Dry fasting

What the words mean

In 2025, Ioannis-Eleemon Papagiannopoulos-Vatopaidinos, Maria Papagiannopoulou, and Eleni Dotsika used dry fasting to mean choosing to take in no food and no fluid for a set time. They published that description in Metabolism Open, and they cite a 2024 consensus paper by Daniela Koppold and colleagues for the wording. Water, coffee, and tea are all off. Some people also refuse a shower or a bath, because water on the skin counts to them as breaking the stretch. That extra rule does not replace the water lost in breath, sweat, and urine.

A fast that still includes drinking is a different practice. On a daily window you drink water, and black coffee or plain tea can stay if you want them. Hydration and electrolytes while fasting is about that window. A stretch past a day still includes water. Water and minerals past a day is about when salt becomes a louder question on that stretch.

A holy day can also include no water, and that is not this practice. Ramadan runs from dawn to sunset, with food and water at night. Yom Kippur and Tisha B’Av run about a day, with no food and no water, inside a calendar that already exempts illness, pregnancy, travel, and hard labor. Those calendars stay in Religious and spiritual fasts. A summer day with no water is not a template for several days you chose on your own.

What goes wrong

The NHS page on dehydration says dehydration means the body loses more fluid than it takes in, and that if it is not treated it can get worse and become a serious problem. Babies, children, and older adults are more at risk. It also happens more easily with diabetes, vomiting or diarrhoea, heat, alcohol, heavy sweat, a high temperature, or medicines that make you pee more. Those medicines are called diuretics, and people often call them water pills.

Symptoms the NHS lists for adults include thirst, headache, feeling light-headed, dark yellow strong-smelling pee, peeing less often than usual, dizziness, tiredness, and a dry mouth, lips, and tongue. The NHS says you should drink fluids if you have those symptoms. On a dry fast, that is the conflict. The step that treats the problem is to drink. Waiting it out is how the problem gets worse.

Ask for urgent help if you are unusually tired, if dizziness when you stand does not go away, if your pee is dark yellow or you are peeing less than normal, or if you are breathing quickly or your heart is racing. The NHS lists those as signs of serious dehydration. Call emergency help if, along with dehydration, the skin, lips, or tongue look blue, grey, pale, or blotchy, the skin feels cold, breathing is difficult, the person is confused, or they are sleepier than normal or hard to wake. The NHS says those can be signs of shock. On brown or black skin, a colour change can be easier to see on the palms or the soles.

The National Kidney Foundation lists severe dehydration, meaning not enough water for what the body needs, among the causes of acute kidney injury. That is a sudden drop in how well the kidneys filter. It can leave no clear symptom until the drop is already large. When signs show, they can include peeing less or not at all, swelling, tiredness, shortness of breath, confusion, and pain in the side between the ribs and the hip. Thirst does not tell you which of those is starting.

Heat makes the same shortage worse. The NHS page on heat exhaustion and heatstroke says a person with heat exhaustion needs to be cooled down and given fluids, and that children, older people, and people with long-term conditions such as diabetes or heart problems are more at risk. A dry fast in hot weather stacks two reasons the body is short of water.

The study people cite

In 2013, Ioannis A. Papagiannopoulos, Vassilis I. Sideris, Michael Boschmann, Olga S. Koutsoni, and Eleni N. Dotsika watched 10 healthy adults through 5 days with no food and no water. They published the results in Complementary Medicine Research. In that group they reported blood pressure, heart rate, oxygen in the blood, glucose, potassium, sodium, chloride, urea, creatinine, and the concentration of the blood as stable. Creatinine clearance, a measure of how much the kidneys filter, increased up to 167 percent in their report, and they called that improved kidney function. They also reported a mean weight loss of about 1.4 kilograms a day. They called the 5 days safe.

That paper is what people quote when they say a dry fast of several days has been studied and found safe. Ten healthy adults is a small group. The same circle of authors later named three risks of this kind of fast: too little fluid in the circulation, blood that is too concentrated, and low blood sugar. The weight leaving the scale during days with no drinking is largely water. It is not a week of body fat, and it is not a result to chase.

In 2025, Papagiannopoulos-Vatopaidinos, Papagiannopoulou, and Dotsika measured another 10 adults through 5 days with no food and no water. Three were men and seven were women, ages 30 to 65. They were already experienced with multi-day dry fasting, they were on no medicine, they could walk and work, and the study kept out kidney failure, adrenal failure, type 1 diabetes, and a body mass index under 19.5. Daily blood and urine tests were part of the watch. A selected group in that setting is not a home method, and it is not evidence that five days is a safe length for someone else.

Who should not, and when to drink

Children, pregnancy, and breastfeeding are already outside what this clock is for. Kidney disease, heart disease, diabetes, an eating disorder, and medicines that make you pee more each change the risk. Those situations are listed in Who should be careful with fasting.

If you are already going without water and you are thirsty, dizzy, confused, unusually tired, or peeing less, drink. The NHS treats those as dehydration, and the treatment it names is fluid. Chest pain, fainting, and a heartbeat you cannot explain are reasons to stop and get help. Those signs are in When to stop a fast. Do not add hours to finish a number you saw in a comment.

There is no safe length printed here, no list of steps to prepare, and no schedule for the first drink afterward. Ten adults watched in a clinic are not a plan you can copy. A holy day’s length is not that plan either. If the wish is a fast, the fast described in the rest of this library includes water. The timer counts hours. It cannot tell you that going without water is safe.