Water Fasting: Benefits, Weight Loss, And How ... - Medical News Today

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Medical News TodaySubscribeAll you need to know about water fasting

Medically reviewed by Marie Lorraine Johnson MS, RD, CPT — Written by Claire Sissons and Mandy French — Updated on May 20, 2025

Overview
How it works
Benefits
What to expect
Safety
Summary

Water fasting involves a person eating no food and drinking only water. It has various uses, including losing weight, for spiritual or religious reasons, or to combat particular health problems.

While water fasting may provide some health benefits, it also poses certain risks. It also may not be suitable for everyone.

To ensure that water fasting is safe, people should prepare properly and choose a good time to go without food, such as when the body does not require too much energy. Individuals should always speak with a healthcare professional before beginning any new program, such as water fasting.

Individuals considering water fasting for any reason should consult a healthcare professional first. This is especially true for those with underlying health conditions.

What is water fasting?

Multiple glasses of water against a blue background — Elizabeth Fernandez/Getty Images

A water fast involves a person not eating or drinking anything other than water.

There is no set time for water fasting. However, individuals should discuss fasting with their healthcare professional to determine the longest period that is safe and healthy for them. Most fasting regimes suggest people fast for no more than 24 hours at a time.

Throughout history, people have undertaken fasts for spiritual or religious reasons. However, in recent years, people have also begun to use fasting as an overall health-promoting practice. Individuals use fasting to aid in weight loss, longevity, and various health conditions.

Water fasting may also help to promote autophagy, which is the body breaking down and recycling old parts of the cells that can be potentially harmful or dangerous. Autophagy can help with various conditions, including:

metabolic conditions
neurodegenerative conditions, such as Huntington’s disease
cancers
infectious diseases

»Learn more about fasting

How does water fasting work?

There are no set guidelines for water fasting. Certain people should consider not fasting or should only do so under the supervision of a healthcare professional. These include children and adolescents, adults over the age of 75, people who are pregnant or nursing, and individuals with an eating disorder.

Who should not fast?

Water fasting is not safe for everyone. People who fasting may not be safe or appropriate for include:

those who are underweight
those who have a heart condition or issues with their heart
those who have type 1 diabetes
those have uncontrolled migraine
those who are undergoing a blood transfusion
those who are taking medications

Water fast

During fasting, an individual does not eat or drink anything besides water.

Some experts recommend consuming 2-3 liters of water daily, especially mineralized water. This is so the body can replace essential minerals lost during fasting.

Most types of fasting do not last for more than 24 hours. Anyone who would like to fast for longer should discuss the risks and benefits with a healthcare professional.

Healthcare professionals may recommend avoiding strenuous and highly physical activities during fasting.

Post-fast period

Experts recommend reintroducing food in small increments after fasting. They also recommend preparing one’s body for this by practicing it before beginning fasting. This can include dividing food intake into five smaller meals a day: breakfast, lunch, dinner, and two snacks.

They also recommend that a person avoid processed foods before and after fasting. Some say that following a Mediterranean diet can help. This includes eating a diet of whole grains, fresh fruits and vegetables, legumes, and nuts. It also typically includes lean proteins, such as fish, over red meat.

Following fasting, especially longer-term fasting, it is important to introduce food to the body slowly to prevent developing refeeding syndrome. This serious condition occurs when there are medical complications due to rapid changes in fluid and electrolyte levels.

Quick tips for water fasting

If someone has not fasted before, they should consider starting with a shorter fast to ensure no adverse effects. Longer fasts should only occur after seeking the advice of a healthcare professional.

Fasting can be mentally and physically tiring, so people should consider preparing 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, drinking enough water and spreading it out throughout the day is essential. Drinking more than usual when fasting may be tempting, but this can be harmful and should be avoided.

Benefits of water fasting

People with risk factors for certain conditions could benefit from short-term or intermittent fasting because it aids in weight loss. These include:

heart disease
high blood pressure
high cholesterol
diabetes
overweight or obesity

However, it is essential to eat a healthy and balanced diet between fasting times. It is also important to get regular exercise and physical activity. The Centers for Disease Control and Prevention (CDC) states that those who experience a gradual weight loss are likelier to maintain that loss than those who lose weight quickly.

While more research is necessary on how fasting can affect the body, many believe there are other potential benefits than weight loss. These include:

hormone modulation
ketosis
autophagy
stress reduction and resistance
inflammation reduction

What to expect

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.

It is also important to drink enough water to prevent dehydration.

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

Safety during water fasting

Although there are potential health benefits to fasting, there are considerable risks if a fast happens 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 healthcare professional and consider undertaking a fast under supervision.

Water fasting may not be safe for everyone.

What are the rules for water fasting?

There are no set “rules” for people to follow, but a water fast involves someone eating no food and only drinking water for a certain period of time.

What does water do to the body when fasting?

Water does not affect blood insulin levels, maintains hydration, and supports different functions in the body, without food.

What are the disadvantages of water fasting?

Disadvantages of water fasting include:

reducing the amount of calcium in bones
imbalance of minerals
loss of sodium and potassium in the body

Summary

Water fasting may have various health benefits. Some believe it can aid in weight loss, stress and inflammation reduction, and hormone modulation. People also think it may help in longevity. However, more research is necessary on exactly how fasting affects the body.

Water fasting may not be safe for everyone. Children, older adults, and people with conditions, such as eating disorders, should not participate in fasting. Anyone who is considering fasting should speak with a healthcare professional first.

Options like intermittent fasting may be safer and more beneficial for some people than long-term fasting.

Surgery
Nutrition / Diet
Obesity / Weight Loss / Fitness

How we reviewed this article:

SourcesMedical News Today has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical journals and associations. We only use quality, credible sources to ensure content accuracy and integrity. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Attina A, et al. (2021). Fasting: How to guide.https://pmc.ncbi.nlm.nih.gov/articles/PMC8151159/
Bagherniya M, et al. (2018). The effect of fasting or calorie restriction on autophagy induction: A review of the literature.https://pubmed.ncbi.nlm.nih.gov/30172870/
Steps for losing weight. (2025).https://www.cdc.gov/healthy-weight-growth/losing-weight/?CDC_AAref_Val=https://www.cdc.gov/healthyweight/losing_weight/index.html
Ogłodek E, et al. (2021). Is water-only fasting safe?https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8369953/
Persaud-Sharma D, et al. (2022). Refeeding syndrome.https://www.ncbi.nlm.nih.gov/books/NBK564513/
Shabkhizan R, et al. (2023). The beneficial and adverse effects of autophagic response to caloric restriction and fasting.https://pmc.ncbi.nlm.nih.gov/articles/PMC10509423/
Vasim I, et al. (2022). Intermittent fasting and metabolic health.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8839325/

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Medically reviewed by Marie Lorraine Johnson MS, RD, CPT — Written by Claire Sissons and Mandy French — Updated on May 20, 2025

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