and does not advocate carbohydrate restriction for the general population. This does not mean low-carbohydrate or fasting approaches are contraindicated, but it does mean they fall outside the standard dietary framework recommended for most people.

For individuals interested in medically supervised weight management, a range of options exists, including structured lifestyle programmes, dietitian-led support and, where clinically appropriate, pharmacological treatment. Bolt Pharmacy is a GPhC-registered online pharmacy in Great Britain offering online weight-management consultations, with prescriber review and pharmacist support, where a prescriber may consider appropriate treatment options following a clinical assessment.

Anyone considering intermittent fasting or a ketogenic diet as part of managing a health condition should discuss this with their GP in the first instance, who can refer to a registered dietitian or specialist service where needed. Suspected adverse effects from any dietary supplement used alongside fasting can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Scientific References

1. Diabetic ketoacidosis.
2. Diabetic hyperglycaemic emergencies | Treatment summaries.
3. Ketone bodies: a review of physiology, pathophysiology and application of monitoring to diabetes (Laffel, 1999).
4. Lipolysis-Driven Communication Between Adipose Tissue and Liver: A Key Axis in Metabolic Regulation. Journal of Lipid Research.
5. Hepatic lipid accumulation: cause and consequence of dysregulated glucoregulatory hormones. The Journal of Endocrinology.
6. Understanding Refeeding Syndrome in Critically Ill Patients: A Narrative Review. Nutrients.
7. Combined Assessment of Gastrointestinal Hormones and Metabolomic Profiling Following Mixed-Meal Tolerance Tests in Patients at Increased Risk of Refeeding Syndrome: Study Protocol. Journal of Personalized Medicine.
8. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
9. Life-threatening dapagliflozin-associated euglycaemic diabetic ketoacidosis in a postoperative patient. BMJ Case Reports.
10. Clinical and biochemical characteristics and analysis of risk factors for euglycaemic diabetic ketoacidosis in type 2 diabetic individuals treated with SGLT2 inhibitors: A review of 72 cases. Diabetes & Metabolic Syndrome.
11. Type 2 diabetes in adults: management (NG28) – Dietary advice and interventions.
12. Prevention of Gallstones After Bariatric Surgery using Ursodeoxycholic Acid: A Narrative Review of Literatures. Journal of Metabolic and Bariatric Surgery.
13. Nutritional intervention in the control of gallstones and renal lithiasis. Nutricion Hospitalaria.
14. Cardiometabolic trade-offs of carbohydrate- and fat-focused diets: a network meta-analysis of randomised clinical trials. European Journal of Nutrition.
15. Comparative effect of dietary patterns on selected cardiovascular risk factors: A network study. Scientific Reports.
16. The Effects of Time Restricted Feeding on Overweight, Older Adults: A Pilot Study. Nutrients.
17. Phosphate Excretion Differentiates the Amount of Nephroprotective Effect of Amino Acid Ketoanalogues Treatment with Low Protein Diet in Chronic Kidney Disease. Journal of Clinical Medicine.

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