Water Fasting: Benefits and Risks

Piggybacking on the popularity of intermittent fasting, water fasting is making waves in the popular diet world once again. Far from a new approach, this diet, which dates back to ancient Greece, largely lacks scientific evidence, though many naturopathic doctors tout its potential short-term benefits. (1) Yet this approach isn’t one to start willy-nilly, as depriving your body of food can pose serious health consequences if you don’t take the right precautions, healthcare professionals warn.

Here’s a detailed look at what we know — and don’t — about this type of fasting, along with exactly what steps you’d be smart to take if you’re considering this type of diet yourself.

What Is Water Fasting?

There may be no easier diet to define than water fasting, for this approach involves exactly what it says. “Water fasting is complete absence of all substances, except pure water, in an environment of complete rest,” says Alan Goldhamer, doctor of chiropractic and founder of the TrueNorth Health Center in Santa Rosa, California, who runs medically supervised water fasts that last anywhere from 5 to 40 days (the latter of which is a length of time most healthcare professionals do not endorse). The idea behind being at complete rest (aka not exercising), Dr. Goldhamer says, is your body won’t need as many calories. 

As you may suspect, an aspiration to lose weight is a huge reason people try water fasting. “People are tired of battling with their weight and are looking for the next best weight loss tool, which is one reason water fasting has become so hot,” says Kristen Smith, RD, an Atlanta-based spokesperson for the Academy of Nutrition and Dietetics.

Some people may even do it in hopes of treating health conditions, such as high blood pressure, type 2 diabetes, and heart disease, all of which can involve excess weight, Smith adds. And of course, other people may do a water fast for religious reasons or prior to a medical test such as a colonoscopy. (1,2)

Water fasts are a buzzy topic in the holistic health world, but let’s get one thing out of the way: Despite whatever you’ve read online suggesting otherwise, there’s a lack of rigorous scientific evidence to support using this approach for weight loss, disease management, and the like. In fact, water fasts may pose serious health risks to certain people.

Taking a break from eating, however, can lower your blood pressure, according to one study. In that study, 174 people with high blood pressure underwent a water fast for 10 to 11 days on average, and by the end of that period, about 90 percent of participants had lowered their blood pressure to normal levels. (3)

Whether it’s the fasting approach or simply not eating that’s behind that benefit is unclear. But Goldhamer, who coauthored the study, offers a theory for why: “With these diseases, people are either taking in more calories than they should, especially from animal fat and protein or refined carbohydrates, and they’re eating food that’s toxic,” he says. By fasting, though, he says you’re giving your body a chance to eliminate toxic by-products.

It’s true that the World Health Organization has labeled red meat — and particularly processed red meat, such as hot dogs and bacon — as a carcinogen that contributes to cancer risk. (4) And a wealth of research has linked eating red meat with a higher risk for hypertension, which, along with high cholesterol and being overweight, is a risk factor for heart disease. (5,6)

Although more research is needed, another potential benefit of water fasting may be a reduction in oxidative stress, which, according to an article published in 2017, is a phenomenon that contributes to various chronic diseases. (7) In fact, a small study published in 2018 found that participants saw a reduction in blood pressure, body weight, and oxidative stress following 11 days of water fasting. (8)

What’s unclear is whether those effects are long-lasting. But any level of stress can age you, says Taz Bhatia, MD, a holistic doctor in Atlanta. Consequently, you may notice excessive fatigue, brain fog, wrinkles, gray hair, poor eyesight, headaches, sensitivity to noise, and muscle and joint pain, she says.

You don’t have to be a scientist to know that when you consume nothing but water, you could wind up in trouble. Food, after all, isn’t an indulgence but fuel for your body.

So what are the dangers of water fasting? That depends on two questions: “Who’s doing it and for how long?” says David L. Katz, MD, MPH, founding director of Yale University’s Yale-Griffin Prevention Research Center. Certain people, including anyone with type 1 diabetes, who are dependent on insulin, or anyone with a heart issue, should avoid water fasts.

Meanwhile, a water fast that lasts more than 48 hours could put your body in serious trouble, as evidenced from nutritional history when very-low liquid calorie diets (VLCD) were the rage. “These produced rapid weight loss in people by dropping their calories below 800 calories,” Dr. Katz says. Trouble is, people also tended to die, because when fuel supplies dip too low, the body starts shopping for calories from its own stores. “That can create changes in the cardiac architecture, which is enough to disrupt electrical flow and lead to fatal cardiac arrhythmias,” he says. “If this is truly just water for extended weeks, people will die.” (9,10)

But that’s not all. Dr. Bhatia says water fasting can strain the kidneys and lead to nutrient deficiencies as well. That same muscle wasting that water fasting can trigger can lead to excess protein delivery to the kidneys, which makes these organs have to work harder. (8)

Other side effects may include fainting, brain fog, fatigue, and in women, alterations in hormone levels if done for a prolonged period, she says. Fainting, brain fog, and fatigue is likely the result of low blood sugar from not eating. (11) Yet more research is needed on whether fasting alters hormone levels in women. For example, a study published in 2014 found that after one month of Ramadan fasting, the hormonal changes in 30 women were statistically insignificant. (12) However, Ramadan fasting happens between dawn and sunset, so people can eat at night, unlike the water fasting described in this article.

One thing about water fasting is for sure: “Because it’s self-imposed starvation, which isn’t a healthy or maintainable way to lose weight, you go back to eating food after the fast and the weight is likely to come back, perhaps even more than before,” says Kelly Kennedy, RDN, staff nutritionist at Everyday Health. This can set you up for yo-yo dieting, which wreaks havoc on your metabolism and can have other negative health effects. Even worse? “It can trigger disordered eating in some people,” she says. (13)

There are no proven benefits of water fasts, so most registered dietitians, including Kennedy, advise against doing them.

Certain individuals should avoid them because they can pose serious health harms. Those people include those with kidney disease, those on multiple medications, or women who are pregnant or nursing, Bhatia says. She also cautions people with diabetes, heart disease, and liver disease from doing this on their own. This approach also isn’t for anybody who has a history of eating disorders, Kennedy says. (14)

If you want to get to a healthy weight, look at water fasting not as a diet but as a tactic that you try for a few days before returning to eating. Perhaps you’ve tried to exercise and use portion control. Maybe you’ve even eaten the highest quality foods that fill you up on the fewest calories. Not confident that those two options are doing the trick? This is where water fasting might come into play. But note: “There’s no metabolic magic in fasting, and it may only work as well as caloric restriction,” Katz says.

The problem with fasting, whether it’s water fasting or another type? Many people assume that if they’re not eating one or two days a week, they can eat whatever they want on the other days, and that’s just plain wrong. “Food isn’t just about losing weight,” Katz says. “Everything in your body depends on the quality of your diet.”

You might see that three- and seven-day water fasts are popular online, but that’s longer than most experts advise. If you don’t have any conditions that would make a water fast a no-go for you — and you’ve cleared this approach with your healthcare team first — Bhatia recommends not doing it any longer than 24 to 48 hours. “This is when issues of brain fog and fatigue really start to set in,” she says. Check with your physician first, and if at any point you don’t feel well during the fast, call your doctor.

During these 24 to 48 hours, make sure you’re getting adequate water intake. “You get a lot of water from food on a normal day of eating, so during a water fast, you’re going to have to drink more water than you normally would in a day,” Smith says. Aim to drink 9 to 13 cups of water a day, divided throughout the day, and use reminders to make you sip, Smith says. For instance, set a reminder on your phone or use a water bottle that lights up at set intervals to tell you to drink.

Also, take a break from physical activity, and because of the fatigue you may feel, you may want to curtail driving or operating heavy machinery during these times, Smith adds.

If you want to use water fasting as a weight loss tactic at home and don’t have any contraindications, “it could be acceptable,” Katz says. Just check with your doctor first and make sure you don’t do it for any longer than 48 hours. Anything over that period should be done under constant medical supervision to prevent unwanted consequences, Bhatia says. Regardless, if you experience any side effects, like dizziness, light-headedness, or fainting, stop fasting immediately.