IF YOU’RE SICK of overly-involved diets, water fasting—consuming only water for two or more days you consume is water for two or more consecutive days—may sound appealing. But is it safe? Can guzzling H2O actually help you drop pounds?
“Water fasting creates significant calorie restriction, which results in rapid weight loss. Initially this loss is due to water and glycogen depletion and ultimately fat loss,” says Trista K. Best, M.P.H., R.D. That said, it’s not something you want to engage in: “It is not a sustainable practice and may result in rebound weight gain once ended.”
You might think, hey, it’s only water, how bad could it be? MH advisor, Robert Glatter, M.D., a professor of emergency medicine at Lenox Hill Hospital Northwell Health, puts it this way: “Having worked in the emergency department for over two decades taking care of people engaged in water fasting, my advice is simple: avoid it at all costs. It’s just not worth the associated risks, and it could cost you your life.”
Here are the dangers of water fasting and what you should know about this risky practice.
What is water fasting?
Simply put, water fasting is a water-only diet that lasts two or more days.
Fasting itself is hardly a new idea. Humans have incorporated fasts into their lives for thousands of years, often as part of religious traditions. In the mid-1800s, a few doctors on the fringe started experimenting with fasts for health reasons, and this continued into the 20th century, with a bit of a revival in the 1960s.
Today, on TikTok, you can find all kinds of pseudo-experts touting the benefits of a water fast. Often the goal is weight loss or some other health benefit, like a reduction in cancer risk or an improvement in type two diabetes or high blood pressure. As with any health advice, check where it’s coming from. Influencers don’t often have the credentials to be able to comment on health effects.
How do you water fast?
There are no hard and fast rules for a water fast, mostly because there isn’t enough research to establish safety guidelines. It’s worth noting that a water fast is not the same thing as a liquid diet.
“There’s not a lot of published science about it,” says Krista Varady, Ph.D., professor of nutrition at the University of Illinois, Chicago. In the research studies that exist on water fasting, study participants (usually a small number of them) are closely and constantly monitored by medical professionals during the fast and then watched and guided as they are re-introduced to regular food.
Water fasts range from two to 41 days with an average of around seven days. In some versions of the fast, participants are allowed a small amount of liquified food, like fruit juice and vegetable soup, each day.
As part of their treatment, study participants might also be given complimentary care, like enemas to clear their guts since the fast blows up their usual poop schedule. Sometimes they stay at a spa and are encouraged to get massages. In other words, these guys aren’t usually at home toughing it out alone.
“In some controlled water fasting studies, participants consume two to three liters daily, much greater than recommended daily water intake. While participants were typically young and healthy, without underlying kidney disease, they still experienced multiple metabolic derangements which if undetected, could lead to serious neurological complications, which unrecognized had untreated, may ultimately prove fatal,” Glatter says.
Is water fasting safe?
“I would say it’s not a safe choice—definitely not—for people to do on their own,” says Varady.
If you wanted to try just drinking water for one day, which is also known as zero-calorie alternate day fasting, that’s a possibility, says Varady. (Still, check with your doctor first.) But she doesn’t recommend multiple-day water fasts.
“I would say it’s not a safe choice—definitely not—for people to do on their own.”
One reason: Although you might lose weight on a water fast, you probably will shed more than fat. Water fasting will likely result in lean muscle wasting, or muscle mass loss that occurs when you don’t take in protein, she says. To compensate, your body starts to break down muscles. Over a long fast, you might develop other nutrient deficiencies as well.
Another problem: “I’d be worried about eating disorder propensity,” says Varady. “That seems a little worrisome for me, if you’re not eating, it might change your psychology around food if you haven’t eaten anything for 30 days.”
And it’s not just the fast that’s potentially dangerous–it’s the period of time when the fast is over and you’re ready to eat everything in sight. “When you re-feed the person after like, 20 days of water fasting, that can be kind of dangerous, because the person’s body doesn’t know how to deal with glucose and fat and all this stuff,” says Varady.
This could be even more dangerous if you already have a metabolic problem like type 2 diabetes. “If somebody has type two diabetes, I would absolutely not do a complete water fast for days unless they’re under the careful supervision of a doctor,” says Varady.
Complications of water fasting include low blood sugar or hypoglycemia, Glatter points out. “If abrupt and sustained, this may also result in seizures and ultimately death if not rapidly treated,” he says.
What are the risks of water fasting?
Research published in the journal BMC Complementary Medicine Therapies showed that the most common adverse events from water fasting include:
- Quality of life problems like fatigue
- Heart problems like high blood pressure
- Musculoskeletal problems like arthritis
- Gastrointestinal problems like colitis, gastritis, and constipation
- Endocrine problems like thyroid trouble or diabetes
- Neurological problems like headache, neuropathy, and dizziness
- Urologic and renal problems
- Psychiatric problems like depression and anxiety
- Pulmonological problems like inflammation, labored breathing, and infection
Two people in the studies analyzed required hospitalization, one with severe dehydration and another with dangerously low sodium levels.
In a recent study of middle-aged men from Poland who did an eight-day water fast, patients lost weight and perceived a reduction in stress, but their bodies experienced some negative consequences including dehydration, low sodium levels, low blood sugar levels, and an increase in uric acid in their blood.
“The body needs electrolytes to keep sodium levels stable, and most of these electrolytes come from fruits and vegetables, such as bananas, spinach and watermelon,” says
Farhad Modarai, D.O., chief clinical officer of population health at Strive Health in Denver, which specializes in kidney care.
A balanced diet is a safer way to boost your health, says Glatter. As is listening to your thirst. “The key marker of when you need to drink fluids is your thirst, a highly reliable mechanism that is based on feedback from special receptors in the hypothalamus (in your brain),” he says. “These receptors detect a higher concentration of salt or sodium in your blood stream, and then trigger the thirst mechanism so you take in more fluids. It also leads to release of a hormone known as ADH (antidiuretic hormone) that enables your kidney to reabsorb water to lower your serum sodium, addressing the early warning signs of dehydration.”
Is there a safer way to fast?
Yes. Intermittent fasting, where you eat real food normally during an eight-hour window of the day and then drink water for the rest of the day, is much safer than drinking only water for days on end.
“If somebody wants to lower their fat mass while maintaining a lot of their muscle mass, you know, it makes more sense to just do simple intermittent fasting,” says Varady.
During intermittent fasting, people tend to naturally reduce their energy intake by about 500 calories, so they lose about a pound or two per week, she says. In her research studies on intermittent fasting, constipation is a common side effect, but it usually resolves in a couple of weeks—no daily enemas needed.
“I do not recommend that healthy people, and especially those with known kidney disease engage in water fasting; it’s not only dangerous at best, but could ultimately prove fatal,” Glatter says. “While many dietitians have touted the potential health benefits of short term water fasting under the guise of a ‘detox’ or cleanse, ramifications of this fad to improve health, reduce inflammation, and promote longevity can be deadly.”
Is it dangerous to drink too much water?
“Water fasting is not a good idea for most people, especially for a prolonged period of time. If someone isn’t eating solid foods, they will feel hungry and they may drink more and more water to achieve some level of fullness,” says Modarai. “This can be very dangerous, as hyponatremia, or dangerously low sodium levels in the blood, can happen.”
Low serum sodium, Glatter explains, “can lead to seizures, but also brain swelling, brain herniation, and ultimately death if not recognized and treated urgently. Low blood potassium, another risk of water fasting, could adversely affect nerve, muscle and heart function.” Low blood calcium and low blood magnesium can happen, too, and that’s dangerous since these electrolytes are vital to nerve and muscle function.
Modarai also points out that drinking large amounts of water in short periods of time can also overload the kidneys and cause your cells to swell. If you’re fasting and only drinking water, you may chalk symptoms like headaches, nausea, weakness, and fatigue up to your hunger, but it could actually be a life-threatening problem if left untreated, he says.
Some of the claims about water fasting specifically revolve around the idea that fasting can induce autophagy, your body’s natural process of cellular cleanup and removal of damaged cells. There is limited evidence, often from animal studies, that short-term fasting can increase autophagy, but there’s no published research in humans demonstrating that water fasting is a safe or effective way to induce it.
Why do people water fast?
There’s a slew of reasons people may be drawn to the dangerous practice of water fasting. Best breaks down some of the more common ones below.
Weight loss
This might be the most common reason people are intrigued by the concept of a water fast. Biologically speaking, water fasting for an extended period of time leads to significant calorie restriction; hence the rapid weight loss, explains Best. As mentioned above, it’s not sustainable—and not safe.
Health Benefits
Others are drawn to water fasts because they think it may result in autophagy, a process in the body where damaged cells are cleared, says Best. “It is thought that this process is triggered during water fasting and can reduce the risk of chronic diseases,” Best says. There’s no concrete research to back this up—so don’t want to starve yourself for this sake.
Spiritual Practice
One of the more legit reasons for water fasting, especially if you’re limiting it to a very occasional occurrences and doing an abridged water fast for, say, a few hours or day, is for spiritual or religious reasons. “The concept of fasting is a common practice in many religions as a spiritual discipline, promotes spiritual growth and provides focus on the divine during the fasting period,” says Best.
Mental Clarity
Perhaps you’ve seen folks bragging on X or Instagram about their hyperfocused, smooth-sailing brains post-water fast. “Those who take part in water fasting report experiencing enhanced mental clarity,” says Best. Don’t buy it: “This is likely due to the lack of blood sugar spikes.”
