Water fasting: what is it, benefits, how to do it

Fasting diets, a group that includes a type of fast known as water fasting, are popular and it’s likely you know someone who is currently on a variation of a fasting diet (Freire, 2019). These diets may provide a number of wellness benefits with research linking them to a reduced risk for several major diseases and many other potential health improvements (Finnell, 2018). 

Some of these diets also appeal to common sense. Human beings evolved to endure periods without food intake, so it stands to reason then that eating throughout the day, every day, might not be the only or best approach to food consumption. 

Most of the recent research on fasting has looked at intermittent fasting, as opposed to water fasting. But studies have found that water-only fasting, like other fasting diets, may be associated with potential benefits (Finnell, 2018). 

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Different experts define “fasting” in slightly different ways. But generally, fasting means that you’re choosing to restrict your food or calorie intake for a longer-than-usual period, with many considering 12 hours or longer as the acceptable threshold for a fast (Sen, 2018; Patterson, 2017). 

During water fasting, also known as water-only fasting, you eat nothing and drink only water (Ogłodek, 2021). This is different from other fasting diets that may allow you to consume non-caloric drinks—such as coffee or tea—or even some food or nutrient supplements (Patterson, 2017). 

Research has linked the practice of drinking water (and swallowing nothing else) to several potential health benefits. These include (Ogłodek, 2021):

• Reduced blood pressure among people with hypertension (Finnell, 2018)

• Lower stress 

• Weight loss 

• Improved body composition 

• Improved blood sugar levels 

• Improved cholesterol scores (Horne, 2013)

• Higher red blood cell count (Horne, 2013)

• Greater sense of wellbeing 

• Improved kidney functioning (Mojto, 2018)

Research has also found that water fasting may help with the treatment of some health conditions, including appendicitis and heart disease (Finnell, 2018). 

Looking beyond water-only fasting, there’s evidence that fasting diets, in general, provide a range of health benefits. Apart from the ones already mentioned above, these may include (Finnell, 2018; Sen, 2019): improvement of arthritis symptoms, lower risk for cardiovascular disease, reduced chronic pain symptoms, improved energy and quality of life, improved skin, and reduction in symptoms of chronic inflammatory disorders.

While all of that sounds amazing, it’s important to note that most studies on water fasting in people involve a week or more of fasting. It’s not clear if shorter water-only fasts provide some or any of these benefits. Also, it’s not certain that water-only fasting offers benefits above and beyond other forms of fasting (Paoli, 2019).

While research on water fasting has turned up some promising evidence of health benefits, the practice can also cause a number of adverse health events or unwanted side effects. 

These include (Ogłodek, 2021; Finnell, 2018): 

• Dehydration

• Elevated blood uric acid, which is a major risk factor for gout

• Low blood sodium, which can lead to headaches, nausea, poor balance, and other symptoms

• Fatigue

• Insomnia

• Weakness or lightheadedness

• Upset stomach

• Back pain

• Nutrient deficiencies

Research has also tied water fasting to serious adverse events. These include hospitalizations due to dehydration and sodium deficiency (Finnell, 2018).

Nearly all of the research on water fasting involves regular (or constant) health monitoring in a medical clinic or another healthcare facility. People in these studies aren’t fasting for days at home and alone. Also, nearly all of the studies on water-only fasting have been small—just a handful of adults. The medical community needs to conduct more research to iron out all the potential risks or benefits (Finnell, 2018).   

There’s no single “right” way to water fast. Different studies have used different protocols. 

It’s important to remember that research on water-only fasting has looked at the effects of fasts that last anywhere from two days to 22 days (or even longer), although 8–10 day water fasts seem to pop up a lot in the published research. Most of the benefits of water fasting are tied to these extended periods of fasting, which may not be safe to try on your own, and, as you might expect, the risks of adverse events or side-effects increase along with the length of the fast (Finnell, 2018). 

It’s worth saying again that almost all studies of water-only fasting include regular check-ups or full-time medical supervision. You can get into trouble when you suddenly cut your body off from its normal supply of nutrients. While the risks go up as time passes, it’s not clear that shorter water fasts—just a couple of days—are safe to do on your own (Finnell, 2018).

Also, most water-fasting protocols have people eat specialized diets before and after their fast. Some researchers ask people to consume only raw fruit or steamed vegetables for at least two days before their fast, which may help prepare the body for the experience. Another recommendation is adopting a similar refeeding diet following your fast, which may help your body reacclimate to food with minimal side effects (Finnell, 2018). It's not clear whether these measures are necessary if people are attempting short-term water fasts. 

Fasting may be risky for specific groups. Research on fasting therapy (or fasting to address medical issues) has found that the diet may be unsafe for people who have ever had an eating disorder, thyroid disease, organ disease, muscle weakness or wasting, or other chronic diseases. Some experts also recommend that you stay away from fasting diets if you’re pregnant or breastfeeding, depressed, on medication, underweight, or have diabetes (Wilhelmi de Toledo, 2013). 

Even if you’re only fasting for a short period of time and don’t fall into one of the at-risk groups above, it’s a good idea to talk with your healthcare provider before starting a water-fasting protocol.

To sum up, water fasting may have some health benefits, but there are also risks to be aware of. Researchers have yet to work out how to do it optimally or whether it’s superior to other, less extreme forms of fasting like intermittent or alternate-day fasting.

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Current version

October 18, 2021

Written by

Health Guide Team

Fact checked by

Felix Gussone, MD

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About the medical reviewer

Felix Gussone, MD, Ro

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.