Water fasting: what it is, benefits, and the real risks
Water only for 24, 48, or 72 hours. A powerful but demanding fast that isn't for everyone. Everything to know before you try it.
Part of the complete guide How to start intermittent fasting and pick your protocol
What the science saysWater Fasting: What It Is, Benefits and Risks
Water fasting means consuming nothing but water for 24-72 hours or more. In trials running 5-20 days it produces weight losses of 2-10%, higher ketones, and lower blood pressure, but close to two thirds of the weight lost is lean mass and the metabolic benefits fade within 3-4 months. Long fasts are best done under medical supervision.
- Fasts of 5-20 days: weight loss of 2-10%, roughly two thirds of it lean mass; metabolic benefits were not maintained at 3-4 months (Ezpeleta 2024). [1]
- In medically supervised water fasting, 75% of adverse events were mild and only 2 visits recorded a serious event (Finnell 2018). [2]
- In 11 lean people, resting energy expenditure rose from 3.97 to 4.53 kJ/min on day three of an 84-hour fast (Zauner 2000). [3]
What water fasting is
Water fasting means drinking nothing but water for an extended stretch (usually 24, 48, or 72 hours). It's fasting in its purest, most powerful form, but also the most demanding and the one that calls for the most caution.
Possible benefits
- Peak autophagy (cellular cleanup) from 24-48 hours on.
- A sharp rise in growth hormone.
- A "reset" of insulin sensitivity.
Keep in mind: a lot of this data comes from small studies or animal research. Don't romanticize it.
The real risks
- Electrolyte loss (sodium, potassium, magnesium): causes dizziness, headaches, and heart palpitations.
- Muscle loss if you stretch it too far.
- Dizziness, fatigue, low blood pressure.
- Risk of "refeeding syndrome" if you break a long fast the wrong way.
Who it is NOT safe for
Don't water fast if you're pregnant or breastfeeding, under 18, underweight, have diabetes on medication, have a history of eating disorders, or have any medical condition without supervision. For fasts longer than 48 hours, ideally do it with professional monitoring.
How to break it safely
Never break a long fast with a huge meal. Start with broth, a little fruit, or light protein, wait 30-60 minutes, and reintroduce food gradually. The longer the fast, the gentler the reintroduction.
How Ayúname helps
Ayúname tracks your hours precisely and shows you each phase, which helps on long fasts too so you know when you hit deep autophagy. Remember: extended fasts call for good judgment and solid health.
A plan for you · Extended fasting, 48-72hYour first 72-hour fast, guided and done right →
Free intermittent fasting app: phase-by-phase timer, autophagy, streaks and reminders. No ads.
Frequently asked questions
How long can you water fast?
Is water fasting dangerous?
Scientific sources (4)
- Efficacy and safety of prolonged water fasting: a narrative review of human trials. Nutrition Reviews, 2024.
- Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting. BMC Complementary and Alternative Medicine, 2018.
- Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. The American Journal of Clinical Nutrition, 2000.
- Refeeding syndrome: what it is, and how to prevent and treat it. BMJ, 2008.