Water Fasting: What to Know Before You Start

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.

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.

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, roughly two-thirds from lean mass and one-third from 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. 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]. 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 self-reported adverse effects, 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 metabolic acidosis, headaches, insomnia, and hunger (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.

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. Funded by the National Institutes of Health. 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."
• 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, or manage diabetes with medication. Reported side effects include metabolic acidosis, headaches, insomnia, and hunger; stop and seek care if symptoms are severe or you feel unwell.