Extended fasting: three days without food won't make you superhuman. Done wrong, it can hurt.
Water fasts of 36, 48, 72 hours and beyond are the influencer's ultimate flex. Some clinics do it under supervision with decent safety records. At home, alone, on medication? Different story.
The short version
- What it is
- Going 36 hours to several days with no food, or almost none.
- Good for
- Supervised clinic programmes. Not much else that a shorter fast can't do.
- Bad at
- Protecting muscle. Being safe on medication. Being safe to stop.
- Evidence
- Weak. Mostly observational, often from clinics that sell it.
- Our take
- Medical territory. We don't plan it. If you want it, do it with a doctor.
What extended fasting actually is
Extended fasting is any fast longer than a day: 36 hours (skip a whole day, eat breakfast the day after), 48 hours, 72 hours, and in clinics, a week or more. Pure water fasts allow only water and maybe black coffee or tea. Clinic-style fasts, such as the German Buchinger method, allow a small amount of food, about 200 to 250 calories a day.1
It's not the same as Eat-Stop-Eat, where a fast lasts 24 hours and you eat dinner both days. And it's not the same as the fasting-mimicking diet, which uses a set low-calorie meal plan for five days. Extended fasting is the real thing, no food.
How it works in your body
Day one: your body uses up the sugar stored in liver and muscle, along with the water bound to it. The scale drops fast. Day two: fat becomes the main fuel and your liver makes ketones, which your brain can use. Hunger often dips after the first couple of days, which is why fasters describe a "high". In a large clinic study, most people reported no hunger at all.1
What the influencers leave out: your body also breaks down protein. In a carefully measured German study, 16 healthy men fasted for 10 days. They lost about 6 kg. About 60% of that was lean tissue, not fat, although much of it was water and stored sugar. Their resting metabolism dropped by 12%. Their muscles still worked fine, but lean tissue was still below baseline after they started eating again.2
What you eat, what you don't
During the fast
- Water, lots of it
- Herbal tea, plain green tea
- Black coffee, in small amounts
- In supervised programmes: clear vegetable broth, as your doctor or clinic says
- Salt and minerals only as advised by your doctor
Forget it
- Starting a multi-day fast on your own when you take medication
- Hard training during the fast
- Breaking a long fast with a huge meal
- Alcohol, at any point
- Sweeteners, energy drinks, "fasting" supplements
- Repeating long fasts back to back
Thinking about it anyway? Ask these first
If you and your doctor are seriously considering a fast longer than a day, these are the questions a decent programme should answer without blinking:
- Who is supervising? A doctor you can reach during the fast, not a coach in a group chat.
- What happens to my medication? Every tablet, day by day, written down before you start.
- Which checks are done? Blood tests and blood pressure before, and during longer fasts.
- How do I stop? A written plan for breaking the fast, step by step, not "eat something light".
- When do I stop early? Clear warning signs, and who to call.
No clear answers? Then it's selling an experience, not medical care.
In Austria, Germany and Switzerland
Fasting clinics and "Heilfasten" weeks are common here, from medically run clinics to hotel packages and fasting hikes. They're not the same. Ask whether a doctor is on site and how refeeding is handled before you book.
In Singapore
Many people here know fasting from faith, such as Ramadan. That's a different pattern, with daily meals before dawn and after sunset, and we cover it respectfully in religious fasting. Hot, humid weather makes fluids and salts one more thing to plan with your doctor.
Alcohol and extended fasting
OK, now and then
- Nothing during the fast
- After the fast, once you're back to normal meals for a few days: one glass of dry wine with food
Out
- Any alcohol during a multi-day fast
- Alcohol as part of the meal that breaks the fast
- "Celebration" drinks after a long fast
50 Club: no drink during the fast, full stop. Alcohol on an empty stomach after days without food is a bad idea, and while your liver deals with alcohol, fat burning stalls anyway.7 All the details are in our alcohol guide.
What extended fasting is good for, with proof
Supervised clinic fasting, with a decent safety record
The largest study we have followed 1,422 people at a German fasting clinic who fasted for 4 to 21 days on about 200 to 250 calories a day, with medical supervision. They lost between about 3 kg (5 days) and 8.6 kg (20 days). Most reported no hunger, and side effects were reported in under 1%.1 The honest caveats: there was no control group, and the lead author is on the clinic's board. Two people were hospitalised, one with a heart attack.
Muscle function held up, in healthy men
In the 10-day study, strength held steady in some muscles and even rose in others, despite the lean tissue loss.2 That's reassuring for healthy young men under supervision. It says nothing about older people, people with illness, or people fasting alone at home.
The uncomfortable truth
"A 72-hour fast resets your whole body." There's no good human evidence for a "reset". The best-studied effects are the boring ones: weight loss, which comes back if you go back to old habits, and changes in blood markers during the fast. Meanwhile you lose real lean tissue and your metabolism slows.2 For everyday weight and health goals, shorter fasts and daily eating windows have more trial evidence behind them.3
The risks
- Refeeding problems. After a long period of little or no food, eating normally again can shift salts like phosphate and potassium in dangerous ways. UK guidance says anyone who has eaten little or nothing for more than 5 days should restart food carefully, at no more than half their needs at first.4 This is why long fasts need a plan to stop, not just to start.
- Diabetes medication. Insulin and tablets that lower blood sugar can cause dangerous lows during a fast. SGLT2 drugs (empagliflozin, dapagliflozin, canagliflozin) can trigger a serious acid build-up in the blood, sometimes with normal blood sugar.5 Never fast for days on these drugs without your doctor.
- Muscle loss. More than half the weight lost in a 10-day fast was lean tissue.2 For older people, that muscle may not come back easily.
- Dizziness and fainting, especially when standing up. Get up slowly and don't drive if you feel light-headed.
- Eating disorders. Multi-day fasts are a known trigger. If you've ever had one, this isn't for you.
- Other medication. Many drugs need food or need dose changes when you don't eat. Ask before, not after.
Allergies: watch the staples
The fast has no allergens. The broths used in clinic fasting and the meals used to break the fast can.
| Staple | Allergen (EU list of 14)6 | Swap |
|---|---|---|
| Vegetable broth with celery | Celery | Broth without celery or celeriac |
| Stock cubes | Often celery, sometimes cereals containing gluten, milk | Home-made broth, checked labels |
| Refeeding: yoghurt | Milk | Soy yoghurt, stewed fruit |
| Refeeding: porridge, bread | Cereals containing gluten | Rice porridge, potatoes |
| Refeeding: eggs | Eggs | Tofu, lentil soup |
Don't do extended fasting if you…
- are pregnant or breastfeeding
- are under 18
- have ever had an eating disorder
- have diabetes, or take insulin, sulfonylureas, SGLT2 or GLP-1 drugs
- are underweight, frail or older and losing muscle
- have kidney, liver or heart disease, gout, or low blood pressure
- take any regular medication, unless your doctor is supervising the fast
Healthen me asks about all of these and gives you a different plan if any apply.
How we use extended fasting in your plan
We don't. Extended fasting is a medical topic for us: Healthen me screens for it and we refer you to a doctor if you're interested. Our plan never asks you to fast longer than a day.
What we use instead: a daily eating window, as long as you comfortably can, and one fasting day after your weekly Rebel Day, Eat-Stop-Eat style. That gives you most of the habit benefits of fasting without the muscle loss, the medication risks and the refeeding headaches. And no, we won't count a 72-hour fast as extra credit.
This article is general information, not medical advice. If you have a medical condition or take medication, talk to your doctor before changing how you eat.
Sources
- Wilhelmi de Toledo F et al. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One 2019;14(1):e0209353. doi.org/10.1371/journal.pone.0209353
- Laurens C et al. Is muscle and protein loss relevant in long-term fasting in healthy men? A prospective trial on physiological adaptations. Journal of Cachexia, Sarcopenia and Muscle 2021;12(6):1690–1703. doi.org/10.1002/jcsm.12766
- Garegnani LI et al. Intermittent fasting for adults with overweight or obesity. Cochrane Database of Systematic Reviews 2026, Issue 2, CD015610. doi.org/10.1002/14651858.CD015610.pub2
- National Institute for Health and Care Excellence (NICE). Nutrition support for adults: oral nutrition support, enteral tube feeding and parenteral nutrition. Clinical guideline CG32, 2006, updated 2017. nice.org.uk/guidance/cg32
- US Food and Drug Administration. Drug Safety Communication: FDA warns that SGLT2 inhibitors for diabetes may result in a serious condition of too much acid in the blood, 15 May 2015 (archived copy). web.archive.org: fda.gov/Drugs/DrugSafety/ucm446845.htm Published case series: Mistry S, Cocks Eschler D. Euglycemic diabetic ketoacidosis caused by SGLT2 inhibitors and a ketogenic diet: a case series and review of literature. AACE Clinical Case Reports 2021;7(1):17–19. doi.org/10.1016/j.aace.2020.11.009 (PMC7924151)
- Regulation (EU) No 1169/2011 on food information to consumers, Annex II (the 14 allergens). eur-lex.europa.eu/eli/reg/2011/1169/oj
- Siler SQ, Neese RA, Hellerstein MK. De novo lipogenesis, lipid kinetics, and whole-body lipid balances in humans after acute alcohol consumption. American Journal of Clinical Nutrition 1999;70(5):928–936. pubmed.ncbi.nlm.nih.gov/10539756