The danger of intermittent fasting is a topic that deserves careful, evidence-based consideration before anyone embarks on this increasingly popular dietary approach. Intermittent fasting — which alternates defined periods of eating and fasting — has gained significant traction across the UK, promoted through social media and wellness platforms as a tool for weight management and metabolic health. Whilst it may offer benefits for some healthy adults, it carries real risks that are frequently underplayed in popular discourse. From medication interactions and hypoglycaemia to disordered eating and hormonal disruption, understanding who should and should not fast is essential for safe practice.

What Is Intermittent Fasting and How Is It Practised in the UK?

Intermittent fasting alternates defined eating and fasting periods; common UK methods include 16:8, 5:2, and alternate-day fasting. Adequate hydration must always be maintained, and unsupervised fasting carries specific risks not always communicated in popular media.

Intermittent fasting (IF) is a dietary pattern that alternates between defined periods of eating and fasting. Rather than specifying which foods to eat, it focuses on when to eat them. Over recent years, it has grown considerably in popularity across the UK, promoted widely through social media, wellness platforms, and mainstream press as a strategy for weight management and metabolic health. NHS Better Health acknowledges IF as an approach that may help some people, whilst noting it is not suitable for everyone.^[1]

The most commonly practised methods in the UK include:

• 16:8 method – fasting for 16 hours and eating within an 8-hour window each day

• 5:2 diet – eating normally for five days and restricting calorie intake to approximately 500–600 kcal on two non-consecutive days^[1]

• Alternate-day fasting – alternating between normal eating days and very low-calorie or complete fasting days

• 24-hour fasts – abstaining from food for one or two non-consecutive days per week; fluids (water, plain tea, black coffee) must be continued throughout

It is important to emphasise that IF does not mean restricting fluids. Adequate hydration must be maintained at all times; so-called 'dry fasting' — avoiding both food and water — is unsafe and should never be attempted.

Whilst some individuals adopt IF under medical supervision, many do so independently, without professional guidance. This is an important distinction, as unsupervised fasting carries specific risks that are not always communicated clearly in popular media. The approach is not inherently dangerous for healthy adults, but it is far from universally appropriate. Understanding the potential dangers of intermittent fasting is essential before beginning any such regimen, particularly for those with pre-existing health conditions or who are taking regular medications.

Recognised Health Risks and Side Effects of Intermittent Fasting

Intermittent fasting can cause headaches, fatigue, dizziness, acid reflux, and disrupted sleep, with longer-term risks including muscle loss, gallstone formation, gout flares, and disordered eating behaviours in susceptible individuals.

Although intermittent fasting may offer benefits for some individuals, it is associated with a range of recognised side effects and potential health risks. Many of these arise from the physiological effects of calorie restriction and altered eating patterns.

Common short-term side effects include:

• Headaches and difficulty concentrating

• Fatigue, irritability, and low mood

• Dizziness or light-headedness, particularly when standing

• Nausea, heartburn, acid reflux, and gastrointestinal discomfort

• Constipation (particularly if fibre and fluid intake are reduced)

• Halitosis (bad breath)

• Disrupted sleep patterns

• Intense hunger and preoccupation with food

These symptoms often occur during the initial adaptation period but may persist in some individuals. More concerning are the longer-term risks associated with sustained or extreme fasting practices. Prolonged calorie restriction — particularly when protein intake is insufficient — can contribute to muscle mass loss (sarcopenia), which is of particular concern in older adults.^[3] Bone health may also be affected, primarily in the context of low energy availability and associated hormonal disruption (see functional hypothalamic amenorrhoea, below), rather than from fasting alone.

Rapid weight loss associated with very restrictive fasting patterns may increase the risk of gallstone formation, and can trigger gout flares in susceptible individuals due to changes in uric acid metabolism.^[4] These risks are worth discussing with a GP before starting.

Intermittent fasting has also been linked to disordered eating behaviours in susceptible individuals. The rigid structure of fasting windows can trigger or exacerbate binge eating, food anxiety, and obsessive dietary thinking. NICE guideline NG69 on eating disorders highlights the importance of recognising restrictive eating patterns as potential precursors to or features of clinical eating disorders.^[2] Evidence on the effects of meal skipping on cognitive performance and mood in adults is mixed; any impact is likely to vary between individuals and contexts, and should not be overstated.

If you are concerned that your relationship with food or eating may be affected, speak to your GP promptly. You can also contact Beat (the UK's eating disorder charity) on 0808 801 0677 or at beateatingdisorders.org.uk.

Who Should Avoid Intermittent Fasting on Medical Grounds?

Intermittent fasting is contraindicated in pregnant or breastfeeding women, children, frail older adults, those with eating disorder histories, people who are underweight, and those with conditions such as type 1 diabetes, advanced kidney disease, or active cancer.

Intermittent fasting is not appropriate for everyone, and certain groups face significantly elevated risks. Healthcare professionals, including GPs and registered dietitians, generally advise against intermittent fasting in the following populations:

• Pregnant and breastfeeding women – calorie restriction during pregnancy or lactation can compromise foetal development and milk production; tailored dietary advice from an NHS dietitian is recommended

• Children and adolescents – growing bodies require consistent nutritional intake; fasting may impair development and establish unhealthy relationships with food

• Frail older adults or those at risk of malnutrition or sarcopenia – rather than applying a strict age cut-off, the key concern is frailty, low muscle mass, and nutritional vulnerability; unsupervised fasting in this group is inadvisable

• Individuals with a history of eating disorders – including anorexia nervosa, bulimia nervosa, or binge eating disorder, where fasting may precipitate relapse (see NICE NG69 for recognition and referral pathways; Beat helpline: 0808 801 0677)

• People who are underweight or malnourished – further calorie restriction is clinically contraindicated

• Those with certain chronic conditions – including type 1 diabetes, advanced kidney disease, liver disease, or active cancer

• People with a history of gallstones – rapid weight loss may increase the risk of further gallstone-related complications

• People with gout – fasting-related metabolic changes may precipitate acute flares

• People taking SGLT2 inhibitors (e.g., dapagliflozin, empagliflozin, canagliflozin) – see the medications section below for important safety information

It is also worth noting that individuals with a history of disordered eating may not always self-identify as such, making it important for healthcare professionals to screen carefully before endorsing any fasting regimen. The psychological dimension of intermittent fasting is frequently overlooked in popular discourse, yet it represents one of the most significant dangers for vulnerable individuals.

Anyone considering intermittent fasting who falls into one or more of the above categories should seek advice from their GP or a registered dietitian before making any dietary changes. Self-directed fasting in these groups carries a meaningful risk of harm.

Impact on Medications, Blood Sugar, and Existing Health Conditions

Fasting significantly increases hypoglycaemia risk with insulin and sulphonylureas, raises DKA risk with SGLT2 inhibitors, and can affect the safety of warfarin, lithium, antihypertensives, and NSAIDs; always review medicines with a GP or pharmacist before fasting.

One of the most clinically significant dangers of intermittent fasting relates to its interaction with medications and pre-existing health conditions. Fasting alters the timing and volume of food intake, which can have direct consequences for drug absorption, efficacy, and safety. Before starting any fasting regimen, check the patient information leaflet (PIL) for each of your medicines, or speak to your pharmacist or GP.

Medications particularly affected include:

• Insulin and sulphonylureas (e.g., gliclazide, glibenclamide) – fasting significantly increases the risk of hypoglycaemia (low blood sugar) in people with type 1 or type 2 diabetes. The Summary of Product Characteristics (SmPC) for sulphonylureas explicitly warns that missed or irregular meals increase hypoglycaemia risk.^[6] Diabetes UK advises that any dietary changes in people on insulin or insulin-stimulating agents must be discussed with a clinician before implementation, and that medication doses may require adjustment under medical supervision.

• SGLT2 inhibitors (e.g., dapagliflozin, empagliflozin, canagliflozin) – the MHRA has issued a Drug Safety Update warning that SGLT2 inhibitors are associated with a risk of diabetic ketoacidosis (DKA), including euglycaemic DKA (where blood glucose may not be markedly elevated).^[7][8] Low calorie intake, dehydration, and illness — all of which may occur during fasting — increase this risk. People taking SGLT2 inhibitors should seek clinical review before fasting and should be familiar with their sick-day rules, which may include temporary cessation of the medicine.

• Metformin – typically taken with food to reduce gastrointestinal side effects; fasting may worsen nausea and discomfort.

• Anticoagulants (e.g., warfarin) – significant changes in dietary vitamin K intake (found in green leafy vegetables) can affect INR stability. The key principle is to keep vitamin K intake as consistent as possible rather than to avoid these foods. Anyone taking warfarin should discuss any major dietary change with their anticoagulation clinic before proceeding.

• Antihypertensives, particularly diuretics and alpha-blockers – dehydration and reduced food intake during fasting may lower blood pressure further, increasing the risk of postural (orthostatic) hypotension and falls. This risk is most pronounced with diuretics and alpha-blockers.

• Lithium – dehydration, which can occur during fasting, may increase lithium plasma concentrations and the risk of toxicity. People taking lithium should maintain adequate fluid intake and seek medical advice before fasting.

• NSAIDs and corticosteroids – these should generally be taken with food; fasting may increase the risk of gastric irritation or ulceration.

For individuals with type 2 diabetes, intermittent fasting may affect glycaemic control unpredictably. Blood glucose monitoring should be intensified if fasting is undertaken, and medication doses may require adjustment under medical supervision. People with cardiovascular disease, epilepsy, or adrenal insufficiency should also exercise particular caution.

If you experience a suspected side effect from a medicine or medical device, you can report it to the MHRA via the Yellow Card Scheme at yellowcard.mhra.gov.uk.

Warning Signs That Require Medical Attention

Fainting, chest pain, severe confusion, or persistent vomiting during fasting require immediate 999 or A&E attendance; amenorrhoea, obsessive food thoughts, or persistent fatigue should prompt urgent GP review.

Whilst mild side effects such as hunger and fatigue are common during the early stages of intermittent fasting, certain symptoms should prompt immediate or urgent medical review. If significant symptoms develop, stop fasting, rehydrate, and eat something if it is safe to do so.

Call 999 or go to A&E immediately if you experience:

• Fainting or loss of consciousness – may indicate severe hypoglycaemia or dehydration

• Chest pain or palpitations – electrolyte imbalances caused by prolonged fasting or severe dehydration can affect cardiac rhythm

• Severe confusion or difficulty speaking – potential signs of hypoglycaemia or serious neurological compromise

• Persistent vomiting – may lead to dangerous dehydration and electrolyte disturbance

Contact NHS 111 or your GP urgently if you experience:

• Severe dizziness or inability to stand safely – risk of falls, particularly in older adults

• Symptoms of hypoglycaemia (shakiness, sweating, rapid heartbeat, confusion) that do not resolve promptly with food or glucose; people with diabetes should follow their personalised hypoglycaemia management plan and NHS guidance on recognising and treating low blood sugar

Contact your GP if you notice:

• Significant unintentional weight loss beyond your intended goal

• Worsening mood, anxiety, or obsessive thoughts about food

• Irregular menstrual cycles or cessation of periods (amenorrhoea)

• Persistent fatigue that does not resolve after the initial adaptation period

• Hair thinning or loss, which may indicate nutritional deficiency

Amenorrhoea in women of reproductive age is a particularly important warning sign. It may indicate that calorie restriction has become severe enough to disrupt the hypothalamic-pituitary-ovarian axis — a condition known as functional hypothalamic amenorrhoea (FHA). FHA has implications for bone health and fertility and should be assessed by a GP. Similarly, any signs consistent with an eating disorder — such as extreme food restriction beyond the fasting window, purging, or significant psychological distress around eating — should be discussed with a GP without delay. Support is also available from Beat (beateatingdisorders.org.uk; helpline: 0808 801 0677).

NHS and NICE Guidance on Safe Dietary Approaches

NICE does not recommend intermittent fasting above a structured 600 kcal calorie-deficit diet combined with physical activity; those with obesity or type 2 diabetes should be referred to supervised weight management programmes rather than fasting unsupervised.

NICE does not specifically recommend intermittent fasting over other dietary approaches. NHS Better Health acknowledges IF as an option that may suit some people, but it is not routinely recommended above a balanced, sustainable calorie-deficit diet. NICE guideline CG189 on obesity and NICE guideline PH53 on weight management lifestyle services for adults recommend a structured reduction in calorie intake — typically around 600 kcal below estimated daily requirements — combined with increased physical activity, as the first-line approach for most adults.^[10] This is considered safer and more achievable for the majority of people.

The NHS Eatwell Guide remains the cornerstone of dietary advice in the UK, promoting a varied diet rich in fruits, vegetables, wholegrains, lean proteins, and healthy fats, consumed at regular intervals throughout the day. This framework supports stable blood glucose levels, adequate micronutrient intake, and long-term dietary adherence — all of which may be compromised by restrictive fasting patterns.

For individuals with obesity or type 2 diabetes who are interested in dietary modification, NICE recommends referral to structured weight management programmes or specialist dietetic services rather than self-directed fasting. The NHS Low Calorie Diet Programme, available in some areas of England, offers a medically supervised very low-calorie approach for eligible patients with type 2 diabetes — a clinically distinct and closely monitored intervention that differs meaningfully from unsupervised intermittent fasting. Referral routes vary by area; your GP can advise on local Tier 2 and Tier 3 weight management services.

In summary, whilst intermittent fasting is not inherently harmful for all healthy adults, the dangers of intermittent fasting are real and clinically significant for many individuals. Anyone considering this approach is strongly encouraged to consult their GP or a registered dietitian beforehand, particularly if they take regular medications, have a chronic health condition, or have any history of disordered eating. Safe, evidence-based dietary change is always preferable to unsupervised restriction.

Frequently Asked Questions