Extended Fasting: 24, 36, 48 and 72 Hours Compared

Quick answer: Extended fasting means going longer than a day without food. The useful range for someone doing this without medical supervision is 24 to 72 hours, and longer is not better — the benefits flatten out while the costs keep climbing. For most people the honest recommendation is a weekly 24-hour fast, an occasional 36 or 48-hour one, and 72 hours rarely if ever. This page compares the tiers and links the full guide for each.

Every extended-fasting question eventually collapses into one: how long?

The answer that gets clicks is "longer". The answer supported by what has actually been measured in humans is more boring and more useful, and it is below.

What counts as extended fasting

Anything past roughly 24 hours. Below that you are doing time-restricted eating — 16:8, 18:6, 20:4 — where you eat every day and vary the window.

The line matters because the two are different activities. Daily fasting is a habit you build. Extended fasting is an event you plan: it needs preparation, electrolytes, a clear day, and a way to end it. It also needs a screen first, which is the next section.

The screen that comes before any of this

Extended fasting is optional for everyone. If any of the following is true of you, the right next step is a conversation with your doctor, not a shorter fast:

• Diabetes, or any medication that lowers blood sugar
• Any prescription medication that must be taken with food
• Pregnancy or breastfeeding
• Any history of an eating disorder, current or past
• Under 18
• Being underweight, or losing weight without meaning to
• A heart, kidney or liver condition, or a history of fainting or low blood pressure

The extended fast planner runs the same screen before it draws anything, and a hit stops it rather than warning you. Nothing on this page is worth a hypoglycaemic episode or an eating-disorder relapse.

The tier map

24 hours

The workhorse. One meal to the same meal the next day, once a week, which is exactly the structure of Eat Stop Eat. Long enough that the metabolic switch Anton and colleagues describe is well behind you, short enough to slot into an ordinary week without rearranging anything.

The direct evidence at this length is decent for a fast this short: a randomised cross-over trial in 30 healthy volunteers found a single day of water-only fasting acutely raised growth hormone and HDL cholesterol and lowered triglycerides — with most markers back at baseline within 48 hours. That last part is the point. A 24-hour fast is a stimulus you repeat, not a switch you flip.

Start here. Full guide: 24 hour fast.

36 hours

Dinner one evening to breakfast two mornings later. It buries two nights of sleep inside the fast, which is why many people find 36 hours barely harder than 24. It is the natural next step, and the rhythm inside alternate day fasting.

Full guide: 36 hour fast.

48 hours

Two full days. This is where the fast stops fitting around your week and your week starts fitting around the fast.

It is also a duration researchers have used, so there is direct human data — including magnetic resonance measurements of triglyceride moving into the livers of men and the muscles of women across a 48-hour fast, while the whole-body glucose and oxidative responses stayed identical between the sexes.

Worth doing occasionally if you are experienced. Not worth doing weekly.

Full guide: 48 hour fast.

72 hours

Three days, and the first tier where a real physiological cost is measurable rather than theoretical. In eight healthy men fasted for 72 hours, net phenylalanine release from skeletal muscle rose and mTOR signalling fell by roughly half — muscle breakdown outpacing synthesis.

Meanwhile the benefits most people are chasing at this length — the autophagy and immune claims — have not been quantified in humans at any specific hour. That combination, a measured cost against an unquantified benefit, is why this tier is rare rather than aspirational.

Full guide: 72 hour fast.

Beyond 72 hours

This site stops at three days, and the planner will not draw anything longer.

The reason is in the safety data. The reassuring numbers on multi-day fasting — under 1% adverse effects across 1,422 people fasting 4 to 21 days, and 75% mild adverse events across 768 water-only fasting visits — come from supervised programmes. The 1,422-person cohort was not water fasting at all: participants took 200 to 250 kcal a day and saw a physician two or three times a week. That evidence says long fasts are well tolerated when someone is checking the bloods. It says nothing about doing the same thing alone at home.

So how long should I fast?

Here is the honest version, and it will disappoint anyone hoping the answer is a bigger number.

Length buys you depth, not multiples. A 72-hour fast is not three times as useful as a 24-hour one. The big metabolic transition — glycogen down, fat and ketones up — is largely complete inside the first day. Everything after that is time spent in a state you already reached, and the extra hours add cost faster than they add benefit.

Frequency beats duration. A weekly 24-hour fast gives you 52 fasts a year. A quarterly 72-hour fast gives you four. Which of those do you think moves your body composition and your relationship with food more?

Nothing here beats ordinary dieting on the scale. The most rigorous long-run test of a fasting protocol — a one-year randomised trial of alternate-day fasting against daily calorie restriction — found no advantage for fasting on weight loss at 6 or 12 months, and the fasting group had the highest dropout rate at 38%. Fasting works roughly as well as careful daily restriction. Its advantage is that some people find it far easier to keep doing, which is a real advantage, and not the same as a metabolic one.

Your metabolism is not the reason to worry. It does not fall during a short fast. Measured across 84 hours of starvation in lean healthy people, resting energy expenditure rose rather than fell, tracking a large increase in noradrenaline. The metabolic slowdown people fear belongs to months of undereating, not to two days without food.

So: weekly 24 hours, occasional 36 or 48, 72 rarely. If you want a different answer, the honest one is that the evidence does not support paying more for it.

What every extended fast needs, whatever the length

Electrolytes, on a schedule. Falling insulin makes the kidneys flush sodium, with potassium and magnesium going with it. That single mechanism explains most of what goes wrong on a long fast — the headaches, the cramps, the fog, the head-swim on standing. Get amounts from the fasting electrolyte calculator and read electrolytes during fasting for the why.

A plan for ending it. The gut quietens down during a long fast, and a huge first meal is the fastest route to feeling awful. Small, easy, protein-and-fat first; wait; then a normal meal. The how to break a fast tool builds the sequence around your actual fast length.

An honest read on refeeding risk. For a well-nourished adult, fasts in the 24 to 72-hour range do not reach the criteria NICE uses to flag high risk — those start at little or no intake for more than 10 days on its own, or more than 5 days combined with a second factor such as a low BMI or a history of alcohol misuse. If you are in one of those groups, the risk is real and the answer is a clinician. Refeeding syndrome, and who is actually at risk has the full criteria.

A stopping rule. Faint, confused, breathless, or a racing or irregular heartbeat means eat now, regardless of the timer.

Where the hour markers come from

Every fasting chart, including the zone model on this site, puts hours on the stages of a fast. Some of those are well measured in humans — the metabolic switch past roughly 12 hours, the rise in ketones, the rise rather than fall in metabolic rate. Others, autophagy above all, are models built from animal work and human signalling proxies rather than direct human measurement.

That distinction is worth carrying into any fasting content you read, including this site's. A number on a chart is not the same kind of claim as a number from a trial.

Choosing a rhythm you will keep

If you are new to this, a sensible year looks like: build a comfortable daily fasting habit first, add a weekly 24-hour fast once that is boring, try a 36-hour fast after a couple of months, and only then consider 48. Most people never need to go further, and what actually changes week by week is a better guide to whether it is working than any single long fast.

The extended fast planner will lay out any of these hour by hour, with electrolyte checkpoints, sleep and training placement, and a refeeding section it will not let you skip.

How Fasted helps

Fasted runs the timer for any of these lengths with the metabolic phase shown as you cross it, so a long fast has landmarks rather than just a rising number. Electrolyte logging and reminders keep the minerals on schedule, weight logging gives you the before and after, and every extended fast lands in the same history as your daily ones — which is the only way to see whether the long ones are earning their place.

Frequently Asked Questions

What is the best length for an extended fast?

For most people, 24 hours done weekly. It reaches full fat and ketone metabolism, costs one awkward day, and can be repeated fifty times a year — which matters far more than depth. Move up to 36 or 48 hours occasionally if you want a bigger stimulus, and treat 72 hours as a rare event rather than a goal.

Is a longer fast always better?

No. The major metabolic transition is largely complete within the first day, so extra hours extend a state you have already reached rather than unlocking a new one. Meanwhile the costs keep rising: by 72 hours, muscle protein breakdown is measurable in humans. Longer fasts buy depth at an increasing price, and past three days they belong under medical supervision.

How often can I do extended fasts?

A 24-hour fast weekly is sustainable for most healthy adults. A 36-hour fast fits weekly to monthly. Forty-eight hours is a monthly ceiling at most, and 72 hours is best kept to once or twice a year. Doing long fasts frequently makes it hard to eat enough protein and repeats the muscle cost without evidence of extra benefit.

Do I need electrolytes on every extended fast?

On anything past about 24 hours, almost certainly. When insulin falls the kidneys excrete more sodium, and water, potassium and magnesium follow, which is why headaches, cramps, dizziness and fatigue are usually mineral loss rather than hunger. Zero-calorie electrolytes do not break a fast, and taking too little is the most common reason a long fast goes badly.

Is extended fasting safe without a doctor?

For a healthy adult with none of the contraindications, fasts up to 72 hours are widely done alone and the published safety data is reassuring — but that data comes from supervised programmes, so it supports a careful solo attempt rather than proving one is safe. If you have diabetes, take medication with food, are pregnant or breastfeeding, are under 18, or have any history of an eating disorder, extended fasting is a medical decision and not a lifestyle one.

Does extended fasting slow your metabolism?

Not over the durations covered here. Measured through 84 hours of starvation in lean healthy people, resting energy expenditure rose rather than fell, alongside a large increase in noradrenaline. The metabolic adaptation people worry about comes from sustained undereating over months, and the way to avoid it is eating properly between fasts rather than fasting for less time.

What to Read Next

• 24 Hour Fast: What Happens and How to Do It
• 48 Hour Fast: Benefits, Risks and a Realistic Plan
• Refeeding Syndrome: What It Is and Who Is Actually at Risk