Water fasting means taking nothing but water for a set period, usually 24 to 72 hours. Some people do it for religious reasons, weight loss or “detox”. It may have some short-term benefits, but the dangers of fasting without food are real, especially for longer fasts or for people with health conditions. Here is what the evidence says, who should avoid it, and how to fast more safely.
What is water fasting?
During a water fast you eat no food and drink only water. It is different from intermittent fasting, where you eat within set hours each day, and from “dry fasting”, where you take neither food nor water. Dry fasting is more dangerous because it quickly leads to dehydration.
Water fasting benefits: what the evidence says
Most research on fasting has looked at intermittent fasting or short fasts, and many studies are small or in animals. Possible benefits include:
- Short-term weight loss. You will lose weight quickly, but much of it is water and stored carbohydrate, and it is often regained once you eat normally.
- Better blood sugar control. Fasting can improve how the body responds to insulin, at least in the short term.
- Lower blood pressure in some people.
- Autophagy. Fasting triggers a natural “clean-up” process in which cells break down and recycle damaged parts. This has been shown mainly in animal studies, and its effects in humans are not yet clear.
There is no good evidence that water fasting “detoxes” the body. Your liver and kidneys do that every day.
Dangers of fasting
- Dizziness and fainting, especially when standing up quickly, due to drops in blood pressure
- Low blood sugar, causing shakiness, sweating, confusion or collapse, particularly in people on diabetes medicines
- Salt (electrolyte) imbalances, which can affect the heart
- Dehydration, because a lot of daily water normally comes from food
- Muscle loss during longer fasts
- Gout attacks, as fasting can raise uric acid levels
- Refeeding syndrome, a dangerous shift in body salts that can happen when eating again after a long fast. It is a key reason long fasts need medical supervision
- Headaches, tiredness, irritability and poor concentration
- Triggering disordered eating in people at risk
Who should not water fast
Do not water fast, or only do so under medical supervision, if you:
- Are pregnant or breastfeeding
- Have diabetes, especially if you take insulin or tablets that lower blood sugar
- Have kidney disease, heart disease, low blood pressure or gout
- Have, or have had, an eating disorder
- Are under 18, older, frail or underweight
- Take regular medicines, which may need adjusting
How to fast more safely
- Talk to your doctor first, especially if you have a health condition or take medicines.
- Keep it short. Don’t go beyond 24 to 72 hours without medical supervision.
- Drink enough water throughout. See the signs you’re not drinking enough water.
- Rest and avoid hard exercise, driving long distances or operating machinery if you feel weak.
- Break the fast gently with small, light meals such as soup, fruit or pap, rather than a heavy meal.
- Stop if you feel faint, confused, very weak, or have chest pain or a racing heartbeat, and seek help.
Religious fasting
Many Nigerians fast during Ramadan, Lent or church programmes. If you have diabetes, high blood pressure or another long-term condition, speak to your doctor before the fasting period so they can adjust your medicines and tell you when it is safer to break the fast. Most faiths exempt people who are ill, pregnant or travelling.
Safer alternatives for weight loss
For lasting weight loss, approaches such as eating within a 10 to 12 hour window, or reducing portions and sugary drinks, are easier to keep up and carry fewer risks than long water fasts.
Frequently asked questions
How long is it safe to water fast?
Most healthy adults can manage up to 24 to 72 hours. Longer fasts should only be done under medical supervision.
How much weight can you lose by water fasting?
You may lose 1 kg or more a day at first, but much of it is water and is regained after the fast.
Is water fasting the same as intermittent fasting?
No. Intermittent fasting limits when you eat each day. Water fasting means no food at all for a day or more.
This article is for general information and does not replace advice from your doctor.
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