Water Fasting: Potential Benefits, Real Risks and When It Becomes Dangerous
Nothing but water, for several days. For some it feels like a reset, for others it is a risk. Both can be true. Here you can read what studies show, where the limits are, and for whom water fasting is not something to do alone at home.
Online, water fasting is often told like a hero's journey: five days of nothing but water, and you come out a new person. I consider fasting a tool worth taking seriously. That is exactly why it deserves honest instructions for use. The art is not the days without food. The art is knowing when you need to stop.
Maybe you have seen a video. Someone talks about day three, about sudden clarity, a light head and new energy. And you think: I want that too. Next week. Just do it.
I understand that wish very well. There is often something deeper behind it than weight. The need to trust your own body again. To press pause for once. To feel that you can handle more than you thought.
And that is exactly why I want to tell you the other half of the story. The one with the dizziness when you stand up. With the skipped heartbeats on day four. With the blood glucose reading that suddenly drops far too low while you are on medication. Not to scare you. But so that you can decide on what basis you start.
What you can expect in this article
- What switches over in your metabolism during a water fast
- Which potential benefits studies show, fairly assessed
- Electrolytes, circulation, uric acid, low blood sugar, muscle mass
- Refeeding syndrome: NICE criteria, phosphate, thiamine
- The list of contraindications, with reasons
- Warning signs that mean you should stop immediately
- Four KPNI lenses on water fasting
- Three levers before you even start
What switches over inside you during a water fast
Imagine your body as a house with two heating systems. One runs on sugar, fast and convenient. The other runs on fat and ketone bodies, slower, but with a large tank in the basement.
During the first one to two days, the liver's sugar store empties. Then the second heating system kicks in. Insulin drops to a very low level. The liver builds ketone bodies from fatty acids, which your brain can also use as fuel.
In 2014, Longo and Mattson summarized in Cell Metabolism what is known about cellular responses to fasting. They describe adaptations that could reduce oxidative damage and inflammation and strengthen cellular protection programs.
The limitation matters: the protective effects against diabetes, cancer or neurodegeneration come from rodent studies. In humans, the authors cite indications in overweight, high blood pressure, asthma and rheumatoid arthritis.
What this means for you: the biology is fascinating and plausible. But no promise for you can be derived from mice.
Longo VD, Mattson MP. Cell Metab. 2014;19(2):181-192. DOI: 10.1016/j.cmet.2013.12.008At the same time, your salt and water balance shifts. With low insulin, the kidneys excrete more sodium and water. That is one reason why the scale drops so quickly in the first days. And it is the beginning of most of the problems we will talk about in a moment.
Water fasting is not a break for the body. It is a renovation while the building stays in use. Hormones, kidneys, circulation and muscles all switch over at the same time. A renovation can do a lot of good. But it needs someone who keeps an eye on the structure. And now you know why supervision is not a luxury.
The potential benefits, fairly assessed
I want to be honest: there is data that makes me curious. However, most of it comes from specialized clinics, with physicians on site and daily monitoring. That is important to keep in mind when you read it.
In 2001, Goldhamer and colleagues supervised 174 people with high blood pressure as inpatients at the TrueNorth center. After two to three days of fruit and vegetables, they fasted on water alone for an average of ten to eleven days, followed by six to seven refeeding days.
An average reduction of 37/13 mmHg was observed. Almost 90 percent were below 140/90 mmHg at the end. With very high starting blood pressure, the average reduction was 60/17 mmHg.
What this means for you: under medical supervision, water fasting may noticeably influence blood pressure. Without a control group, it remains open how much of this was due to the fasting itself.
Goldhamer A et al. J Manipulative Physiol Ther. 2001;24(5):335-339. DOI: 10.1067/mmt.2001.115263In 2024, Zeiler and colleagues studied 29 people with grade 1 and 2 hypertension without type 2 diabetes in Nutrients. They fasted for a median of eleven days, with a range of 7 to 40 days, followed by a median of five refeeding days.
At the end, median blood pressure was below 130/80 mmHg, and weight had dropped by more than five percent. Most adverse events were mild and transient, and no severe ones occurred.
What this means for you: the direction is confirmed. But this study also comes from the same fasting center and has no comparison group.
Zeiler E et al. Nutrients. 2024;16(22):3959. DOI: 10.3390/nu16223959Pure water or Buchinger: a useful comparison
With Buchinger fasting, you get a little vegetable broth, juice and honey every day. That is only a few calories, but it makes a difference for electrolytes and circulation.
In 2019, Wilhelmi de Toledo and colleagues documented in PLoS One, over one year, 1422 people who fasted for 4 to 21 days on 200 to 250 kilocalories per day, accompanied by exercise and medical care.
Reductions in weight, waist circumference and blood pressure were observed, as well as improvements in blood lipids. Adverse effects occurred in less than one percent.
What this means for you: modified fasting within a structured setting can be well tolerated. The word supervised is the condition here, not the fine print.
Wilhelmi de Toledo F et al. PLoS One. 2019;14(1):e0209353. DOI: 10.1371/journal.pone.0209353A review by Ezpeleta and colleagues in Nutrition Reviews evaluated human studies on fasting periods of 5 to 20 days in 2024. Weight loss was 2 to 10 percent, and blood pressure dropped consistently.
But: three to four months after fasting, the metabolic benefits were no longer detectable in the studies that examined this, even when weight was maintained. Fasting can be an impulse. It is not a permanent state.
If the effects fade after months, water fasting is not an end point. It could be a door through which you walk into a different way of eating and a different daily life. If you see fasting as the goal, you fast again and again. If you see it as a door, you ask: what comes next? And now you know why the refeeding phase counts for more than the record.
The real risks, one at a time
May I ask you an uncomfortable question? On day three, would you know whether your dizziness is normal or a warning sign? Most people don't. And that is not a criticism. It is exactly why supervision matters.
In 2018, Finnell and colleagues reviewed in BMC Complementary and Alternative Medicine the records of 768 stays in which adults consumed only water for at least two days, as inpatients under medical supervision.
75 percent of the documented adverse events were mild and known fasting reactions. In 555 stays, the most severe event remained at grade 2 or below. In 212 stays there was a grade 3 event, once a grade 4 event, and twice a serious adverse event.
What this means for you: even under the best conditions, more than one in four stays involved a grade 3 event. In a clinic, something like this is recognized early. Alone at home, often not.
Finnell JS et al. BMC Complement Altern Med. 2018;18(1):67. DOI: 10.1186/s12906-018-2136-6In 2025, Herman and colleagues compiled in Nutrients the data on hormonal changes during prolonged fasting. They describe a recurring pattern of dampened anabolic signals and activated stress programs.
As possible risks they list temporarily low sex hormones, a relative downregulation of the thyroid, elevated cortisol, orthostatic intolerance, electrolyte disorders, loss of lean mass and problems during refeeding.
Herman R et al. Nutrients. 2025;17(24):3949. DOI: 10.3390/nu17243949Electrolyte derailment
Sodium, potassium, magnesium and phosphate are lost through the kidneys without being replenished. Blood levels can look normal for a long time while the stores inside the cells shrink. Muscle cramps, skipped heartbeats and leaden weakness can be the first signs.
Circulation and dizziness
Less salt, less water in the blood vessels, lower blood pressure. When you stand up quickly, blood briefly pools in your legs. This orthostatic hypotension can lead to blacking out and falls, especially at night on the way to the toilet.
Uric acid and gout
Ketone bodies and uric acid use similar excretion pathways in the kidneys. When ketones rise, uric acid can rise with them. In people with a history of gout, this can favor an attack.
Low blood sugar on medication
If you keep taking insulin or sulfonylureas without food, you give your body a signal to lower sugar even though no sugar is coming in. This can trigger dangerous hypoglycemia.
Uric acid: what the Buchinger data show
In 2024, Grundler and colleagues tracked in Nutrients the daily ketone levels of 1610 people who fasted for 4 to 21 days following the Buchinger Wilhelmi program, supplemented with about 75 to 250 kilocalories per day.
From day four onward, ketones were detectable in the urine of more than 95 percent. In the group with strongly pronounced ketosis, blood uric acid rose more. Pathological ketone levels were not reached.
What this means for you: the deeper the ketosis, the more uric acid. With pure water fasting, ketosis is usually deeper than with broth and juice. If you know gout, you should not try this alone.
Grundler F et al. Nutrients. 2024;16(12):1849. DOI: 10.3390/nu16121849Low blood sugar: the underestimated medication risk
In 2018, Corley and colleagues had 41 people with type 2 diabetes eat very few calories on two days per week for twelve weeks, published in Diabetic Medicine. All took metformin or glucose-lowering medication, which had been adjusted beforehand.
Despite the medication reduction, the rate of hypoglycemia rose during fasting, with a relative risk of 2.05. With education and adjustment, events remained less frequent than expected.
What this means for you: even on individual very low calorie days, the risk nearly doubled. A water fast of several days on insulin or sulfonylureas without a medical plan is therefore not a good idea.
Corley BT et al. Diabet Med. 2018;35(5):588-594. DOI: 10.1111/dme.13595In 2019, Grajower and Horne published a review in Nutrients on fasting with diabetes. They emphasize that the evidence is very thin and go through each diabetes medication individually regarding its safety during fasting. Their conclusion: with supervision and blood glucose monitoring, fasting may be possible even with diabetes. Without that supervision, a relevant risk remains.
Muscle loss: how large is the share?
In 2025, Gabriel and colleagues used DXA in Obesity Science & Practice to measure what adults with overweight lose during water fasting. They fasted for a median of 14 days, followed by refeeding and about six weeks of follow-up.
At the end of the fast, 74 percent of the weight loss was fat-free mass. By follow-up, this had partly recovered, and fat mass then made up 67 percent of the total loss.
What this means for you: fat-free mass also includes water and glycogen, not just muscle. Still, the share is large. If you have little muscle reserve, you pay a higher price.
Gabriel S et al. Obes Sci Pract. 2025;11(4):e70086. DOI: 10.1002/osp4.70086The review by Ezpeleta arrives at a similar picture: with fasting periods of 5 to 20 days, about two thirds of the weight loss was lean mass and one third fat. The authors see this as an indication of increased muscle protein breakdown and explicitly call it a concern.
Five kilos in one week feels like success. But a large part of it is water, glycogen and lean mass. For a young, muscular person this is usually easy to cope with. For an older person with little muscle, it can cost strength that has to be rebuilt with effort. And now you know why the question is not how much you lose, but what.
Refeeding syndrome: the most dangerous phase comes after the fast
It sounds paradoxical. The riskiest moment in water fasting is often not day five without food. It is the moment you start eating again.
During the long break, the body pulls phosphate, potassium and magnesium from the cells into the blood to keep blood levels stable. It is like a bank account that still shows a balance because money is constantly being topped up from the reserve. When food suddenly arrives, especially carbohydrates, insulin rises. Insulin pushes sugar and these minerals abruptly back into the cells. Blood levels may crash.
Phosphate is the raw material for ATP, the energy currency of every cell. If it is missing, the heart, respiratory muscles and nerves can fail. Then there is thiamine, meaning vitamin B1. Sugar metabolism uses it up quickly during refeeding, and the stores are small after a longer fast.
In 2020, the American professional society ASPEN published a consensus definition. A drop in phosphate, potassium or magnesium of 10 to 20 percent is considered mild, 20 to 30 percent moderate, and more than 30 percent or with organ dysfunction severe. What counts is onset within five days of resuming calorie intake. Thiamine deficiency is explicitly included.
da Silva JSV et al. Nutr Clin Pract. 2020;35(2):178-195. DOI: 10.1002/ncp.10474The NICE criteria: who is at high risk
The British guideline body NICE has formulated criteria that Mehanna and colleagues summarized clearly in the BMJ in 2008. High risk exists if at least one of the following criteria applies:
One criterion is enough
- Body mass index below 16
- Unintentional weight loss of more than 15 percent in the last three to six months
- Little or no food intake for more than ten days
- Low levels of potassium, phosphate or magnesium before refeeding
Or two of these criteria
- Body mass index below 18.5
- Unintentional weight loss of more than 10 percent in the last three to six months
- Little or no food intake for more than five days
- Alcohol misuse or certain medications, including insulin, chemotherapy, antacids and diuretics
Look at the list once more. A water fast of more than five days meets one criterion by definition. If low body weight or a diuretic is added, you are formally in the high-risk group.
In 2023, Heuft and colleagues summarized in the Deutsches Ärzteblatt why refeeding syndrome is so easily overlooked: the symptoms are nonspecific, and awareness of the risk is often low.
In the literature they evaluated, malnourished patients with refeeding syndrome had higher mortality after six months, with an odds ratio of 1.54, and a higher risk of admission to intensive care, with an odds ratio of 2.71.
What this means for you: the data come from hospitals, not from fasting courses. But they show how serious the metabolic situation during refeeding can be.
Heuft L et al. Dtsch Arztebl Int. 2023;120(7):107-114. DOI: 10.3238/arztebl.m2022.0381What makes refeeding safe
I am deliberately not giving you a recipe with grams and calories here. The right amount depends on how long you fasted, your weight, your medication and your lab values. The directions, however, are clear:
Directions for refeeding after a water fast of several days
- Start small and warm: small, easily digestible portions spread across the day.
- Increase carbohydrates slowly: they in particular drive insulin and with it the shift of minerals.
- Plan enough time: in the TrueNorth studies, refeeding lasted about half as long as the fast.
- Think about thiamine: for people at risk, vitamin B1 belongs before the first meal, determined by a physician.
- Check lab values: phosphate, potassium and magnesium in the first days of refeeding, daily if at risk.
Many people plan their fasting days precisely and their refeeding days not at all. From a metabolic point of view, it is almost the other way around. The days without food usually go quietly. The days afterwards decide whether your body finds its way back gently or starts to spin out. And now you know why a good fasting plan is most detailed about refeeding.
Who should not water fast alone
Now it gets specific. The following list is not meant to be restrictive for its own sake. It describes situations in which the adaptations of fasting meet a body that has no buffer for them. In some cases I advise against it entirely, in others only with close medical guidance.
| Situation | Why caution |
|---|---|
| Type 1 diabetes | Without food and with adjusted insulin, there is a risk of either hypoglycemia or ketosis getting out of control. The line to ketoacidosis is thin. Grajower and Horne 2019 |
| Insulin or sulfonylureas | These medications lower sugar even when none is coming in. Even on fasting days, the risk of hypoglycemia nearly doubled. Corley 2018, Grajower and Horne 2019 |
| Pregnancy and breastfeeding | Energy and nutrient requirements are increased, and ketosis and deficiency also affect the child. There are no safety data on water fasting during this phase. Clinical consensus, no safety studies |
| Eating disorder, including a past one | Fasting can trigger restrictive eating behavior again. In addition, people with eating disorders belong to the high-risk group for refeeding syndrome. Mehanna 2008, Heuft 2023 |
| Low body weight | A BMI below 18.5 is a NICE criterion; below 16 it is enough on its own for a high refeeding risk. Mehanna 2008 |
| Impaired kidney function | The kidneys regulate sodium, potassium, acid and uric acid during fasting. If their reserve is small, electrolytes and acid-base balance can derail more quickly. Herman 2025, Ezpeleta 2024 |
| Gout or elevated uric acid | With deeper ketosis, uric acid rises more, and a gout attack becomes more likely. Grundler 2024 |
| Heart rhythm disorders | Potassium and magnesium stabilize the heart rhythm. If they drop during fasting or refeeding, this can favor arrhythmias. Herman 2025, da Silva 2020 |
| Diuretics | Water pills increase the loss of salt, potassium and magnesium and are among the NICE risk factors. Mehanna 2008 |
| Lithium | The kidneys handle lithium similarly to sodium. Losses of salt and fluid can cause its level to rise. This is a basic pharmacological rule, not a fasting study. Pharmacological inference |
| Blood pressure medication | Because blood pressure can drop markedly during fasting, the medication may suddenly act too strongly. In the TrueNorth studies, it was reduced or stopped by physicians. Goldhamer 2001, Zeiler 2024 |
| Advanced age, frailty | Smaller muscle reserve, higher risk of falls with dizziness, often several medications at once. Gabriel 2025, Herman 2025 |
Skipped heartbeats or a racing heart at rest. Fainting or near fainting. Confusion or unusual drowsiness. Muscle cramps that don't ease, or pronounced muscle weakness. A suddenly hot, swollen joint. Blood glucose readings that fall below your agreed limit. Shortness of breath or fluid retention in the legs during refeeding.
These are not signs of detox. They are signs that your body needs more right now than endurance.
A water fast that went well a few years ago says little about the next one. What matters is what has changed since then: a new medication, an older age, uric acid that has risen in the meantime, a blood pressure medication.
Often it is not the fasting that has changed, but the body that is fasting. That is why every water fast of several days deserves a fresh assessment beforehand.
If you recognize yourself in the table, it does not mean fasting is off limits for you. It means that pure water alone at home is not the right form. There are often gentler paths: modified fasting with broth, a fasting mimicking diet, a longer eating window during the day. And now you know why the question is not whether you may fast, but how.
Four lenses on water fasting
In Clinical Psychoneuroimmunology, we look at an event through four windows at once. Water fasting is a good example of how closely these systems are interlinked.
Nervous system
Fasting pushes you toward the sympathetic side: alert, focused, sometimes restless. The brain increasingly uses ketone bodies. At the same time, it reacts sensitively to sodium deficiency and low blood pressure. Clarity and lightheadedness are therefore closer together than many people think.
Immune system
Animal studies suggest that fasting might dampen inflammatory processes. In humans, this is less clear. Conversely, rising uric acid can trigger, via crystals in the joint, exactly the kind of inflammation you actually wanted to calm.
Metabolism
The liver switches to ketone production, insulin falls, and the cells break down proteins to provide sugar for the organs that depend on it. That is the price paid in lean mass. During refeeding, this system flips back within hours, with all the mineral shifts that come with it.
Hormonal system
Cortisol rises, thyroid hormones and sex hormones are dialed down. This makes sense for saving energy. For people with cycle disorders, thyroid disease or chronic stress, however, this additional pressure can be too much.
This is exactly the point where it becomes bigger than a symptom. It is not about holding out for a few days. It is about whether you have more strength after the fast or less. Whether you trust your body more afterwards or fear it. That is the freedom I care about: self-chosen, well prepared, without nasty surprises.
Three levers before you even start
If you still feel like water fasting after this article, I think that's good. Then do it with preparation. Three things are enough to start with.
1. Check beforehand
Have your blood pressure, kidney values, electrolytes, uric acid and blood glucose measured. And go through your medication list with your physician before you begin the first day.
2. Start small
Begin with 24 to 36 hours before you think about several days. You learn how your circulation reacts without entering refeeding risk.
3. Supervision for more than two days
From about two to three days onward, you need someone who sees you, measures and stops you if in doubt. Including planned refeeding days.
Water fasting is not a test of courage. It is a medical intervention with real potential benefits and real risks. Courage means asking yourself beforehand who it is right for.
And now you know why I would rather give you an honest list than a nice story.
Where else this topic plays a role
Water fasting touches blood sugar, circulation, kidneys, muscles and hormones all at once. You can find more articles on types of fasting, breaking a fast and gentler alternatives collected in the Fasting section.
Water fasting
Benefits, risks, contraindications
this articleFasting Guide
All articles on types of fasting, effects and practice
Blood sugar
Insulin, hypoglycemia and medication during fasting
Muscle mass
Lean mass, protein and strength after fasting
Frequently asked questions about water fasting
Is water fasting dangerous?
For healthy adults under medical supervision, water fasting may be well tolerated. In an analysis of 768 supervised water-only fasting stays, most adverse events were mild to moderate and known fasting reactions.
It can become dangerous with pre-existing conditions, certain medications, low body weight, eating disorders, or when a fast of several days is done alone at home without monitoring of circulation and lab values.
How long can you water fast before it becomes risky?
There is no fixed limit, because the risk depends more on your starting point than on the number of days. For healthy adults, 24 to 36 hours are usually unproblematic.
From about two to three days onward, the demands on electrolytes, circulation and medication planning increase. Under the NICE criteria, little or no food intake for more than five days is already a risk factor for refeeding syndrome. A water fast of several days therefore belongs under medical supervision.
Who should not water fast?
People who should not fast at all, or only under close medical guidance, include those with type 1 diabetes, people on insulin or sulfonylureas, pregnant and breastfeeding women, people with an eating disorder or low body weight, impaired kidney function, gout or heart rhythm disorders, and people taking diuretics or lithium.
Children and adolescents as well as frail older adults should not do a water fast without supervision either.
What is refeeding syndrome in water fasting?
Refeeding syndrome is a metabolic derailment that can occur when food intake resumes after a longer period without food. Insulin rises and rapidly pushes phosphate, potassium and magnesium into the cells, and blood levels may crash.
The ASPEN consensus recommendations explicitly include thiamine deficiency. This is why a slow reintroduction of food, lab checks and, for people at risk, thiamine before the first meal belong in the plan.
Why do I feel dizzy during a water fast?
During the first days of fasting, the kidneys excrete more sodium and water, and blood pressure drops. When you stand up quickly, blood can briefly pool in your legs before your circulation adjusts. This is called orthostatic hypotension.
A review of hormonal adaptations to prolonged fasting lists this orthostatic intolerance among the potential risks. People taking blood pressure medication are particularly affected, because the medication may then act too strongly.
Can water fasting trigger a gout attack?
Yes, that is possible. Ketone bodies compete with uric acid for excretion in the kidneys, so uric acid in the blood may rise.
In a cohort of 1610 people fasting in a Buchinger clinic, the group with pronounced ketosis showed a larger rise in uric acid. If you have had a gout attack before or know that your uric acid is elevated, you should only fast under medical supervision.
Do I lose muscle during a water fast?
Part of the weight lost during a longer water fast is fat-free mass. A review in Nutrition Reviews found that about two thirds of the weight loss was lean mass. This also includes water and glycogen, not only muscle protein.
In a study with a subsequent change in diet, fat-free mass partly recovered during follow-up. For older adults and people with little muscle reserve, this is still a point to take seriously.
Can I water fast if I have diabetes?
With type 1 diabetes, I advise against water fasting. With type 2 diabetes, fasting is only justifiable with a medically adjusted medication plan and blood glucose monitoring.
In a randomized trial in type 2 diabetes, the rate of hypoglycemia on fasting days nearly doubled, even though medication had been reduced. Insulin and sulfonylureas carry the greatest risk here.
What can water fasting do for blood pressure?
In a clinical series of 174 people with high blood pressure, blood pressure during medically supervised water fasting fell by an average of 37/13 mmHg, and almost 90 percent were below 140/90 mmHg at the end. A newer single-arm study with 29 participants pointed in a similar direction.
Neither study had a control group, and both come from the same fasting center. The result is encouraging, but not proof in the strict sense.
How do I break a water fast safely?
Slowly, in small amounts and under control. After a fast of several days, most programs start with small, warm, easily digestible portions and increase quantity and carbohydrates over several days.
In the TrueNorth study, the refeeding phase lasted a median of five days after eleven fasting days. For people at risk, electrolyte checks and thiamine belong in the first days of refeeding.
Scientific sources
All sources were checked via PubMed for existence, authorship, journal, year and abstract content. The study type is given in square brackets so that you can judge the strength of the evidence yourself.
- Goldhamer A, Lisle D, Parpia B, Anderson SV, Campbell TC. Medically supervised water-only fasting in the treatment of hypertension. J Manipulative Physiol Ther. 2001;24(5):335-339. DOI: 10.1067/mmt.2001.115263 [Case series, clinical, n=174, no control group]
- Zeiler E, Gabriel S, Ncube M et al. Prolonged Water-Only Fasting Followed by a Whole-Plant-Food Diet Is a Potential Long-Term Management Strategy for Hypertension and Obesity. Nutrients. 2024;16(22):3959. DOI: 10.3390/nu16223959 [Real-world, single-arm interventional study, n=29]
- Finnell JS, Saul BC, Goldhamer AC, Myers TR. Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting. BMC Complement Altern Med. 2018;18(1):67. DOI: 10.1186/s12906-018-2136-6 [Real-world, retrospective chart review, 768 stays]
- Gabriel S, Ncube M, Goldman DM, Scharf E, Goldhamer AC, Myers TR. Prolonged Water-Only Fasting Followed by a Whole-Plant-Food Diet Promotes Fat-Free Mass Recovery and Continued Fat Mass Loss in Adults With Overweight or Obesity. Obes Sci Pract. 2025;11(4):e70086. DOI: 10.1002/osp4.70086 [Real-world, secondary analysis with DXA]
- Ezpeleta M, Cienfuegos S, Lin S, Pavlou V, Gabel K, Varady KA. Efficacy and safety of prolonged water fasting: a narrative review of human trials. Nutr Rev. 2024;82(5):664-675. DOI: 10.1093/nutrit/nuad081 [Review, human studies]
- Herman R, Trsan J, Lipar L, Jensterle M, Janez A. Endocrine Adaptations to Prolonged Fasting: From Physiology, Clinical Uncertainties, Translational Challenges to Healthspan Implications. Nutrients. 2025;17(24):3949. DOI: 10.3390/nu17243949 [Review, narrative review]
- Grundler F, Mesnage R, Ruppert PMM, Kouretas D, Wilhelmi de Toledo F. Long-Term Fasting-Induced Ketosis in 1610 Subjects: Metabolic Regulation and Safety. Nutrients. 2024;16(12):1849. DOI: 10.3390/nu16121849 [Real-world, observational study, n=1610]
- Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. 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: 10.1371/journal.pone.0209353 [Real-world, prospective observational study, n=1422]
- Corley BT, Carroll RW, Hall RM, Weatherall M, Parry-Strong A, Krebs JD. Intermittent fasting in Type 2 diabetes mellitus and the risk of hypoglycaemia: a randomized controlled trial. Diabet Med. 2018;35(5):588-594. DOI: 10.1111/dme.13595 [RCT, n=41, type 2 diabetes]
- Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. DOI: 10.3390/nu11040873 [Review, expert-based recommendations]
- Longo VD, Mattson MP. Fasting: molecular mechanisms and clinical applications. Cell Metab. 2014;19(2):181-192. DOI: 10.1016/j.cmet.2013.12.008 [Mechanistic review, mostly animal models]
- Mehanna HM, Moledina J, Travis J. Refeeding syndrome: what it is, and how to prevent and treat it. BMJ. 2008;336(7659):1495-1498. DOI: 10.1136/bmj.a301 [Review, with NICE criteria]
- da Silva JSV, Seres DS, Sabino K et al. ASPEN Consensus Recommendations for Refeeding Syndrome. Nutr Clin Pract. 2020;35(2):178-195. DOI: 10.1002/ncp.10474 [Review, consensus recommendation]
- Heuft L, Voigt J, Selig L, Stumvoll M, Schlögl H, Kaiser T. Refeeding Syndrome. Dtsch Arztebl Int. 2023;120(7):107-114. DOI: 10.3238/arztebl.m2022.0381 [Review]
Transparency about the evidence. The studies on the potential benefits of water fasting come mostly from specialized fasting centers, are mostly single-arm without a control group, and several authors work for the institutions studied. Randomized long-term studies on hard endpoints are lacking. The data on refeeding syndrome come mostly from the hospital treatment of malnourished people and are applied to fasting here. This transfer is physiologically well founded, but it is an inference. The notes on pregnancy and lithium are based on clinical consensus and basic pharmacological rules, not on fasting studies.
No substitute for medical advice. This article is for information only and does not replace an individual medical examination, diagnosis or treatment. Do not stop or change any medication on your own, and plan any fast of several days in advance with your physician.