Fasting and Intermittent Fasting

Fasting means going without food, or restricting food to particular periods of the day or week. Some fasts permit water, tea, coffee and other unsweetened drinks throughout. Some religious fasts restrict both food and drink during particular hours.

Fasting is a chosen, time-limited pattern of eating. It is different from starvation, which is an involuntary and prolonged lack of food that damages health. Prolonged fasting without suitable medical supervision can be dangerous, and the risk rises the longer a fast lasts and the more it restricts fluids.

If you cannot afford enough food, that is not fasting and there is practical help available. See Food Banks and Emergency Food Help.

These are the patterns people most often describe. They are listed for information only — this page does not recommend any particular plan.

• Overnight fasting. Leaving a normal gap between the evening meal and breakfast.
• Time-restricted eating. Eating within a set daily window, such as 8 or 10 hours.
• 16:8. Fasting for 16 hours and eating during an 8-hour window.
• 5:2. Eating normally on five days and substantially reducing calorie intake on two non-consecutive days.
• Alternate-day fasting. Alternating ordinary eating days with days of very little or no food.
• Religious or cultural fasting. Patterns set by faith or tradition, which vary widely.
• Prolonged fasting. Fasts lasting 24 hours or longer.

• The body first uses energy from recently eaten food and from stored glucose.
• As the fasting period continues, the body may use a greater proportion of stored fat for energy.
• Hunger, energy levels, concentration and mood can vary considerably between people, and from day to day in the same person.
• The total quality and amount of food eaten across the week still matters.

Claims that a specific number of fasting hours switches on “fat-burning mode”, triggers cell repair, resets the immune system or prevents disease go well beyond what is established. Anyone promising an exact hourly timetable of internal changes is overstating the evidence.

• Some people find a fixed eating window simpler than counting calories.
• It may help some people reduce their total calorie intake.
• It may lead to weight loss when it creates a sustained energy deficit.
• Improvements in blood glucose, blood pressure or cholesterol may accompany weight loss in some people.
• Some people report less late-night eating or snacking.

Results vary a great deal between individuals, and intermittent fasting is not consistently proven to be better than other balanced, calorie-controlled approaches over the longer term. The approach a person can keep up comfortably and safely usually matters more than the particular method. No benefit can be promised. NICE guidance on overweight and obesity management (NG246) sets out the range of support available in the UK.

• Hunger
• Headaches
• Irritability
• Tiredness or weakness
• Poor concentration
• Dizziness or feeling light-headed
• Constipation
• Sleep disturbance
• Dehydration
• Overeating when the fast ends
• Low blood sugar in people taking insulin or certain diabetes medicines

Persistent or severe symptoms are not something to “push through”. They are a signal to end the fast and get advice.

• Missing or delaying meals can cause low blood sugar in people using insulin or medicines such as gliclazide or glimepiride.
• Fasting can also contribute to high blood sugar if medication is missed or a very large meal is eaten afterwards.
• Dehydration may increase the risk of serious complications, particularly for people with kidney problems.
• A blood glucose reading below 4 mmol/L is considered low and must be treated promptly.
• A safe plan should be agreed with the diabetes team before fasting begins.

Nobody should stop taking insulin in order to fast. Doses and timings can sometimes be adjusted, but only by or with the diabetes team or prescriber. See also Diabetes and Chronic Kidney Disease.

Fasting forms part of many faiths and traditions, and the rules, timings, exemptions and customs differ between them. This page offers health information only; it does not interpret religious law and does not judge whether anyone should fast.

• Health conditions can alter the safety of fasting for an individual.
• A healthcare professional can advise on medicine timing and health risks.
• A trusted faith leader can advise on religious exemptions or alternatives.
• Many traditions already recognise exemptions for illness, pregnancy, older age and childhood.

Nobody should feel pressured by this page either to fast or not to fast. Where a condition is involved, condition-specific planning helps: the Diabetes UK guidance on diabetes and Ramadan is one example of how a clinical plan can be agreed alongside a religious fast, and similar planning applies to other faith fasts and other conditions.

Rigid food rules, strict eating windows and prolonged restriction can trigger or worsen disordered eating. For some people, a fasting plan becomes a way of hiding restriction rather than a neutral eating pattern.

If you have a current or previous eating disorder, or you are experiencing bingeing, purging, compulsive restriction, intense fear around food, or rapid weight loss, please do not begin a fasting plan without specialist advice.

This page deliberately gives no fasting targets to beat, no calorie limits and no weight-loss challenges. See also Mental Health and Nutrition.

Fasting does not replace a balanced diet. What is eaten during the eating periods still shapes health. A reasonable pattern includes:

• Vegetables and fruit
• Wholegrain or higher-fibre carbohydrates
• Beans, pulses, fish, eggs, lean meat or other protein sources
• Suitable dairy foods or alternatives
• Unsaturated fats in modest quantities
• Regular fluids whenever the chosen fast permits drinking
• Sensible portions to end a fast, rather than one very large meal

This page does not set a calorie target or a meal plan; those depend on the individual. The NHS Eatwell Guide shows the general balance of foods for most adults.

Signs of dehydration include:

• Thirst
• Dark yellow, strong-smelling urine
• Urinating less often than usual
• Dry mouth or lips
• Headache
• Dizziness or light-headedness
• Unusual tiredness

Older adults, people with diabetes, people with kidney problems and people taking diuretics may be at greater risk of harm from dehydration.

A GP, practice nurse, pharmacist, dietitian or diabetes team can all help. You may find it easier to use wording you have prepared beforehand:

“Hello, I am considering fasting or time-restricted eating. I take the following medicines and have the following health conditions. Could you tell me whether fasting is safe for me, whether any medicine is affected by missing meals and what warning signs would mean I should stop?”

To find a GP or pharmacy near you, use Find Local Services, or see GP Services for help with booking and preparing for an appointment.

• NHS — The Eatwell Guide
• NHS — Low blood sugar (hypoglycaemia)
• NHS — Dehydration
• Diabetes UK — Diabetes and Ramadan — Diabetes UK is an independent charity, not part of the NHS or government.
• NHS MyHealth London — Fasting
• NHS England — Health Survey for England 2024
• NICE NG246 — Overweight and obesity management

This page provides general information and cannot diagnose a condition or replace professional medical advice. Always speak to a GP, pharmacist, dietitian or other qualified health provider about your own health.

This page provides general information and does not recommend fasting for any individual. It cannot assess whether fasting is safe alongside a health condition, pregnancy, an eating disorder history or prescribed medicine. Speak to an appropriate healthcare professional before making significant changes to eating patterns.