Is dry fasting safe?
In one study of 34 healthy adults, dry fasting for 11 to 12 hours a day looked safe. Longer fasts are barely tested: two 5-day studies had 10 people each.
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Is dry fasting safe?
For a short daytime fast in healthy adults, one 2021 study says yes. In Berlin, 34 healthy Bahá’í volunteers went without food or drink from sunrise to sunset for 19 days.
Each fast lasted about 11 to 12 hours. Their hydration stayed largely in the normal range, and the researchers called the fast safe.
Dry fasts of several days are a different story. Only two small studies have tested them, with 10 people each and no comparison group.
| Study | Who and how long | What happened |
|---|---|---|
| Bahá’í fast, 2021 | 34 healthy adults. No food or drink for about 11 to 12 hours a day, for 19 days. | Hydration stayed largely normal. Weight fell. |
| 5-day dry fast, 2013 | 10 healthy adults. No food or water for 5 days. | Lost about 1.4 kg a day. Blood pressure and blood sugar stayed steady. The authors called it safe. |
| 5-day dry fast, 2020 | 10 people. No food or water for 5 days. | Lost about 7 kg. Stress hormone, uric acid and an inflammation marker rose. |
Sources: Koppold-Liebscher 2021; Papagiannopoulos 2013; Papagiannopoulos-Vatopaidinos 2020
So if you try it, keep each fast to about 11 to 12 hours. That’s the only pattern tested in more than 10 people.
Dry fasting vs water fasting: what’s the difference?
A water fast cuts food but lets you drink. A dry fast cuts both, so you take in no fluid at all.
That matters because food carries water too. The NHS says we get some fluids from the foods we eat. On a dry fast, both sources stop.
The best-known dry fasts are religious and daytime only, such as Ramadan. A Ramadan fast can run 11 to 22 hours a day, depending on the season and where you live. Summer days mean longer fasts.
Thinking of a longer fast with water instead? See the 24-hour fast and 48-hour fast guides.
Does dry fasting help you lose weight?
The scales drop fast. In the 2020 study, 10 people lost about 7 kg in 5 days. The researchers tracked where the weight went.
Most of that 7 kg left the body as pee and water through skin and breath.
The daytime fast was gentler. Median weight went from 75.3 kg to 72.75 kg by week 3, and body fat fell a little. Most changes had gone 3 months later.
Looking for weight loss that lasts? See intermittent fasting for weight loss, where you still drink.
What does dry fasting do to your body?
Going without fluid can make you dehydrated. The NHS lists the signs of dehydration as thirst, headache, feeling dizzy or tired, a dry mouth, and dark, strong-smelling pee.
Over 5 days, several blood tests changed sharply.
| Blood test | What it is | Average rise |
|---|---|---|
| Cortisol | Your main stress hormone. | 495% |
| CRP | A marker of inflammation. | 167% |
| Uric acid | A chemical in your blood. Too much can cause gout. | 103%, about double |
| Haematocrit | The share of your blood made of red cells. | 11% |
Sources: Papagiannopoulos-Vatopaidinos 2020; NHS: gout
The NHS says getting dehydrated can trigger a gout attack. Not drinking enough water also makes kidney stones more likely.
Have kidney stone symptoms? Get an urgent GP appointment or call 111 if you’re in severe pain. The same goes for a high temperature, or feeling hot, cold or shivery.
Got a fasting blood test booked? Keep drinking water. In 15 healthy people, fasting with no fluids gave 8.1% higher total cholesterol than fasting with salt and water.
How to make a dry fast safer
Before and during a daytime dry fast
- Keep each fast to about 11 to 12 daytime hours, not whole days.
- Drink enough sugar-free, decaffeinated fluids before the fast starts.
- Fit the NHS guide of 6 to 8 cups or glasses of fluid a day into your eating hours.
- Check your pee between fasts. It should be pale.
- Skip hard exercise and hot, dry rooms while you fast.
- Stop and drink if you get signs of dehydration, such as dizziness or dark pee.
Sources: NHS: dehydration; NHS: water and drinks; Diabetes UK; Campbell 1994
The tips on fluids before a fast come from Diabetes UK’s Ramadan advice. The heat and exercise tip comes from the Calgary team behind the cholesterol study, who gave it for fasting before blood tests. The NHS says to drink fluids if you have symptoms of dehydration.
Skip dry fasting if you’re pregnant, breastfeeding, under 18, older or unwell. These groups are exempt from the Bahá’í fast or were not studied.
The study only included adults aged 18 to 69. It left out anyone with an eating disorder or serious illness.
When to speak to a GP
Talk to a GP or your care team before you fast if:
- You have diabetes. Diabetes UK says to fast only after talking to your diabetes team, as some tablets and insulin can cause low blood sugar. Break your fast if your blood sugar drops below 4 mmol/l.
- You take water tablets (diuretics), or have had gout or kidney stones.
- You eat very little or have strict rules around food. The NHS says see a GP as soon as you can, or call the Beat helpline on 0808 801 0677.
Get an urgent GP appointment or call 111 if you feel dizzy when you stand and it doesn’t go away. The same goes if you’re unusually tired, peeing less, have dark yellow pee, or are breathing fast or have a fast heart rate.
Call 999 if someone with signs of dehydration is confused, very sleepy or hard to wake. Also call if they struggle to breathe or are taking lots of quick breaths. The same goes for cold or blue, grey, pale or blotchy skin, lips or tongue.
Never stop or change a prescribed medicine without your prescriber.