What Is Water Fasting and How Can It Harm Your Health?

Water fasting can harm your heart health.
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Fasting diets have become buzzy in the weight-loss world, and water fasting is one of the most extreme. But what is water fasting, exactly? And can such a restrictive diet harm your health?


The short answer is yes, water fasting is dangerous. Despite the popularity of various fasting methods, there's no quick fix when it comes to weight loss. In fact, water fasting — or fasting of any kind — is not usually recommended without supervision from a health professional.

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Here, we break down the negative effects of water fasting, why you shouldn't try water fasting for weight loss and safer alternatives.


Ditch the fad diets and talk to your doctor about how to lose weight safely.

What Is Water Fasting?

Water fasting is pretty much what it sounds like:You don't eat any food or drink any beverages except water.

These fasts typically last anywhere from 24 to 72 hours, but they've also been attempted on other timelines, too — for instance, some programs teach you how to water fast for 5 days or even tout a (very dangerous) month-long water fast.


No matter the length of the water fasting diet, though, one thing is for sure: Starving yourself isn't a casual or safe fat-loss method, according to the Mayo Clinic. Despite the recent popularity of different fasting plans (like alternating between fasting and feeding days, according to Harvard Health Publishing), cutting out food entirely is not a safe or sustainable way to lose weight.

Indeed, water fast dieting has many side effects and dangers (more on that later), according to the Mayo Clinic, which is why doctors don't recommend it as a weight-loss technique.


So unless your doctor prescribes water fasting — and doctors rarely endorse such extreme techniques for weight loss — it's safe to assume that this diet is not for you. That's especially true if you have diabetes, liver or kidney issues, are pregnant or have experienced disordered eating in the past.

Side Effects of Water Fasting

Now that we've established water fasting is not good for you, let's get into the specifics. Here are the side effects of fasting this way:


1. It Can Harm Your Kidneys

Water fasting can have a negative affect on kidney function, according to a small February 2018 study in the ​Bratislava Medical Journal​. The researchers found that after people followed an 11-day water fast, they had excess uric acid in their blood, which can lead to kidney stones or gout.


2. It Can Lead to Dangerous Blood Sugar Crashes

A water fast can be especially risky if you have diabetes. Fasting can lead to an extreme drop in blood sugar called hypoglycemia, according to a January 2013 study in ​​Postgraduate Medicine​​.


Per the Mayo Clinic, hypoglycemia can cause symptoms like:

  • Irregular or rapid heartbeat
  • Fatigue
  • Pale skin
  • Shakiness
  • Anxiety
  • Sweating
  • Hunger
  • Irritability
  • Tingling or numbness of your lips, tongue or cheek

If left untreated, it can lead to seizures or loss of consciousness.



If you have diabetes and experience hypoglycemia that doesn't respond to treatment (like eating or drinking a sugary snack), seek medical care immediately, according to the Mayo Clinic.

3. It Doesn't Provide Enough Fuel

For many, water fasting is a form of crash diet, which throws your body into an extreme and unexpected calorie deficit. Over time, this severe calorie restriction can trigger your body to go into starvation mode, according to University of California, Berkeley.

When you don't get enough fuel, you can experience symptoms like:


  • Fainting
  • Weakness
  • Thinning hair
  • Loss of coordination
  • Loss of strength and endurance

Not eating enough calories can also put you at risk for malnutrition, according to the U.S. National Library of Medicine.

4. It Can Harm Your Heart

Eating too few calories for too long forces your body to break down muscle for fuel, including the muscle of your heart, according to the National Eating Disorders Association. This can lead to heart problems such as:

  • Slower pulse
  • Irregular heartbeats
  • Low blood pressure
  • Higher risk for heart failure


5. It Can Trigger Disordered Eating

If you have a history of eating disorders like anorexia or bulimia, extreme diets like a water fast may trigger the condition, according to the Mayo Clinic. Learning how to water fast can also lead to disordered eating for those with no history of a disease.


Other diets like intermittent fasting may trigger disordered eating, as well. That's why it's important to talk to your doctor before you try any program.

How Much Can You Lose on a Water Fast?

Extreme diets like this one often promise rapid and effective weight loss. But does a water fast help you lose weight, really?

While it's possible to lose weight on this program, water fasting results are by no means safe or sustainable: Cutting all calories can result in weight loss that exceeds the safe, expert-recommended pace of 1 to 2 pounds per week, per the Mayo Clinic.

Losing weight any faster than that can put you at risk for the above complications and weight regain. That's because the science of slimming down is a little more complex than just cutting calories.

Your metabolism is the process by which your body burns calories in order to keep you breathing, keep your blood circulating — basically, keep you alive, per the Mayo Clinic. How fast it works is largely determined by your body composition: The more muscle and less fat you've got, the speedier your metabolism.

Although fasting means you eat fewer calories overall, your body also starts to burn those calories more slowly. Crash diets like fasts can cause your body to burn muscle for energy instead of carbs or fat, which leads to a slower metabolism, according to the National Health Service.

Fasting has also been shown to have little benefit over standard calorie-restricted dieting, according to a July 2017 study in ​​JAMA​​. After comparing people who followed an alternate-day fast to those who followed a standard calorie-restricted diet, researchers found that avoiding food altogether had no advantage in weight loss or weight maintenance.


Much of the long-term research on fasting has been done in rodents, though, so more research is needed to replicate the results in humans, according to the Harvard T.H. Chan School of Public Health.

And even if you decide to subject yourself to the dangers of this diet, once you stop, you're likely to gain all the weight back (and, often, additional weight), according to September 2017 research in ​Perspectives on Psychological Science.

Alternatives to Water Fasting

Water fasting isn't the answer for shedding fat or extra pounds. However, it's possible that more moderate fasting techniques — like the 16:8 diet where you eat during an eight-hour period, then fast for the remaining 16 hours of the day — may have some benefits​, per Johns Hopkins Medicine.

Here are the potential perks of intermittent fasting:

  • It may burn fat to support weight loss
  • Protection against chronic diseases like type 2 diabetes, heart disease and age-related neurodegenerative disorders
  • Improved blood pressure and resting heart rate
  • Better memory

Still, it's always best to discuss your diet plans with a medical professional to make sure it's safe for you.




Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.