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Water Fasting: What to Know Before You Start

Water fasting means consuming only water for a set period, typically 24 to 72 hours, with no food or caloric drinks. Studies report side effects such as headaches and hunger and, in fasts of 3 days or less, a rise in LDL cholesterol; some groups, including people managing diabetes, should not attempt it without a clinician.

By the fasting.com team

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A glass of water on a wooden kitchen table, lit by low morning sun.

Medical caution

This is general information about fasting, not medical advice. Fasting can affect blood sugar, blood pressure, medication timing, and nutritional status. Talk to a clinician before starting or changing a fasting practice if you are pregnant, under 18, living with diabetes or another chronic condition, taking prescription medication, or have a history of an eating disorder.

What Water Fasting Is

Water fasting means consuming only water, with no food and no caloric drinks, for a set period, typically 24 to 72 hours. It differs from intermittent fasting, which cycles between eating and fasting windows most days, and from a fasting-mimicking diet, which allows a small amount of specific food.

Is "Japanese Water Fasting" a Real Fast?

"Japanese water fasting" is not a fast. As the term is commonly used online, it usually refers to "Japanese water therapy": drinking several glasses of water on waking, then eating normal meals with set waiting periods around them. It involves no period of food restriction, and no clinical trial has tested this specific routine. A separate randomized trial of 84 primary-care adults with obesity found that drinking about 500 ml of water before main meals was linked to 1.3 kg more weight loss than the comparison group at 12 weeks, though after adjusting for ethnicity, deprivation, age, and gender the difference (1.2 kg) was no longer statistically significant (Parretti et al., 2015) [11]. That trial is about mealtime fullness, not fasting, and it is not evidence for the named routine itself. Drinking a large volume of water quickly can cause water intoxication (hyponatremia), so more is not better. If you are looking for an actual water fast, see what water fasting is and how long you can water fast on this page.

How Long Can You Water Fast

24 and 36 Hour Fasts

A single 24-hour water-only fast significantly increased human growth hormone, red blood cell count, hematocrit, and total and HDL cholesterol, and decreased triglycerides and weight, compared with a day of normal eating, in a randomized crossover trial of 30 apparently healthy volunteers; in the 16 participants who fasted first, most of these changes, including growth hormone and cholesterol, were back to baseline after a following day of normal eating, except weight, with triglycerides also staying suggestively lower (Horne et al., 2013) [8]. A 36-hour fasting study in untrained and endurance-trained adults measured muscle autophagy markers, not the weight, blood pressure, or blood-sugar outcomes discussed on this page; its authors' summary is that muscle autophagy was "only modestly affected" (Dethlefsen et al., 2018) [9]. Everyday context helps calibrate scale: an overnight fast of 8 to 10 hours is already normal for most people, and even extended daily fasting patterns show generally small overall metabolic effects (Maughan et al., 2010) [5]. This page makes no outcome claim specific to 36 hours.

48 and 72 Hour Fasts

The most detailed day-by-day human data in this set comes from a 7-day trial, longer than the 48 to 72 hour range in this section, which found an average weight loss of 5.7 kilograms in 12 healthy volunteers, with distinct, body-wide protein changes appearing only after day three of complete calorie restriction (Pietzner et al., 2024) [2]. That is a small, 12-person study with no clinical-outcome measures, useful for understanding what happens inside the body day by day, not for a specific outcome claim at 48 or 72 hours. The pooled review of human trials cited throughout this page includes fasts as short as about 5 days (Ezpeleta et al., 2024) [1], still longer than the 48 to 72 hour range most readers mean by these durations. The lipid meta-analysis described below is the one exception, and it covers blood lipids only (Çamli et al., 2026) [10].

What Happens in Your Body During a Water Fast

As glycogen stores run down, the body shifts toward ketone-body metabolism, the same adaptation that makes multi-day fasting survivable at all (Rebello et al., 2025) [3]. This shift is a coordinated, multi-system response to falling insulin, described in the broader fasting-physiology literature as affecting metabolism throughout the body (Longo and Mattson, 2014) [4]; most of the disease-specific findings in that broader literature, for conditions such as diabetes, cancer, heart disease, and neurodegeneration, come from rodent studies, and this page does not extend those findings to a claim about human disease outcomes.

What the Research Actually Shows

A narrative review of human trials of prolonged water-only fasting (5 to 20 days) found mild to moderate weight loss of 2% to 10% of body weight; in the two studies in that review that measured body composition (an 8-day and a 10-day fast, one by bioimpedance, which the authors note is unreliable when hydration changes), roughly two-thirds of the weight lost was lean mass and one-third fat mass (Ezpeleta et al., 2024) [1]. The authors flag that lean-mass proportion as a concern suggesting increased muscle protein breakdown, not simply an incidental side effect of weight loss. What happens to that lean mass after the fast ends is not settled: a DXA-based follow-up of 29 adults with overweight or obesity after a median 14-day supervised water-only fast found fat-free mass partly regained by a follow-up visit about 6 weeks after refeeding, with fat loss making up most (67%) of the weight still off (Gabriel et al., 2025) [12], while a 10-day supervised fast with a small daily calorie allowance and daily supervised exercise in 16 healthy men found body weight and lean tissue still below baseline at 3 months while fat mass was no longer significantly different from baseline (Laurens et al., 2021) [13]; see "Where the studies disagree" above for how the two studies differ and why this page does not reconcile them. Blood pressure consistently decreased across the trials reviewed. Glycemic measures improved in people with normal blood sugar but did not improve in people with type 1 or type 2 diabetes, and lipid effects were inconsistent from trial to trial. Metabolic improvements were no longer detectable three to four months after the fast ended, even when the weight loss itself was maintained, so the research's own conclusion is that prolonged fasting is "moderately safe" and can produce meaningful weight loss over days to weeks, while whether the metabolic improvements last is still an open question (Ezpeleta et al., 2024) [1].

The lipid finding in that review, effects "inconsistent" from trial to trial, now has a duration-dependent shape. A 2026 meta-analysis of 32 water-only-fasting studies, mostly small before-and-after studies without a control group, found that in fasts of 3 days or less, the range this page covers, LDL cholesterol (Hedges' g=0.322, 95% CI 0.151 to 0.494) and total cholesterol (g=0.152, 95% CI 0.013 to 0.291) rose, HDL cholesterol did not change (g=-0.013, 95% CI -0.230 to 0.204), and triglycerides showed a downward trend that was not statistically significant (g=-0.335, 95% CI -0.684 to 0.013). In longer fasts, HDL fell and the LDL and total-cholesterol rises were larger, though the difference between the two duration groups was not statistically significant for LDL or total cholesterol. The authors rate the certainty of this evidence low for HDL and very low for the other lipids, report signs of publication bias for LDL and total cholesterol, and describe the duration thresholds as hypothesis-generating rather than clinically actionable (Çamli et al., 2026) [10]. Its finding of no HDL change in short fasts differs from the single 24-hour trial above, which found HDL rose (Horne et al., 2013) [8]. A separate, large observational study of a supervised, modified fasting program (1,422 participants, 4 to 21 days) reported favorable shifts in blood pressure, weight, and lipids, and a low rate of physician-documented adverse effects (self-reported mild symptoms were more common), though that program allowed a small daily caloric intake rather than strict water only and had no control group (Wilhelmi de Toledo et al., 2019) [6]. A chart review of 768 medically supervised, strictly water-only fasting visits (at least 2 consecutive days, with a refeeding period equal to half the fast length) found that the highest-grade adverse event was severe (grade 3) in 212 visits and life-threatening (grade 4) in 1 visit, with no deaths; 2 of 768 visits involved a serious adverse event (Finnell et al., 2018) [7].

Electrolytes and Hydration

Plain water contains no electrolytes, and multi-day fasting can shift the body's sodium, potassium, and magnesium balance as glycogen and its associated water stores deplete. This is a practical, general caution rather than a finding from a specific trial in this set; anyone water fasting beyond a day should pay close attention to how they feel and should not treat thirst or fatigue as something to push through.

Side Effects to Expect

Reported adverse events across the human trials reviewed include headaches, insomnia, hunger, and dry mouth; the review reports no serious adverse events, metabolic acidosis, or deaths in those trials, though it notes more severe events, including metabolic acidosis, documented in earlier reports (Ezpeleta et al., 2024) [1]. A chart review of 768 medically supervised, water-only fasting visits found most adverse events were mild to moderate, a severe (grade 3) event in 212 visits, a single life-threatening event, and no deaths; 2 of 768 visits involved a serious adverse event (Finnell et al., 2018) [7]. This is a list of what has been reported in the cited trials, not an exhaustive list of everything that can happen during a water fast.

Who Should Not Water Fast Without a Clinician

People with type 1 or type 2 diabetes should not attempt a water fast without a clinician: the one citation testing glycemic measures directly in people with diabetes found they did not improve during prolonged fasting (Ezpeleta et al., 2024) [1]. Separately, as a matter of general clinical knowledge rather than a finding from that review, extended periods without food carry their own hypoglycemia and ketoacidosis risk on glucose-lowering medication. Pregnancy, being under 18, and a history of an eating disorder are not directly studied in any citation reviewed for this page; these are reasons for general clinical caution rather than sourced findings, and a clinician should be involved before attempting a water fast in any of these situations. Receiving cancer treatment is also a reason to involve a clinician before attempting a water fast, as a matter of general clinical caution.

How to Break a Water Fast Safely

Longer fasts carry higher refeeding risk when they end. See how to break a fast safely for the full guide to refeeding, including why electrolyte shifts matter.

Authors and Funding of the Cited Studies

  • Longo and Mattson, 2014 [4]: Valter D. Longo (University of Southern California) is first and corresponding author; Mark P. Mattson was at the National Institute on Aging. Funding: "This work was supported, in part, by the Intramural Research Program of the National Institute on Aging, by the Glenn Foundation for Medical Research, and by NIH/NIA grants AG20642, AG025135, and AG034906 to VDL." No competing-interest statement appears in the paper's PubMed or PMC record.
  • Wilhelmi de Toledo et al., 2019 [6]: the lead author, FWT, is a member of the Directory Board of the Buchinger Wilhelmi Clinic (BWC), where the study was performed, and, as managing director of Amplius GmbH, executes the scientific leadership at BWC; Amplius GmbH "conceives, coordinates and develops fasting research on behalf of BWC." Co-author FG is currently employed, and co-author SD was formerly employed, at BWC. Co-author AM is a consultant at BWC and receives financial compensation for that role from Amplius GmbH.
  • Finnell et al., 2018 [7]: "ACG is the director of the TrueNorth Health Center, president of the board of the TrueNorth Health Foundation, and member of the board of the National Health Association. He does not receive financial compensation for these roles. BCS and TRM are paid consultants for the TrueNorth Health Foundation, which is funded in part by the TrueNorth Health Center. JSF received funds from the National Health Association grant."
  • Çamli et al., 2026 [10]: funded by the Scientific Research Projects Coordination Unit of Istanbul Yeni Yuzyil University. Disclosure: "The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest."
  • Gabriel et al., 2025 [12]: the paper states, "The authors received no specific funding for this work." Conflicts of interest, quoted from the paper: "D.M.G. consults for Metabite Inc. A.C.G. is owner of TrueNorth Health Center and President of the Board of the TrueNorth Health Foundation. All other authors declare no conflicts of interest." D.M.G. is Goldman; A.C.G. is Goldhamer. The paper's author list gives the TrueNorth Health Foundation as the affiliation of Gabriel, Ncube, and Myers.
  • Laurens et al., 2021 [13]: funded by the CNRS and Amplius GmbH (Überlingen, Germany), with an additional grant from the Laboratoire Therascience (Monaco) for hormonal and metabolic assays; the paper states no additional external funding was received. Conflicts of interest, quoted from the paper: Françoise Wilhelmi de Toledo "is managing director of Amplius GmbH, a company that coordinates the scientific documentation for the Buchinger Wilhelmi Clinics and is co-worker of the Buchinger Wilhelmi Clinic," and Franziska Grundler "is co-worker at the Buchinger Wilhelmi Clinic." The paper states the other ten authors declare no conflict of interest.
  • Disclosures are listed where the full text of the paper was available to us; a study not listed here is not a statement that its authors have no ties.

Safety and Cautions

This is general information, not medical advice. Do not attempt a water fast of any length without talking to a clinician first if you are pregnant, under 18, have a history of an eating disorder, manage diabetes with medication, or are receiving cancer treatment. Reported side effects include headaches, insomnia, hunger, and dry mouth; more severe events, including metabolic acidosis, have been documented in earlier reports, not in the trials that the Ezpeleta et al., 2024 review covered; stop and seek care if symptoms are severe or you feel unwell.

What the evidence actually supports

Across human trials of prolonged water-only fasting lasting 5 to 20 days, a narrative review found mild to moderate weight loss of 2% to 10% of body weight; in the two studies in that review that measured body composition (an 8-day and a 10-day fast, one by bioimpedance, which the authors note is unreliable when hydration changes), about two-thirds of the weight lost was lean mass and one-third fat mass, a split the review's authors flag as a concern suggesting increased muscle protein breakdown (Ezpeleta et al., 2024) [1]. Systolic and diastolic blood pressure consistently decreased across the trials reviewed; glycemic measures (fasting glucose, insulin, insulin resistance, HbA1c) improved in people with normal blood sugar but remained unchanged in people with type 1 or type 2 diabetes, and the metabolic benefits were no longer detectable three to four months after the fast ended, even when weight loss was maintained (Ezpeleta et al., 2024) [1]. A smaller mechanistic study of a 7-day water-only fast in 12 healthy volunteers found distinct, body-wide protein changes appearing only after day three of complete calorie restriction (Pietzner et al., 2024) [2]. The physiological ability to sustain a multi-day fast depends on the shift to ketone-body metabolism once glycogen stores are depleted (Rebello et al., 2025) [3]. A one-year observational study of 1,422 people doing a supervised, modified fast (4 to 21 days, with a small daily intake of 200 to 250 kilocalories, not strictly zero-calorie) found significant reductions in weight, waist circumference, and blood pressure, with adverse effects, documented by the clinic's physicians, in fewer than 1% of participants by the authors' own summary (14 recorded events in 1,422 people, including two hospitalisations, one for a heart attack on fasting day 9 in a 75-year-old man with known coronary artery disease), while self-reported mild symptoms were more frequent (sleep disturbance in 15%, fatigue in 14%, and hunger in 7% of participants who returned the daily symptom questionnaire) (Wilhelmi de Toledo et al., 2019) [6]. A separate chart review of 768 strictly water-only fasting visits under medical supervision found the highest-grade adverse event was severe (grade 3) in 212 visits and life-threatening (grade 4) in 1 visit, with no deaths; 2 of 768 visits involved a serious adverse event (Finnell et al., 2018) [7].

Where the studies disagree

A randomized crossover trial directly studied a 24-hour water-only fast (Horne et al., 2013) [8], and a 36-hour fasting study measured only muscle autophagy markers, which its authors found "only modestly affected" (Dethlefsen et al., 2018) [9]; the shortest multi-day duration in the pooled review of health outcomes is about 5 days (Ezpeleta et al., 2024) [1], and the most detailed day-by-day data comes from a 7-day trial (Pietzner et al., 2024) [2]. The one pooled analysis here that includes fasts of 3 days or less covers blood lipids only, and for those short fasts it found no change in HDL cholesterol, unlike the 24-hour trial, which found HDL rose (Çamli et al., 2026 [10]; Horne et al., 2013 [8]). This page makes no outcome claim specific to 36 hours. The supportive findings above also come with a durability limit worth weighing against them directly: the metabolic improvements (glucose, insulin, lipids) were no longer detectable three to four months after the fast ended, even in people whose weight loss held, so whether the benefits outlast the fast itself is still an open question, not a settled part of the supportive case (Ezpeleta et al., 2024) [1]. The 1,422-person study (Wilhelmi de Toledo et al., 2019) [6] is a large observational study with no control group, so it shows this supervised program was associated with good outcomes and a low rate of physician-documented adverse effects, with self-reported mild symptoms such as sleep disturbance and fatigue reported by up to 15% of participants who returned the daily symptom questionnaire, not that water fasting causes those outcomes in a controlled comparison; the protocol studied also included 200 to 250 kilocalories a day from juice and soup, a modified fast rather than strictly water-only. Most of the disease-outcome evidence linked to fasting physiology in general (diabetes, cancer, heart disease, neurodegeneration) comes from rodent studies; the human evidence for water fasting specifically is limited to the more general outcomes described above, and this page does not extend the rodent findings to a human claim (Longo and Mattson, 2014) [4]. What happens to that lean-mass loss after a fast ends is also unsettled, and the two follow-up studies cited here disagree with each other rather than with this page. One followed 29 adults with overweight or obesity (median age 62, 23 of them women; 29 of 40 enrolled completed the follow-up scan) through a median 14-day supervised water-only fast, a 6-day refeed onto a whole-plant-food diet, and a follow-up visit a median 45 days after that, measuring body composition by DXA: fat-free mass was partly regained after the fast, and by the follow-up visit fat loss made up most (67%) of the weight still off (Gabriel et al., 2025) [12]. The other followed 16 healthy men (mean BMI 26) through a 10-day supervised fast that allowed 200 to 250 kilocalories a day from juice, broth, and honey, not strictly water-only, combined with up to 3 hours a day of supervised low-intensity exercise, with a 3-month follow-up measured by bio-impedance in 12 of them: body weight and lean soft tissue were still significantly below baseline, while fat mass was no longer significantly different from baseline (Laurens et al., 2021) [13]. Neither study had a control group, and both papers attribute part of the lean mass lost during the fast itself to water rather than tissue. The two differ in population, protocol, refeeding diet, measurement method, and follow-up length; this page does not pick a winner between them, and neither result is read here as what happens to all fasters.

References

  1. Ezpeleta M, Cienfuegos S, Lin S, Pavlou V, Gabel K, Varady KA (2024). Efficacy and safety of prolonged water fasting, a narrative review of human trials. Nutrition Reviews. PMID 37377031 doi:10.1093/nutrit/nuad081 Finding: supports
  2. Pietzner M, Uluvar B, Kolnes KJ, Jeppesen PB, Frivold SV, Skattebo Ø, Johansen EI, Skålhegg BS, Wojtaszewski JFP, Kolnes AJ, Yeo GSH, O'Rahilly S, Jensen J, Langenberg C (2024). Systemic proteome adaptions to 7-day complete caloric restriction in humans. Nature Metabolism. PMID 38429390 doi:10.1038/s42255-024-01008-9 Finding: limited evidence
  3. Rebello CJ, Zhang D, Anderson JC, Bowman RF, Peeke PM, Greenway FL (2025). From starvation to time-restricted eating, a review of fasting physiology. International Journal of Obesity. PMID 39369112 doi:10.1038/s41366-024-01641-0 Finding: supports
  4. Longo VD, Mattson MP (2014). Fasting, molecular mechanisms and clinical applications. Cell Metabolism. PMID 24440038 doi:10.1016/j.cmet.2013.12.008 Finding: limited evidence
  5. Maughan RJ, Fallah J, Coyle EF (2010). The effects of fasting on metabolism and performance. British Journal of Sports Medicine. PMID 20484315 doi:10.1136/bjsm.2010.072181 Finding: supports
  6. Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A (2019). Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. PMID 30601864 doi:10.1371/journal.pone.0209353 Finding: supports
  7. Finnell JS, Saul BC, Goldhamer AC, Myers TR (2018). Is fasting safe, a chart review of adverse events during medically supervised, water-only fasting. BMC Complementary and Alternative Medicine. PMID 29458369 doi:10.1186/s12906-018-2136-6 Finding: supports
  8. Horne BD, Muhlestein JB, Lappé DL, May HT, Carlquist JF, Galenko O, Brunisholz KD, Anderson JL (2013). Randomized cross-over trial of short-term water-only fasting: metabolic and cardiovascular consequences. Nutrition, Metabolism and Cardiovascular Diseases. PMID 23220077 doi:10.1016/j.numecd.2012.09.007 Finding: supports
  9. Dethlefsen MM, Bertholdt L, Gudiksen A, Stankiewicz T, Bangsbo J, van Hall G, Plomgaard P, Pilegaard H (2018). Training state and skeletal muscle autophagy in response to 36 h of fasting. Journal of Applied Physiology. PMID 30161009 doi:10.1152/japplphysiol.01146.2017 Finding: limited evidence
  10. Çamli A, Ülker İ, Terzi M, Yağan E, Özbay S, Ulupınar S (2026). Duration-dependent effects of water-only fasting on blood lipids: a systematic review, meta-analysis, and threshold meta-regression. Frontiers in Nutrition. PMID 41994097 doi:10.3389/fnut.2026.1772246 Finding: supports
  11. Parretti HM, Aveyard P, Blannin A, Clifford SJ, Coleman SJ, Roalfe A, Daley AJ (2015). Efficacy of water preloading before main meals as a strategy for weight loss in primary care patients with obesity, RCT. Obesity (Silver Spring, Md.). PMID 26237305 doi:10.1002/oby.21167 Finding: limited evidence
  12. Gabriel S, Ncube M, Goldman DM, Scharf E, Goldhamer AC, Myers TR (2025). 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. Obesity Science & Practice. PMID 40765844 doi:10.1002/osp4.70086 Finding: limited evidence
  13. Laurens C, Grundler F, Damiot A, Chery I, Le Maho AL, Zahariev A, Le Maho Y, Bergouignan A, Gauquelin-Koch G, Simon C, Blanc S, Wilhelmi de Toledo F (2021). 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. PMID 34668663 doi:10.1002/jcsm.12766 Finding: limited evidence