• Posted July 12, 2023

Water Fasting: Is It Safe? Is It for You?

Water fasting has become one of the trendiest new weight-loss sensations, touted by former athletes and wellness gurus.

But whatever weight is lost during a water fast can come back quickly, while other health benefits fade fast, according to a new review of the available scientific evidence.

“I personally wouldn't really recommend this diet,” said senior researcher Krista Varady, a professor of nutrition at the University of Illinois at Chicago. “I know it's become popular for some reason, kind of out of nowhere, over the past year or so. But even if people maintain the weight loss, all the health benefits go away.”

Water fasting involves taking in nothing but water for at least five to 20 days, and sometimes even longer, researchers said in background notes.

Some supervised water fasts allow 250 calories a day, “a little bit of juice at breakfast and a really small amount of soup at lunch,” Varady said.

Last month, former Seattle Seahawks lineman Russell Okung announced on Twitter that he had lost more than 100 pounds by taking in nothing but water for 40 days.

“I fasted for 40 days with nothing but water. Yes, you read right!” Okung tweeted. “The experience was so rich and rewarding that I'm going to do it again…"

And Australian wellness coach Kristine Crouch says a 25-day water fast cleared up her skin and helped her lose 20 pounds.

“I felt lighter in my body, my digestion was incredibly smooth, I had noticeably clearer and smoother skin, brighter eyes, increased energy and mental clarity and, above all, a new sense of peace,” Crouch wrote in an essay for Newsweek.

Stories like these prompted Varady, an expert on intermittent fasting, to conduct a review of the available evidence for water fasting.

“Journalists were contacting me to ask about it,” she said. “I just wanted to review the literature so people could see the science-based evidence supporting this diet.”

Varady and her colleagues identified eight studies on water fasting, and analyzed the data to consider how well this method of weight loss works.

Fasting did spur noticeable short-term weight loss, researchers found.

A five-day water fast caused people to drop about 4% to 6% of their weight; a seven- to 10-day fast produced 2% to 10% weight loss; and fasting 15 to 20 days yielded 7% to 10% loss of total body weight.

But people tended to lose more lean muscle mass than fat mass during a water fast, according to two studies that measured body composition.

About two-thirds of weight loss came from loss of lean mass in those studies, while only a third came from reductions in body fat, the review said.

“This finding is concerning as lean mass is a key predictor of resting metabolic rate,” researchers wrote. “Reductions in lean mass can translate into lower resting metabolic rate after fasting, which can put individuals at risk for future weight regain.”

Only a few of the studies tracked whether participants regained weight they lost during a water fast.

In one study, people gained back all the weight lost during a five-day water fast within three months.

Two other studies found that a small amount of lost weight returned, but participants in those studies had been encouraged to restrict their calorie intake after their fast.

“If people change their eating habits afterwards and they try to get a healthier diet, it seems like they can maintain some of the weight loss,” Varady said.

Water fasting also produced a number of health benefits, such as improvements in blood pressure, cholesterol and blood sugars, the review found.

But these improvements were short-lived, fading away soon after a person started eating again -- even if they didn't put weight back on.

“Their blood pressure just went back to where it was, their ‘bad' LDL cholesterol went back to where it was," Varady said. “They lose all of the metabolic protective effects.”

Further, improvements in blood sugar control did not happen in fasting patients with type 1 or type 2 diabetes, results showed.

Water fasting does appear to be relatively safe, Varady said. The most common side effects were hunger, headaches, nausea, vomiting and insomnia.

Dr. Ivania Rizo, director of obesity medicine at the Boston Medical Center, said she also would be concerned about deficiencies in vitamins and minerals that would occur in people undergoing a sustained water fast.

“In this meta-analysis, I don't believe they looked to see whether there was any nutrition deficiency,” said Rizo, who wasn't part of the study.

Both Varady and Rizo said water fasting simply isn't sustainable.

“Obesity is a chronic disease, as has been emphasized now more and more, and these really short-term, very non-sustainable interventions do not seem particularly as impactful,” Rizo said. “These pretty extreme measures do not really seem like something I want to do for a patient.”

She said patients would be better off looking into the new weight-loss drugs like Ozempic, which reduce hunger and let people think about something other than food.

“Their significant occupation with food is diminished, and they can go ahead and focus all their energy into other parts of their life that they can build on,” Rizo said.

By comparison, water fasting would lead a person to do nothing but think of food, she said.

Varady said she would recommend intermittent fasting as a more sustainable alternative.

“Intermittent fasting is a much lighter form of this, I guess you could say, where people eat within an eight-hour window,” she said. “Basically, when people cut out hours of eating, they tend to just naturally reduce their calorie intake. So I guess I advocate that a lot more. That has a lot more science behind it, and it just seems safer.”

At the same time, Varady said if a person really wants to try water fasting, there's nothing wrong with a short fast.

“If somebody wants to do it and they have no medical conditions, and they want to try it for a couple days, I guess go ahead,” Varady said. “But something tells me that people would get so miserable after a couple of days that most people would quit anyway.”

The social aspects could be problematic, she suggested.

“What do you do when you're sitting with your family? You just drink water?” Varady said. “And it would be really hard to be around people if they're eating, so you can't go to any social things for a couple of weeks. So it just seems kind of weird to me.”

The new evidence review was recently published in the journal Nutrition Reviews.

More information

Harvard Medical School has more about the side effects of fasting.

SOURCES: Krista Varady, PhD, professor, nutrition, University of Illinois at Chicago; Ivania Rizo, MD, director, obesity medicine, Boston Medical Center; Nutrition Reviews, June 27, 2023