The 24 Hour Fast Explained: UK Guide 2026 | Complete Nutrition

Understanding Fasting

The 24 hour fast explained

A 24 hour fast means no calories for one full day with water, black coffee and plain tea allowed throughout. Typical pattern: dinner at 7pm one day, next meal at 7pm the day after. Popularised as eat-stop-eat by Brad Pilon. Done once or twice per week it creates substantial weekly caloric deficit without daily restriction. Reaches early fasting metabolism with insulin near baseline, fat oxidation rising and mild ketone production toward the end of the window.

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Updated:
May 2026

Written by:
Dominic Walton, MD

Reading time:
7 min

The full answer

The structure and physiology of a 24 hour fast

The 24 hour fast occupies a useful middle ground between daily intermittent fasting and longer extended fasts. Four points explain what happens and why people do it.

1. The standard structure

The most common pattern is dinner-to-dinner: eat dinner at 7pm, no calories from 7pm to 7pm the following day, then eat dinner. This puts the entire fasting window during one calendar day plus the overnight period either side. You skip breakfast and lunch on the fasting day and resume normal eating with dinner. The eat-stop-eat protocol popularised by Brad Pilon recommends this pattern once or twice per week. Some people prefer lunch-to-lunch (eat lunch one day, eat lunch the next day) which keeps each day with one meal.

2. The physiology through the 24 hours

Hours 0 to 8: digestion finishing, blood glucose normalising, insulin falling, mild glycogen breakdown beginning. Hours 8 to 16: liver glycogen depleting toward exhaustion, fat oxidation rising substantially, insulin reaching baseline. Hours 16 to 24: liver glycogen exhausted, gluconeogenesis from amino acids and glycerol maintaining blood glucose, significant fat oxidation, mild ketone production starting (blood ketones reaching 0.3 to 0.8 mmol/L by 24 hours). This captures the early fasting metabolic state but does not reach deep ketosis or sustained growth hormone elevation.

3. The hunger and energy pattern

Hunger waves peak around your usual meal times (around noon and again early evening if you eat lunch and an early dinner normally) then subside between them. Most people experience tolerable hunger throughout the day. Energy is typically stable for the first 18 hours. Some people experience slight fatigue, mild headache or irritability in hours 18 to 24. The fast is uncomfortable but not exhausting for most people. The discomfort fades with practice over 2 to 4 weeks.

4. Why people choose 24 hour fasts

Three main reasons. First flexibility: most days you eat normally and only one or two days have the demanding fast. Easier to fit social meals than daily fasting windows. Second cumulative effect: a 24 hour fast creates substantial deficit (around 1500 to 2000 kcal) without needing to track daily calories. Third psychological structure: knowing the fast has a defined end (dinner tomorrow) is easier than daily ongoing restriction for some people.

Practical guidance

How to do a 24 hour fast well

Five practical points for executing a 24 hour fast.

Eat normally the day before

Do not pre-load with extra calories before the fast. A normal balanced meal as your last meal before the fast (containing protein, vegetables and some healthy fat) sets you up well. Avoiding refined carbohydrates and alcohol the night before reduces hunger spikes during the fast. Aim for a meal that satisfies you for the evening but is not unusually large.

Hydrate aggressively

Water through the day, ideally 2.5 to 3 litres or more. Black coffee in the morning if you normally drink it. Plain tea, herbal tea or sparkling water through the afternoon. The hydration does important work: replaces the water normally from food, prevents headaches and reduces hunger by giving your gut something to process.

Plan the fasting day

Pick a day with moderate demands. Work days are usually fine. Heavy training days, important social meals or stressful events make the fast harder. A typical Tuesday or Wednesday when nothing demanding is planned works well. Avoid weekends if those involve social meals or stress.

Ride hunger waves with hot drinks and activity

Hunger peaks around your usual meal times. When a wave hits make a hot drink (tea, coffee, hot water with lemon), do a short activity (walk, errands, brief task) and the wave subsides in 20 to 30 minutes. Fighting the wave or sitting with it makes it harder. Engaging with something else lets it pass.

Break the fast normally

Dinner at the planned time should be a normal balanced meal eaten at normal pace. Protein, vegetables and some healthy fat. Avoid starting with a huge meal or with refined carbohydrates which produce strong glucose spikes after fasting. Eat slowly. Stop when comfortably full. The next day resume normal eating.

Safety

Who should avoid 24 hour fasts

Standard contraindications apply.

History of eating disorders. 24 hour fasts can trigger restrictive patterns. Contraindicated.
Pregnancy or breastfeeding. Not appropriate.
Type 1 diabetes or insulin dependent type 2 diabetes. Hypoglycaemia risk significant. Specialist supervision required.
BMI under 18.5. Not appropriate.
Children, adolescents or adults under 18. Contraindicated.

Anyone on medications, with significant medical conditions or over 65 should discuss with GP first. Stop the fast immediately if you experience severe dizziness, fainting, chest pain, palpitations, severe headache that does not respond to water or other concerning symptoms. End the fast with water and a balanced meal. Seek medical attention if symptoms persist.

For the wider picture on fasting from the gentlest protocols to extended fasts plus the science behind hunger, metabolism and refeeding, our Understanding Fasting hub brings every guide together in one place.

Part of the hub

Back to the Fasting Hub

This article sits inside our complete knowledge base on fasting covering protocols, physiology, safety and practical guidance. Head back to the hub for the full index.

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Keep reading

Compare with other fast lengths

Several pages cover different fast durations. Our piece on 16:8 fasting explained covers daily intermittent fasting. The 36 hour fast explained covers the next step up. And the 48 hour fast explained covers the typical demanding intermediate fast.

Frequently asked

24 hour fast questions

What is a 24 hour fast?

A 24 hour fast means consuming no calories for a full day. Typical pattern: eat dinner at 7pm one day, eat next at 7pm the following day. Water, black coffee and plain tea are allowed throughout. This is sometimes called the eat-stop-eat protocol popularised by Brad Pilon. Done once or twice per week it creates substantial caloric deficit without daily restriction. The 24 hour fast is a stepping stone between daily intermittent fasting and longer extended fasts.

What happens during a 24 hour fast?

Hours 0 to 12: normal digestion fading, glycogen breakdown beginning, mild fat oxidation rising. Hours 12 to 16: liver glycogen near depleted, fat mobilisation accelerating, insulin near baseline. Hours 16 to 24: significant fat oxidation, mild ketone production starting toward end of window. Hunger waves peak around usual meal times then subside. Most people feel hungry but tolerable through the day. Energy is typically stable. Some people feel slight headaches or irritability.

How often should you do a 24 hour fast?

Once or twice per week is the typical recommendation. The eat-stop-eat protocol uses one or two 24 hour fasts per week. Three or more 24 hour fasts per week starts approaching alternate day fasting territory and may be more demanding than necessary. Daily 24 hour fasts (one meal a day OMAD) is a different protocol with different considerations. Most people use 24 hour fasts as a flexible addition to daily eating not as primary protocol.

Is a 24 hour fast safe?

For healthy adults yes. 24 hour fasts in well-nourished people without contraindications carry minimal acute medical risk. Standard contraindications apply: eating disorder history, pregnancy or breastfeeding, type 1 diabetes or insulin dependent type 2 diabetes, BMI under 18.5, children, adolescents and adults under 18. Anyone on medications should discuss with GP first. Refeeding syndrome is not a risk for 24 hour fasts in well-nourished people.

Will I lose weight with 24 hour fasts?

Modest weight loss is typical with regular 24 hour fasts (once or twice per week). The protocol creates about 1500 to 2000 kcal weekly deficit from one 24 hour fast or 3000 to 4000 kcal from two. Over 12 weeks this typically produces 2 to 5 kg loss depending on starting weight and food choices on eating days. Compensatory eating on non-fast days can offset the deficit so total intake matters more than just fast days.

How should I break a 24 hour fast?

With a normal balanced meal eaten slowly. No special protocol needed for healthy adults breaking a 24 hour fast. Avoid starting with very large meals or refined carbohydrates which produce strong glucose spikes. A meal with protein (eggs, fish, meat), vegetables and some healthy fat works well. Eat at normal pace. Stop when comfortably full not stuffed. The next meal can be normal. No phased reintroduction needed.

Can I exercise during a 24 hour fast?

Light to moderate exercise yes. A walk, gentle yoga, easy cycling or light cardio during a 24 hour fast is well tolerated by most people. Heavy resistance training or high intensity exercise is better timed before the fast starts or after it ends. Performance during intense exercise is usually lower in the fasted state especially in hours 18 to 24. Hydration during exercise on fasting days needs more attention than normal because no food water is being consumed.