Water Fasting: What Happens, the Risks and How to Refeed

Water fasting is the strictest form of fasting, with no food and no drink except plain water, usually for 24 to 72 hours. Daily intermittent fasting is different, because you skip food for 12 to 20 hours and then eat in a window every day. The longer a water fast runs, the more the risks grow, and restarting food after a long fast carries its own danger.

Water fasting vs intermittent fasting

The phrase “water intermittent fasting” can mean either one, so the table sets out the difference. Fasts of more than 72 hours are often called prolonged or extended fasts, and periodic fasting means repeating a multi-day fast now and then.

	Daily intermittent fasting	Water fast	Prolonged fast
Length	12 to 20 hours a day (about 23 on one meal a day)	24 to 72 hours	More than 72 hours
Drinks while fasting	Water, often black coffee or plain tea	Plain water only	Plain water, under medical supervision
Doctor first?	If you take medicines or have a health condition	Yes: see a doctor before going past 24 hours	Yes, with supervision throughout
Refeeding risk (NICE: more than 5 days of little or no food)	Not reached	Not reached by length alone; other risk factors still count	Reached after day 5

A 36-hour fast, one full day and two nights without food, sits inside the water-fast range and needs the same doctor check. For a daily routine instead, a 12-hour overnight fast is the first step in the starter plan.

What happens to your body hour by hour

The hours below are approximate and differ between people. Your last meal and your activity level both shift the timing.

Time without food	What is happening	How strong the evidence is
0 to 4 hours	Your body is still digesting. Glucose from the meal is the main fuel, with blood sugar and insulin raised.	Strong
About 4 to 12 hours	Insulin drops. The liver draws on its glycogen (stored sugar) so blood sugar holds steady.	Strong
About 12 to 36 hours	The liver’s glycogen gets low, so fat becomes the main fuel and ketone levels climb. This metabolic switch comes somewhere between 12 and 36 hours.	Strong
24 hours and beyond	Fat burning and ketones keep rising. Growth hormone rose during 1-day and 5-day fasts in a study of six men.	Early
First 4 days	The kidneys pass more sodium. In one study of fasting adults with obesity, sodium in urine rose from 35 to 109 mmol a day by day 4.	Early

Autophagy, the body’s recycling of worn-out cell parts, may increase on longer fasts. No human study shows when it starts, and most of the evidence comes from animal and lab studies. The autophagy guide covers why the popular hour figures don’t hold up.

Water fasting weight loss: what the scale shows

The scale drops fast on a water fast, and much of that early loss is water. Stored carbohydrate (glycogen) is held with about 3 to 4 grams of water per gram. As the glycogen is used up, that water goes too. In a study of a 10-day, 800-calorie low-carbohydrate diet, 61% of the weight lost was water. The evidence here is moderate (some human trials), and the water comes back when you eat again.

Worked example, from the water fast weight loss calculator with its default body details and the fast set to 72 hours. Take a sedentary 40-year-old woman who weighs 80 kg (176 lb) and is 168 cm tall. Her estimated maintenance is about 1,787 calories a day. Over a 72-hour fast her body would use about 5,360 calories. Dividing by roughly 7,700 calories for each kilogram of fat gives at most about 0.70 kg (1.5 lb) of fat.

That 7,700 figure is a rough rule, and it overstates the fat lost for leaner people. So the calculator shows a ceiling on fat lost, never a forecast for the scale.

Who should not water fast

Get medical advice before fasting for more than 24 hours, especially if you take medicines, are pregnant or have a health condition. No official body sets a safe number of hours, so this is the site’s own line.

Get your doctor’s advice before a water fast if any of these apply:

• pregnancy, or trying for a baby (losing weight on purpose while pregnant is not recommended);
• breastfeeding;
• being under 18;
• any history of an eating disorder, or being underweight (for either one, we advise against this kind of fast);
• type 1 diabetes, insulin, or a sulfonylurea or other medicine that can push blood sugar too low;
• a medicine you must take with food, or one for blood pressure or fluid balance;
• frailty, older age, or recovery from illness or surgery.

GLP-1 medicines such as semaglutide and tirzepatide raise the risk of low blood sugar when taken with insulin or a sulfonylurea. Never change diabetes medicines yourself to fit a fast. The full list is on the medical disclaimer page.

What breaks a water fast?

Anything other than plain water breaks a water fast. Black coffee, tea, bone broth and electrolyte drinks are allowed on some other kinds of fast, but they make it a different kind of fast. Juice cleanses and liquid diets contain calories, so they are not water fasts at all. You may read that under about 50 calories won’t matter. That figure is only a rule of thumb, with no official limit behind it.

Some people on longer fasts add a pinch of salt to water. A quarter teaspoon of table salt holds roughly 500 to 580 mg of sodium. WHO advises adults to keep sodium under 2,000 mg a day in total. Potassium salt substitutes are not safe for everyone. Skip them if you have kidney disease or take potassium-sparing water tablets, ACE inhibitors, ARBs or potassium supplements, unless your doctor agrees. The electrolytes guide sets out the amounts.

For water, EFSA’s adequate intake is about 2.0 litres a day for women and 2.5 litres for men, counting drinks and food. Food usually supplies about a fifth of that, so on a fast more has to come from drinks. Drink to thirst and check that your urine stays pale yellow. Drinking far more than you need can lower blood sodium (hyponatraemia), which can be a life-threatening emergency.

Warning signs: when to stop and get help

End the fast at once and get urgent medical help if you faint, feel confused or have chest pain. The same goes for repeated vomiting, a change in your vision, or a racing or skipping heartbeat. In an emergency, call your local number, such as 112 in the EU, 999 in the UK or 911 in the US and Canada.

A headache is common. The International Headache Society recognises “headache attributed to fasting”, which usually eases after eating. Its exact cause isn’t known. Low fluid, low salt, caffeine and poor sleep are common things to check. Dizziness or faintness on blood-pressure tablets is a reason to eat and call your doctor.

Refeeding syndrome and how to break a water fast

Refeeding syndrome is a dangerous shift in blood minerals when food comes back after a long time without it. Phosphate, potassium and magnesium levels fall, usually within 5 days of eating again, and it can be fatal. Clinical guidance sets out who is at risk.

High refeeding risk if any one applies	Or if two or more apply
BMI under 16	BMI under 18.5
Over 15% unplanned weight loss in 3 to 6 months	Over 10% unplanned weight loss in 3 to 6 months
Little or no food for more than 10 days	Little or no food for more than 5 days
Low potassium, phosphate or magnesium before eating again	Alcohol misuse, or insulin, chemotherapy, antacids or diuretics

In clinical care, people who have eaten little or nothing for more than 5 days restart slowly. They get no more than half their energy needs for the first 2 days. That rule is for clinical care, so a fast that long needs a doctor before the first meal. This site’s caution is stricter. Get medical advice before breaking any fast longer than 2 to 3 days.

For shorter fasts, start small and gentle. The break-fast and refeed planner gives first meals for fasts up to 24 hours, 24 to 48 hours and 48 to 72 hours. For fasts over 72 hours it gives no home plan and sends you to a doctor. The water fast weight loss calculator asks its safety questions first. It then shows how much energy a 24- to 72-hour fast would use for your body.