Keto and Intermittent Fasting: Benefits and Risks

Keto and intermittent fasting are two of the most widely discussed dietary strategies in the UK, and many people are curious about whether combining them is safe and effective. The ketogenic diet restricts carbohydrates to shift the body into fat-burning ketosis, while intermittent fasting focuses on when you eat rather than what you eat. Together, they share overlapping metabolic effects, particularly on insulin levels and fat metabolism. This article explains how each approach works, what the evidence says about combining them, who should exercise caution, and how to get started safely with support from a GP or registered dietitian.

What are keto and intermittent fasting, and how do they work?

The keto diet restricts carbohydrates to induce ketosis, while intermittent fasting cycles between eating and fasting periods; both lower insulin levels and shift the body away from using dietary carbohydrates as its primary fuel.

The ketogenic (keto) diet and intermittent fasting are two distinct dietary strategies that each alter the body's primary fuel source, though they do so through different mechanisms. The keto diet is a very low carbohydrate, high fat eating pattern that typically restricts carbohydrate intake to around 20 to 50 grams per day, prompting the liver to produce ketone bodies from fat as an alternative energy source, a metabolic state known as ketosis.^[3][4] The British Dietetic Association (BDA) notes that carbohydrate targets vary between individuals and clinical contexts, and that a registered dietitian can help determine an appropriate level for a given person. Intermittent fasting, by contrast, is not defined by what you eat but by when you eat, cycling between periods of eating and fasting.

Common intermittent fasting patterns include:

• 16:8 (fasting for 16 hours, eating within an 8-hour window)

• 5:2 (eating normally for five days and restricting calories significantly on two non-consecutive days)

• Alternate-day fasting (alternating between normal eating days and very low calorie days)

During a fasting period, insulin levels fall and the body begins drawing on stored glycogen and then fat for energy.^[5] This process shares some metabolic overlap with ketosis, which is one reason the two approaches are often discussed together. Both strategies influence insulin sensitivity and encourage the body to shift away from relying on dietary carbohydrates for fuel. Understanding these mechanisms helps explain why some people choose to combine them, and why doing so requires careful consideration of individual health circumstances.

Can you safely combine keto and intermittent fasting?

Most healthy adults can combine keto and intermittent fasting safely, but the combination demands careful planning around protein intake, electrolytes and hydration, and is not appropriate for everyone without medical review.

Most healthy adults can safely combine keto and intermittent fasting, though the combination is more demanding than either approach alone and warrants careful planning. Because both strategies independently reduce carbohydrate availability and lower insulin levels, combining them can accelerate the transition into ketosis. Some people find this helpful for managing appetite, as ketone bodies and the hormonal changes associated with fasting can reduce hunger signals. However, the combined restriction also increases the risk of nutrient gaps, dehydration and electrolyte imbalance if not managed thoughtfully.

For people considering a combined approach, a structured plan is important. Key practical considerations include:

• Adequate protein intake to preserve lean muscle mass during fasting windows

• Electrolyte management, particularly sodium, potassium and magnesium, which are lost more readily on a keto diet due to reduced insulin and increased urinary excretion.^[3] Do not self-supplement potassium or magnesium without first discussing your medicines and kidney function with a clinician, as unsupervised supplementation can be harmful in people with kidney disease or those taking medicines such as ACE inhibitors, angiotensin receptor blockers or spironolactone

• Hydration, as both approaches can increase fluid losses

• Gradual introduction, starting with one strategy before adding the other

People taking SGLT2 inhibitors (such as dapagliflozin, empagliflozin or canagliflozin) should consult their diabetes team before attempting a very low carbohydrate diet or fasting regimen. The MHRA has issued safety advice warning that SGLT2 inhibitors can increase the risk of euglycaemic diabetic ketoacidosis, a serious condition that can occur even when blood glucose appears normal, and this risk may be heightened by significant carbohydrate restriction or prolonged fasting.^[1][2]

People already managing a health condition, taking prescription medicines, or with a history of disordered eating should seek professional advice before combining these approaches. Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, and can help direct individuals towards appropriate clinical guidance. A registered dietitian or GP can assess whether the combination is suitable for a specific individual and help design a plan that minimises risk while supporting the person's health goals.

What are the potential health benefits of doing both together?

Combining keto and intermittent fasting may improve weight management, blood glucose control and triglyceride levels, though most supporting studies are short-term and benefits vary considerably between individuals.

Combining keto and intermittent fasting may offer complementary metabolic benefits, particularly for weight management, blood glucose regulation and cardiovascular risk markers, though the evidence base is still developing and most studies are short-term. Research, including systematic reviews and meta-analyses, suggests that very low carbohydrate diets can improve glycaemic control in people with type 2 diabetes, and intermittent fasting has been associated with reductions in fasting insulin and improvements in insulin sensitivity.^[13][14] Diabetes UK's evidence-based nutrition guidance acknowledges low carbohydrate approaches as one option for adults with type 2 diabetes when undertaken with appropriate clinical support.^[6][8] When combined, these effects may be additive for some individuals, though this has not been established in large long-term trials.

Potential benefits that have been reported in the literature include:

• Weight and fat loss, particularly visceral (abdominal) fat, which is associated with cardiometabolic risk, based on findings from short-to-medium-term randomised controlled trials and systematic reviews

• Improved blood glucose and insulin levels, relevant for people with prediabetes or type 2 diabetes (under medical supervision)

• Reduced triglycerides and improved HDL cholesterol in some studies of ketogenic diets; however, LDL cholesterol can rise in some individuals on a ketogenic diet, which may be relevant for cardiovascular risk assessment and warrants lipid monitoring^[15][16]

• Appetite regulation, with many people reporting reduced hunger once adapted to both approaches

• Possible cognitive effects have been explored in early research, but evidence in humans remains limited and largely observational; no firm conclusions can be drawn at this stage

It is important to note that these benefits are not guaranteed and are influenced by individual factors including baseline health, adherence, overall dietary quality and physical activity levels. NICE guidance on weight management emphasises that sustainable lifestyle change, rather than any single dietary pattern, is the cornerstone of long-term health improvement.^[28][29] The quality of food choices within both approaches matters considerably, and a diet high in processed fats or lacking in vegetables does not become healthful simply by virtue of being low in carbohydrates.

Are there any risks or side effects I should know about?

Risks include keto flu symptoms, hypoglycaemia in people on diabetes medicines, euglycaemic diabetic ketoacidosis with SGLT2 inhibitors, nutrient deficiencies and potential worsening of disordered eating behaviours.

Both keto and intermittent fasting carry recognised side effects, and combining them can intensify some of these, particularly during the initial adaptation period. The most commonly reported short-term effects of starting a ketogenic diet include headache, fatigue, irritability, difficulty concentrating and muscle cramps, a cluster sometimes called the "keto flu". These symptoms typically arise from electrolyte shifts and reduced glycogen stores and usually resolve within one to two weeks, though they can be more pronounced when fasting is added simultaneously.

Other risks to be aware of include:

• Hypoglycaemia, particularly in people taking insulin or sulphonylureas for diabetes, where both strategies can lower blood glucose significantly and medication doses may need adjustment under medical supervision

• Euglycaemic diabetic ketoacidosis, a serious risk for people taking SGLT2 inhibitors who follow a very low carbohydrate or fasting regimen; the MHRA has issued specific safety guidance on this. Anyone on an SGLT2 inhibitor should speak to their diabetes team before making significant dietary changes

• Disordered eating patterns, as prolonged fasting windows and strict food rules can trigger or worsen restrictive eating behaviours in susceptible individuals

• Nutrient deficiencies, including fibre, B vitamins, calcium and vitamin D, if food variety is severely limited

• Kidney stone risk, which evidence suggests may be elevated with long-term ketogenic diets, particularly in those with a personal or family history; discuss this with your GP if relevant^[17]

• Constipation, due to reduced fibre intake on a very low carbohydrate diet

• Uric acid levels may rise transiently during early ketogenic adaptation, which can trigger gout flares in susceptible individuals^[18][19]

If you experience severe or sudden chest pain, chest tightness, pain radiating to your arm or jaw, or shortness of breath, call 999 immediately as these may be symptoms of a heart attack. For other urgent concerns, including severe dizziness, fainting, persistent vomiting or signs of severe dehydration, contact your GP promptly or call NHS 111 for advice.

Suspected adverse effects from any medicine taken alongside these dietary changes can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Who should avoid keto or intermittent fasting in the UK?

Pregnant women, people with type 1 diabetes, those with eating disorders, chronic kidney disease or familial hypercholesterolaemia, and anyone taking SGLT2 inhibitors, lithium or warfarin should avoid these approaches without specialist medical supervision.

Certain groups should avoid keto, intermittent fasting, or both, and should seek medical advice before making significant dietary changes. People with type 1 diabetes face a particular risk of diabetic ketoacidosis on a ketogenic diet, as the metabolic state of nutritional ketosis can be difficult to distinguish from pathological ketoacidosis without careful monitoring. Those with type 2 diabetes taking insulin, sulphonylureas or SGLT2 inhibitors require close medical supervision because these strategies can cause significant reductions in blood glucose or, in the case of SGLT2 inhibitors, increase the risk of euglycaemic diabetic ketoacidosis, necessitating medication review.

Groups for whom caution or avoidance is generally advised include:

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

• Children and adolescents, except under specialist supervision for specific medical indications such as drug-resistant epilepsy

• People with a history of eating disorders, including anorexia nervosa, bulimia or orthorexia, where restrictive dietary rules may be harmful

• Underweight individuals or frail older adults, and those experiencing unintentional weight loss, for whom further caloric or dietary restriction may be harmful and who should seek specialist advice

• Those with chronic kidney disease, as dietary changes including protein and electrolyte intake require individualised planning; the UK Kidney Association recommends dietitian-led guidance for people with CKD, as protein and mineral targets vary by stage of disease^[27]

• People with familial hypercholesterolaemia or markedly elevated LDL cholesterol, as ketogenic diets can raise LDL in some individuals; lipid monitoring and dietitian input are advisable

• People with a history of pancreatitis or gallbladder disease, as high fat diets can exacerbate these conditions

• Those taking certain medicines, including lithium, warfarin, diuretics or SGLT2 inhibitors, where dietary changes can affect drug levels, electrolyte balance or ketoacidosis risk

In the UK, a GP is the appropriate first point of contact for anyone with an existing health condition who is considering a significant dietary change. A referral to a registered dietitian, accessible through the NHS in many areas, can provide personalised, evidence-based guidance.

How to get started safely with guidance from your GP or dietitian

Start by consulting your GP or a registered dietitian to review your health status and medicines, then transition gradually, reducing carbohydrates over one to two weeks and beginning with a 12-hour fasting window before extending it.

Starting keto and intermittent fasting safely begins with a conversation with your GP or a registered dietitian, who can assess your current health status, review any medicines you take and identify any contraindications before you make changes. This step is particularly important if you have an existing medical condition, a family history of metabolic or cardiovascular disease, or if you are taking prescription medicines that interact with dietary changes. In England, NHS dietitian services are available via GP referral, and some Integrated Care Boards (ICBs) commission community weight management programmes that include dietary support.

Practical steps for a safe start include:

• Clinician-directed investigations, such as fasting glucose, HbA1c, lipid profile and kidney function tests, where your GP or clinician considers them appropriate based on your individual risk and health history; for healthy adults with no risk factors, routine blood tests may not be necessary, though they can provide a useful baseline. Monitoring weight, waist circumference and blood pressure may also be helpful

• Gradual transition, reducing carbohydrates progressively over one to two weeks rather than switching abruptly, to minimise keto flu symptoms

• Introducing fasting windows slowly, beginning with a 12-hour overnight fast before extending to 14 or 16 hours

• Keeping a food diary, which helps identify nutrient gaps and supports conversations with a dietitian

• Prioritising diet quality, including non-starchy vegetables, fibre-rich foods and unsaturated fats, consistent with UK dietary principles, even within a low carbohydrate framework

• Regular follow-up, ideally at four to six weeks, to review any blood markers and adjust the approach as needed

Reliable UK resources include the NHS website (nhs.uk), the British Dietetic Association (bda.uk.com) and Diabetes UK for those managing blood glucose conditions. Approaching these dietary strategies as long-term lifestyle adjustments, rather than short-term fixes, is associated with better outcomes and a lower risk of nutritional harm. Sustainable, gradual change supported by professional guidance remains the approach most consistent with NICE recommendations on weight management and metabolic health.

Scientific References

1. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
2. Empagliflozin 10 mg film-coated tablets – Safety Alert: SGLT2 inhibitors: advice on the risk of diabetic ketoacidosis.
3. Medical ketogenic diets.
4. Metabolism and epilepsy: Ketogenic diets as a homeostatic link.
5. Three weeks of time-restricted eating improves glucose homeostasis in adults with type 2 diabetes but does not improve insulin sensitivity: a randomised crossover trial.
6. Diabetes – Type 2.
7. A lower-carb diet is an effective short-term option for type 2 diabetes.
8. Diabetes UK evidence-based nutrition guidelines for the prevention and management of diabetes.
9. Type 2 diabetes in adults: management (NG28) – Dietary advice and interventions.
10. Effect of Intermittent Fasting Diet on Glucose and Lipid Metabolism and Insulin Resistance in Patients with Impaired Glucose and Lipid Metabolism.
11. Intermittent fasting and health outcomes: an umbrella review.
12. Effects of time-restricted eating on markers of glucose metabolism and regulation in individuals with prediabetes or type 2 diabetes: a systematic review and meta-analysis of randomised controlled trials.
13. Effect of Low-Carbohydrate Diets on Glycemic Control in Type 2 Diabetes Mellitus: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.
14. Effectiveness of low-carbohydrate diets on type 2 diabetes: A systematic review and meta-analysis of randomized controlled trials in Eastern vs. Western populations.
15. Cardiometabolic trade-offs of carbohydrate- and fat-focused diets: a network meta-analysis of randomised clinical trials.
16. Ketogenic diet-induced changes in adult lipid metabolism: a comprehensive systematic review and meta-regression of randomized controlled trials.
17. The Relationship between Modern Fad Diets and Kidney Stone Disease: A Systematic Review of Literature.
18. Association between ketogenic diet and gout among U.S. adults.
19. The effect of dietary approaches to stop hypertension and ketogenic diets intervention on serum uric acid concentration: a systematic review and meta-analysis of randomized controlled trials.
20. Continuous Ketone Monitoring Captures Overt Ketosis in Type 1 Diabetes Caused by Empagliflozin, Low-Carbohydrate Diet, and Exercise.
21. Design of a randomized cross-over study evaluating effects of carbohydrate intake on glycemic control in persons with type 1 diabetes.
22. Intractable Seizures in Children With Type 1 Diabetes: Implications of the Ketogenic Diet.
23. Ketogenic diets for drug-resistant epilepsy (NG217) – Evidence review 12.
24. Evidence review: Ketogenic diets for drug-resistant epilepsy (NG217).
25. The efficacy and safety of ketogenic diet therapy among adolescents and adults with drug resistant epilepsy: A systematic review and meta-analysis.
26. Comparative efficacy of the classical ketogenic diet and modified Atkins diet in children with drug-resistant epilepsy: A systematic review and meta-analysis.
27. Healthy Eating with CKD.
28. Overweight and obesity management (NG246).
29. Overweight and obesity management – Quality standards (QS212).

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