What is Water Fasting: Dangers, Benefits, and Tips

by Ahmed Zayed, MD on October 16, 2019
Last updated on May 23, 2021

Fasting is not new to us, especially that it is practiced by many people in all parts of the world for thousands of years. Usually, it is done for religious purposes, but fasting for losing weight is already in demand to the public. And one of the types of fasting is water fasting, which prohibits everything except water.

glass filled with water

Studies have ascertained how water fasting is very beneficial to our entire well-being. It helps in stimulating our old cells and in lowering our risk of various chronic diseases. In this article, we will give you an overview of water fasting, its dangers, and benefits, as well as tips.

What is water fasting?

Water fasting means that you are not allowed eating or drinking anything, except water. This kind of fasting can last from 1 to 3 days. But, water fasting should not be more than the prescribed days without supervision from an expert.

Besides for religious purposes and weight loss, fasting is also used to detoxify your body from harmful chemicals. And patients are usually required to fast while preparing for a medical procedure.

Several studies show how water fasting can provide us with various health benefits. It can lower the risk of heart disease, diabetes, and cancer (1). It may also aid in autophagy, wherein our old cells are broken down and recycled in order to protect us from health risks (2).

Dangers of water fasting

According to Livestrong, fasting is not good for weight loss in the long run as it comes with various health risks (3). Here are a few dangers of water fasting:

Rapid weight loss

Indeed, your main purpose is to lose weight. In fact, it has been shown that you can lose up to 0.9 kg a day if you practice water fasting for 24 to 72 hours (4). But your weight loss is actually from carbs, water, and even muscle mass.

Too much deprivation of calories can cause the water and muscles from your body to lose, which means that your weight loss is not actually from fats. And in order for your body to conserve energy, your metabolism will become slower, which will result in burning fewer calories compared to when you are on your solid-food diet.


It may be ironic how water fasting can make you dehydrated. The reason for this is that the food you eat actually consists of around 20 to 30 percent of your daily water intake (5). So, if you just drink water, without eating foods, then your body doesn’t get enough water it needs.

A study has noted the physiological effects of dehydration including physical performance, cognitive performance, delirium, gastrointestinal function, kidney function, heart and hemodynamic response, headache, skin, and chronic diseases (6).

Orthostatic Hypotension and Hypertension

Orthostatic hypotension is an abrupt drop in blood pressure when you suddenly stand up, which can cause dizziness, lightheadedness, or even fainting. So, if you are water fasting and you are experiencing these symptoms, you should avoid operating or driving heavy machinery as these symptoms could result in an accident.

On the other hand, the opposite of hypotension or low blood pressure is hypertension or high blood pressure. During a fast, patients usually experience adverse symptoms. Aside from headaches, nausea, irritability, and vomiting, orthostatic hypertension is a very usual issue (7).

Nutrient deficiencies

You will most likely experience nutrient deficiencies in any fasting protocol, and water fasting is not an exception. If you restrict your calorie intake, you are also restricting vital nutrients such as vitamins, minerals, amino acids, electrolytes, and fatty acids. These nutrients are actually needed by your body in order to function efficiently.


When the water and salt in our body are lost through sweating, and these are replaced by water alone, we will most likely experience water intoxication. Hyponatremia usually occurs when our blood has an abnormally low level of sodium concentration. This is usually due to drinking too much water.

Remember that sodium is an important electrolyte which helps in regulating the amount of water in our cells. If there is a dilution of sodium in our body, it will result in the rapid rise of our body’s water level, which will lead to the swelling of our cells. This can actually cause mild to life-threatening health issues (8).

Fatigue, dizziness, and brain fog

Calorie restriction can result in fatigue, dizziness, and trouble focusing. If your body does not get the right amount of calories, you will most likely struggle from performing your daily activities at an optimal level. It may be hard for you to focus on your work during water fasting, plus it can cause mild or even severe headaches.

Worsen various medical conditions

Although water fasting is said to be an excellent way to lose weight, people suffering from various medical conditions such as chronic kidney disease, diabetes, eating disorders, gout, and heartburn, should not follow this kind of diet without consulting from an expert as these medical conditions may be worsened by water fasting.

Benefits of water fasting

Water fasting has been associated with several health benefits, both in human and animal studies. Here are a few benefits of water fasting:

Promotes low blood pressure

A study has shown how water fasting is an effective treatment of borderline hypertension, provided that it is medically supervised by experts (9). The study was conducted among 68 participants who have borderline high blood pressure. They are advised to fast for about 14 days as medically supervised by experts. After fasting, 82 percent of the participants experienced a significant drop in their blood pressure to healthy levels.

Another study was conducted among 174 participants suffering from high blood pressure who undergo water fasting for 10 to 11 days. After fasting, 90 percent of the participants saw their blood pressure fall below 140/90 mmHg. It only means that their blood pressure falls to healthy levels.

Improves insulin and leptin sensitivity

Our metabolism is affected by two important hormones- insulin and leptin. The nutrients from our bloodstream are stored in our body with the help of insulin. On the other hand, leptin is responsible for making us feel full.

It was found out that if our body is more sensitive to insulin and leptin, these hormones become more effective. Since water fasting makes our body become more sensitive to these hormones, then these hormones can effectively affect our body’s metabolism (10).

Moreover, the key factor in the development of diabetes is insulin resistance. Since fasting can aid in improving insulin sensitivity, it can help decrease our risk of diabetes.

Promotes autophagy

Autophagy is when our old cells are broken down and recycled in order to protect us from various health risks such as Alzheimer’s disease, heart disease, and cancer (11). Autophagy helps in preventing damaged cells from accumulating, which is one of the many factors causing cancer. This may also help in preventing cancer cells to grow.

Lowers the risk of various chronic diseases

Water fasting can reduce our risk of various chronic diseases such as cancer, diabetes, and heart disease. A study has shown the metabolic and cardiovascular consequences of short-term water fasting. It was found out that after a day of fasting, the participants have lower levels of cholesterol and triglycerides. These are actually risk factors promoting heart disease (12).

Another study also shows that water fasting combats the heart from damages caused by free radicals (13). Free radicals are known to play a major role in various chronic diseases. These are unstable molecules that can be detrimental to our cells.

Promotes weight loss

According to Very Well Fit, water fasting means that you cannot eat or drink anything, except water. It further means that tea, coffee, and even zero-calorie beverages are not allowed. And of course, it also means that any kind of foods is not allowed too (14).

Water fasting is typically up to 3 days, so it’s not safe to exceed beyond the said number of days without medical supervision. And of course, if you restrict your calorie intake for more than one day, you will most likely lose weight. But, your weight loss is not actual fat loss as it is usually in the form of stored carbohydrates and water weight.

Tips for water fasting

Although water fasting is said to have various health benefits, it can also be detrimental to our health, that’s why we need to know who is the diet for.

Not for the underweight

A person who is underweight should not fast because, during water fasting, the body strives to survive using the energy and nutrients it already stored. If you don’t have enough stored fat energy, then you will most likely suffer from various nutrient deficiencies.

According to Medical News Today, if you are underweight, you are most likely to have more health concerns that one who is overweight. It also means that you don’t have enough nutrients that can help your body in building and maintaining your hair, bones, and skin (15).

Usually, a proper diet is required for them to stay healthy. People who are underweight should eat more fruits and vegetables for them to have sufficient energy for their daily activities. Since water cannot provide enough nutrients, water fasting alone is not enough for their diet.

Not for those who have eating disorders

People who are suffering from an eating disorder are not allowed to practice water fasting (16).

Eating disorders such as:

  • Anorexia. A person with this kind of eating disorder fears of gaining weight that’s why such a person tends to eat less. Usually, a person with anorexia uses various methods of losing weight, such as exercising or using laxatives or pills. People with anorexia think that they are fat (even if they are not) that is why they are very strict with their foods.
  • Bulimia. Usually, this is called binge eating. Usually, a person overeats when they are binge eating. A person who has bulimia eats more foods, even if he is not hungry. It only shows that he cannot control his food intake.
  • Since these kinds of eating disorders can strongly affect how a person eats, as well as his entire well-being, it’s best to ask help from a doctor. If there is something wrong with the way you eat, then this kind of diet is not good for you.

Not for pregnant and breastfeeding women

Pregnant and breastfeeding mothers are not allowed to undergo water fasting. According to Brigham Health Hub, pregnant woman and breastfeeding moms should have a healthy diet. These moms, as well as their babies, need to obtain the right amount of foods like fruits and vegetables, which water cannot provide sufficiently (17).


Water fasting is not an appropriate way for everyone to lose weight. Indeed, water fasting can offer us various health benefits, but it also comes with several risks and dangers. It may have detrimental effects on our health, such as rapid weight loss, dehydration, orthostatic hypotension and hypertension, nutrient deficiencies, hyponatremia, fatigue, dizziness, and brain fog.

In order to utilize this diet safely, you need to make sure that it’s alright for you. If you are already underweight, pregnant or breastfeeding, or suffering from an eating disorder, this diet is not for you. And before you should decide to practice fasting, you should seek help from your doctor. No matter how healthy you are, water fasting may have adverse side effects, which might result in various health problems.


PhentermineDoctors has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical associations. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Leave a Reply

Your email address will not be published. Required fields are marked *