Longer fasting: water fasting, therapeutic fasting and fasting-mimicking

How several days without food remodel the gut microbiota, what studies on therapeutic fasting and fasting-mimicking diets show, and where the risks lie.

How solid is the research? Plausible

Well established: longer fasting changes the gut microbiota markedly and temporarily, and supervised fasting is mostly safe for healthy people. Whether this benefits the gut in the long term is open. Studies on water and dry fasting are lacking.

Healthy state

After only about twelve hours without food, the body starts to use more fat. After one to three days the sugar stores of the liver are empty, it burns mostly fat and makes Ketone bodies Energy carriers that the liver makes from fat during fasting or a very low-carbohydrate diet. The brain and muscles can use them instead of sugar. , which the brain can also use as fuel. In Buchinger therapeutic fasting there are about 200 to 250 kcal a day from vegetable broth, juice and a little honey; in water fasting nothing but water. Fasting-mimicking diets give clearly fewer calories over five days, little sugar and protein, but plenty of unsaturated fat. For pure water fasting over several days we found no human study on the Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity”.

The digestive tract switches to a saving mode. Digestive juices are hardly needed, the gallbladder gets no fat signal and empties less often, and hardly any residue is formed. Stools become rarer and smaller. That is normal and not yet constipation, because what is missing is the material, not the movement. The Migrating motor complex A wave of muscle contractions that sweeps the stomach and small intestine empty about every one and a half to two hours when fasting. Eating interrupts it. More in the chapter “Intermittent fasting and meal timing: 16:8, 5:2 and Ramadan” of the small intestine (the migrating motor complex, see intermittent fasting) keeps running in the time without a meal.

The gut microbiota rebuilds too. Bacteria that break down fibre from food, such as Lachnospiraceae and Ruminococcaceae, become fewer. Bacteria that live on the body itself, such as mucus from the gut wall, become more common: Bacteroides, but also Escherichia coli and Bilophila. In a study of 15 healthy men over 10 days, this had disappeared again after three months. You can picture it like this: if no food arrives, the species that live on it are lost first.

In people with high blood pressure and metabolic syndrome, 5 days of fasting followed by a healthy diet lowered blood pressure more after three months than the diet alone. The changes in the gut microbiota also involved genes for making Short-chain fatty acids Acids such as acetate, propionate and butyrate that gut bacteria make from fibre. They nourish the gut wall and act throughout the body. More in the chapter “Butyrate and other short-chain fatty acids”. In 38 healthy people, 5 days of fasting clearly changed gut microbiota and metabolites, and part of the weight loss lasted 12 weeks. How much of this is due to gut bacteria and how much to weight, salt and calories is open.

In a specialist clinic, 1,422 people fasted for 4 to 21 days. Fewer than 1 percent reported side effects, 93 percent had no hunger, and 84 percent of those with complaints reported an improvement. This shows that supervised fasting is mostly well tolerated by healthy people; but it is an observation without a comparison group, partly analysed by the clinic itself.

For people with severe Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis”, an inpatient fasting therapy with 10 days of fasting and 5 days of refeeding was described in Japan. Pain, bloating, diarrhoea and anxiety improved more than in the comparison group. In mice with intestinal inflammation, fasting-mimicking diet helped more than pure water fasting and promoted renewal of the gut lining. Both findings are hints, not treatment recommendations.

During fasting, fibre eaters decline and mucus users increase. After fasting, this reverses.

Disrupted state: causes and effects

Without food, gut bacteria live on the Mucus layer A gel layer of mucins on the gut wall that keeps bacteria at a distance. In the colon, the inner layer is almost free of microbes. More in the chapter “The mucus layer and additives”. In mice without fibre, the protective layer became thinner and pathogens reached the gut wall more easily. Whether this matters in humans after a few fasting days is unclear. It is, however, a reason to deliberately build fibre back up after fasting.

Breaking the fast is a delicate moment. With the first meal the gut's immune response switches back on and inflammatory messengers rise briefly. After a very long fast, or in undernutrition, a Refeeding syndrome A dangerous shift of blood salts such as phosphate and potassium when someone eats again too quickly after long starvation or malnutrition. can occur: blood salts such as phosphate and potassium fall dangerously when eating again, because the body suddenly releases insulin again with the sugar and pulls the salts into the cells. With short fasts in healthy people this is rare; with underweight or very long fasts it is a real risk.

Rapid weight loss promotes gallstones. When the gallbladder gets hardly any fat signal, it empties rarely, the bile thickens and cholesterol can crystallise. In people with severe overweight, 28 percent developed gallstones during 16 weeks of a very low-calorie diet. With the Bile acids Substances made by the liver that make fat digestible. About 95 percent are reabsorbed in the last part of the small intestine; gut bacteria transform the rest. More in the chapter “Bile acids: when bile causes diarrhoea” medicine Ursodeoxycholic acid A bile acid used as a medicine, prescription-only in many countries. Approved among other things for dissolving gallstones and for certain liver diseases. More in the chapter “Bile, lipase and lecithin: what fat digestion really needs” it was only 2 to 8 percent. For a few fasting days the risk is much smaller; the figures cannot be transferred.

The circulation often pays the price: loss of water and salt, falling blood pressure, dizziness, headache, feeling cold and sleep problems are common companions. It becomes more dangerous with medication. Blood pressure medicines and water tablets can lower blood pressure too far, and some diabetes drugs, such as SGLT2 inhibitors, can trigger a dangerous acidification of the blood during fasting. Muscle protein is also broken down during longer fasting; that is a reason not to do it often or for long.

Many promises are unproven. Dry fasting, i.e. without drinking, has not been studied and is risky for kidneys and circulation. Fasting does not cure infections, does not "detox" the gut, and, according to many people's experience, does not "starve out" fungi or parasites; we found no studies on this. Symptoms during fasting are not a sign that toxins are being released. Relying on fasting when you suspect fungi or parasites delays an examination.

The studies on longer fasting are small and mostly from fasting clinics. The gut microbiota usually returns. What remains depends mostly on how you eat afterwards. For people with pre-existing conditions, older people, pregnant women and people who are underweight, the data are thinner still, and the risks are higher in precisely these groups.

Too long, too dry or broken too fast: the mucus layer thins, salts drop, gallstones can form.

What often helps

Lifestyle

Diet, exercise, sleep, social contact and dealing with stress.

Start small, and with broth rather than only water Only if you are healthy, so not with underweight, an eating disorder, pregnancy, breastfeeding or type 1 diabetes: one to three days with vegetable broth and diluted juice are a cautious start. Drink plenty, get salt from the broth, stay physically calm, and stop if you feel dizzy or have palpitations. With illnesses or medication, check with a doctor first, because blood pressure medicines and some diabetes medicines can become dangerous during fasting. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [1]
Break the fast slowly Plan about half a day to a day of refeeding for each day of fasting: first steamed vegetables, soup, porridge, then gradually more fibre and legumes, so the gut microbiota gets food again. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [2]
Change your diet for good afterwards The benefit for blood pressure appeared with a healthy diet afterwards. Use fasting as a starting point for more plants and less ultra-processed food, not as compensation for the rest of the year. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [3]
Do not dry fast Going without water has no proven benefit but real risks for the kidneys and circulation. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
One or two lighter days beforehand Fasting clinics have people eat lighter for one or two days beforehand, without coffee, alcohol and lots of sugar, so the transition is gentler and there is less headache. This is practical experience, not a tested method. Experimental How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
Do not fight rare stools During fasting little comes out; that is normal. Plenty of fluid and light movement are usually enough. Laxative salts or enemas are part of some fasting programmes, but should only be used with guidance, because they shift salts in the body. Experimental How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

Natural supplements

Supplements do not replace a medical check-up. Discuss dosage and interactions with your doctor or pharmacist.

Fasting-mimicking diet as a gentler alternative Five days with few calories, little sugar and protein, but enough fat and vegetables. Well tolerated in a study of 100 people, with better risk markers. On the effects on the gut there are mainly animal studies so far. Ready-made kits exist, but you can also put it together yourself. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [4][5]
Salt and minerals: broth is usually enough During fasting, water and salts are lost in the urine. Salty vegetable broth makes up for this in most healthy people over a few days. Do not take high-dose mineral supplements on your own, especially with kidney or heart disease. Experimental How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

Medical treatments

Information, not a treatment recommendation. Decide together with a doctor. Approval and availability differ from country to country.

From five days on, or with health conditions, under medical supervision Longer fasting is best done in a fasting clinic or with medical supervision. Do not fast with underweight, an eating disorder, during pregnancy or breastfeeding, with type 1 diabetes, kidney or liver disease, or during a flare of inflammatory bowel disease. Discuss medication beforehand. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [1]
Fasting as a clinic therapy for severe IBS In Japan, inpatient fasting therapy is used for IBS that does not respond to other treatments. The evidence is based on a retrospective analysis. Doing it alone at home is not the same. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [6]
Keep gallstones in mind Anyone losing weight very quickly over weeks or who has had gallstones before should discuss this with their doctor. On very-low-calorie diets, ursodeoxycholic acid is sometimes given as prevention. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [7]
Go through medication before fasting Blood pressure medicines, water tablets, insulin and other diabetes drugs (especially SGLT2 inhibitors), blood thinners and drugs with a narrow therapeutic range must be adjusted or paused before fasting. Your doctor decides this, not you alone. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
After longer fasting or with underweight, blood tests when restarting If you have fasted more than about five days, are underweight or have eaten little for a long time, have the refeeding supervised medically, with checks of phosphate, potassium and magnesium, to recognise a refeeding syndrome. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

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How we rate the evidence

Trust score 5 of 5:Established: Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies.
Trust score 4 of 5:Well supported: Several randomised studies in humans with largely consistent results.
Trust score 3 of 5:Promising: Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies.
Trust score 2 of 5:Plausible: Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms.
Trust score 1 of 5:Experimental: Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

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