Water Fasting: What Trials Found and Who Should Avoid It

In 30 seconds

Water fasting means having only water, with no food or other drinks, often for days. In small trials of 5- to 20-day fasts, the benefits had faded 3 to 4 months later, and in the 2 that measured body composition, about two-thirds of the weight lost was lean mass. Many people shouldn’t try it, and Blimbi gives no water-fasting plan.

• Water fasting means water only, usually for longer than intermittent fasting.
• In the 2 trials that measured it (8- and 10-day water fasts), about two-thirds of the weight lost was lean mass.
• Where people were followed up, the benefits were gone 3 to 4 months later.
• Not for pregnancy, under-18s or eating disorders; with diabetes, medicines or older age, only with a doctor.

What is water fasting?

A water fast cuts out all food and every drink except water.

A narrative review classes water-only fasting as prolonged fasting, eating little or no food for several days to weeks, while intermittent fasting uses fasts of about 16 to 36 hours.Source 11

Dry fasting adds the risk of dehydration; see dry fasting.

What did the trials find?

The main summary is a review of 8 small trials, some water-only and some allowing about 200 to 300 kcal a day.

Across fasts of 5 to 20 days, weight fell by 2 to 10%. In the 2 trials that measured body composition, about two-thirds of the weight lost was lean mass, which the authors call a concern.Source 11

Blood pressure fell, and blood sugar markers improved in people without diabetes but not in those with it. Insomnia, fatigue, dizziness, headaches and hunger were frequent.Source 11

In the few trials with follow-up, the metabolic benefits were gone 3 to 4 months after the fast, even when the weight stayed off.Source 11

Most of the 8 trials had no control group and several ran in inpatient clinics. They reported no serious adverse events, but most didn't record oedema, liver function, bone density, malnutrition or acidosis, so whether long fasts cause serious harm is unclear.Source 11

What about 24-, 48- or 72-hour fasts?

Those trials lasted 5 days or more. Blimbi’s sources don’t show that a 24-, 48- or 72-hour water-only fast improves health, so there’s no schedule to give. The fasting timeline explains what changes in the first day or two.

Who shouldn’t water fast?

When should you stop?

Eating again carries its own risk.

A BMJ clinical review describes refeeding syndrome as potentially fatal shifts in fluids and salts when malnourished patients are fed again by tube or drip, and treats any patient who has eaten almost nothing for more than 5 days as at risk.Source 22

It’s written for hospital care, though it lists prolonged fasting among high-risk situations. Blimbi gives no plan for ending a multi-day fast: if you’ve eaten little or nothing for more than 5 days, get medical advice before eating normally again.

Frequently asked questions

Talk to a qualified health professional before fasting if you are pregnant or breastfeeding, under 18, living with or recovering from an eating disorder, have diabetes, take insulin or a medicine affected by meal timing, are an older adult, or are acutely ill. Do not change the dose or timing of any medicine without your clinician. See Blimbi’s safety guide.

Related guides

• Dry Fasting: What It Is and Why It Isn't Safe

  Dry fasting means no food and no fluids, not even water. Why the dehydration risk makes it unsafe, the warning signs to know and what fasting trials allowed instead.

• Who Should Not Do Intermittent Fasting Without Advice?

  Who should not do intermittent fasting without advice? Diabetes medicines, pregnancy and eating disorders need care; few data on children or over-60s.

• Intermittent Fasting Side Effects: When to Stop

  Intermittent fasting side effects may start with hunger, headache, dizziness or irritability. With diabetes, know the warning signs and when to seek care.

• Fasting Timeline: What Happens and Why Timing Varies

  This fasting timeline explains how the body moves from meal glucose to liver glycogen, then to fat and ketones, and why the timing varies.

Frequently asked questions

What is water fasting?

Fasting with water only, often for days. A review classes it as prolonged fasting, not intermittent fasting.Source 11

Is water fasting safe?

It hasn't been shown to be: the trials were small and most didn't check for serious problems. Anyone pregnant, under 18 or with an eating disorder shouldn't try it.Source 11Source 55Source 33

How long can you water fast?

No source here sets a safe length for a water fast on your own, and Blimbi doesn't suggest one. Water-only trials lasted 5 to 17 days under study conditions.Source 11

Sources

1. 1

   Efficacy and safety of prolonged water fasting: a narrative review of human trialsMark Ezpeleta et al. Oxford University Press (OUP), April 12, 2024.DOI: 10.1093/nutrit/nuad081 · PMID: 37377031Narrative or mechanistic review

2. 2

   Refeeding syndrome: what it is, and how to prevent and treat itHisham M Mehanna, Jamil Moledina, Jane Travis. BMJ, June 28, 2008.DOI: 10.1136/bmj.a301 · PMID: 18583681Narrative or mechanistic review

3. 3

   Overview – Eating disordersNHS. NHS, January 23, 2024.Official guidance

4. 4

   Calorie restriction and fasting diets: What do we know?National Institute on Aging. National Institute on Aging, August 14, 2018.Official guidance

5. 5

   Weight gain in pregnancyNHS. NHS, July 6, 2022.Official guidance

6. 6

   Fasting Safely with DiabetesMartin M. Grajower. National Institute of Diabetes and Digestive and Kidney Diseases, August 26, 2020.Official guidance

7. 7

   GliclazideNHS. NHS, August 6, 2026.Official guidance

8. 8

   DehydrationNHS. NHS, May 1, 2026.Official guidance

9. 9

   Low blood sugar (hypoglycaemia)NHS. NHS, August 3, 2023.Official guidance