Water fasting is when a person eats no food and drinks only water. People may undertake water fasting to lose weight, for spiritual or religious reasons, or to try and combat particular health problems.

Research suggests that occasional fasting may help with weight loss, although other methods may be more effective long-term.

To make sure that water fasting is done safely, people should prepare properly and choose a good time to go without food, when the body does not require too much energy.

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Water fasting may be undertaken for a number fo different reasons, including spiritual, diet, or medical reasons.

A water fast is when a person does not eat and drinks nothing other than water.

There is no set time that water fasting should last for, but medical advice generally suggests anywhere from 24 hours to 3 days as the maximum time to go without food.

Throughout history, people have undertaken fasts for spiritual or religious reasons. But, water fasting is now popular in the natural health and wellness movements, often alongside meditation.

People with risk factors for certain diseases could benefit from short-term fasting. These include:

These risks are often related. When the body does not have access to carbohydrates, which are its preferred source of energy, it will use fats. So, a fast can result in weight loss as the body uses up fats in the body for its energy.

According to the Academy of Nutrition and Dietetics, the best way to lose weight is to take it slowly, combining a healthy diet with exercise. It is also important to try and change some eating habits, such as reducing the number of sugary foods and snacks eaten.

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When considering a water fast, medical advice should always be sought, as it may not be safe for everyone.

Although there are potential health benefits to fasting, there are considerable risks if a fast is carried out for too long, or by someone whose health or age puts them at risk of damage to their body.

If someone has health concerns, or is planning to fast for longer than 24 hours, they should seek the advice of a medical professional and consider undertaking a fast under supervision.

Water fasting will not be safe for everyone, and should not be undertaken by older adults, those under 18, or those who are underweight.

An alternative to long periods of fasting can be intermittent fasting. This means eating nothing or very few calories for a certain amount of time and then eating as usual for another set period. An example is the 5:2 diet, where someone eats a regular diet for 5 days in the week, and a quarter of their daily calories on the remaining 2 days.

In a study comparing intermittent fasting and eating an ongoing low-calorie diet, both methods were found to be equally good for weight loss, as well as reducing the risk of cancer, diabetes, and heart disease. Intermittent fasting was found to be as easy to stick to as a low-calorie diet.

Research based on studies with mice and rats suggests that fasting may protect against certain diseases, such as diabetes, and has the potential to delay aging. Fasting regularly for short periods of time has been associated with lower rates of diabetes, a lower BMI, and a reduced risk of coronary artery disease in people being tested for blocked arteries.

There have not been extensive human studies on fasting, although research has found some positive impact on blood pressure, body weight, and improved rheumatoid arthritis symptoms from small studies. Fasting can have adverse effects on the immune system for older adults, so individuals should seek medical advice on whether occasional water fasting could be beneficial.

Water fasting is not safe for everyone. People who should not fast, or who should seek advice from a medical professional before fasting include older adults, those under 18, and those who:

  • have an eating disorder
  • are underweight
  • are pregnant or breast-feeding
  • have heart problems
  • have type 1 diabetes
  • have uncontrolled migraines
  • are undergoing a blood transfusion
  • are taking specific medication; seek the advice of a doctor
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It is best to perform a water fast is when not working or unwell. Ideally, a person will fast in the comfort of their home while resting.

If someone has not fasted before, they should consider starting with a 1-day fast to try it out and make sure there are no adverse effects. Fasting for longer than 3 days should only be carried out after seeking the advice of a medical professional.

Fasting can be mentally and physically tiring, so people must carefully prepare themselves by:

  • eating well before the fast, with foods that are high in energy
  • picking a time that will allow for rest, maybe a day when not at work
  • avoiding fasting if unwell or very tired
  • avoiding demanding exercise
  • considering building up to a fast slowly, perhaps by reducing the size of meals

During the fast, it is essential to drink enough water and to spread this out throughout the day. It may be tempting to drink more than usual when fasting, but this can be harmful and should be avoided.

When ending a water fast, a person should not eat too much at once but build up gradually to avoid a stomach ache or feeling sick.

Fasting deprives the body of the fuel it needs, so expect to feel tired and low on energy. A lack of food can also make people feel dizzy, weak, or nauseous, and if these symptoms are particularly bad, it is important to eat something.

Getting plenty of rest, sitting down, and avoiding intense exercise can help to conserve energy. It is normal to feel irritable or tired from lack of food, but if someone begins to feel disorientated or confused while fasting, they should seek medical advice.

Although there may be some health benefits to water fasting, reducing overall calories is just as effective for weight loss, and is likely to be safer. Alternatives such as intermittent fasting could have more health benefits in terms of reducing the risks for heart disease and diabetes than a long-term water fast for days at a time and may be more sustainable.