The 21 Day Water Fast Guide: How To Prepare, The Risks + Benefits

Water fasting for 21 days is an extreme fasting protocol and rarely advisable.

However, if you are curious about a 21 day water fast, you may have some of the following questions, such as: Is 21 day water fasting safe? What are the best tips for water fasting for 21 days? What are the benefits of a 3 week water fast?

In this article, we will discuss what water fasting for 21 days entails, the benefits of a 21 day water fast, risks and precautions for a 3 week water fast, and tips for how to do it.

We will look at the following: 

  • What Is a 21 Day Water Fast?
  • How to Do a 3 Week Water Fast
  • Benefits of a 21 Day Water Fast

Let’s get started!

A glass of water in a plate with a person sitting in front of it with a fork.

What Is a 21 Day Water Fast?

As the term describes, a 21 day water fast is a 3-week water fast, meaning that for 21 full days, you will only consume water and non-caloric beverages and electrolytes but no food or calorie-containing beverages.

It is imperative that you speak extensively with your doctor before trying a 3 week water fast.

Unless you are water fasting for 21 days for religious or spiritual reasons, there are few—if any—reasons that you would need to take on such an extreme, prolonged water fasting protocol.

Although studies suggest that prolonged fasting can be safe, these studies have not looked at water fasting for 21 days.  

Fasting for 21 days may result in adverse health and metabolic changes such as dehydration, a loss of lean muscle mass, hyperuricemia, hyponatremia, protein sparing, sodium and potassium sparing, decreased serum calcium and magnesium levels, and acidic urine.

A glass of water.

Even shorter water fasting periods can lead to symptoms such as headaches, nausea, vomiting, constipation, the inability to regulate body temperature, muscle loss, lightheadedness, shakiness, poor healing, blurry vision, difficulty sleeping, mood instability, and dangerous nutritional deficiencies.

While we will discuss the benefits of water fasting for 21 days, it would be neglectful and potentially dangerous not to make it abundantly clear that a 21 day water fast is contraindicated for most people and should never be attempted without medical clearance, medical guidance, and proper medical supervision.

There are usually healthier, less severe alternatives for water fasting for 21 days for weight management and perceived health benefits, including shorter fasts, an intermittent fasting diet, and a well-balanced program of diet and exercise.

Even if you receive approval from your doctor to try 21 day water fasting, you should work closely with your provider during the 3 week water fast and afterward to ensure that your body is handling the prolonged abstinence of caloric intake.

A person drinking a glass of water.

How to Do a 3 Week Water Fast

As discussed, water fasting for 21 days involves abstaining from all caloric food and beverages for 21 full days. This is an extended fasting protocol.

Here are the steps of how to do a 21 day water fast:

#1: Consult Your Doctor

As discussed, it is imperative that you receive medical clearance and discuss the risks of 21 day water fasting with your doctor before attempting such a prolonged fasting duration on your own.

Your doctor may want to run lab tests and discuss potentially better alternative options for whatever benefits of 21-day water fasting you are seeking.

In many cases, you can derive the 21 day water fasting benefits from shorter water fasting protocols, such as a 3-day water fast, 5-day water fast, 7-day water fast, or a long 10-day water fast.

These shorter water fasting durations will be safer, physically and mentally easier to get through, and will likely still improve your health and support weight loss when more so than 21 day water fasting plans.

A person meeting with their doctor.

#2: Consider Your Options

Remember, just because you may receive medical clearance to do a 21 day water fast and you might have your goal set on water fasting for 21 days does not mean that you have to go through with it. 

Consider your alternatives, such as water fasting 24 hours once a week for a couple of weeks or doing a shorter prolonged fast like a 2 day water fast.

You can also start your 21 day water fast and end things early if you are not feeling well.

It is vital that you put your ego aside and listen to your body whenever you are doing prolonged fasting.

Just because you want to water fast for 21 days and you have a couple of days under your belt does not mean that you have to or should continue going through with it if you are not feeling well or you have changed your mind.

Intermittent fasting written on a notebook, a clock and a fork and knife.

#3: Try a Shorter Fast

Any time you are going to try extended fasting, it is best if you gradually work up to the length of the fast with some shorter water fasting.

For example, if you want to try a three day water fast, start with just an intermittent fasting diet such as 16/8 intermittent fasting diet.

If that goes well, you can try a 24 hour water fast.

After a week or so for recovery, you can then try a 48 hour water fast, and so on.

Jumping right into prolonged fasting—whether a full 3 week water fast or even something comparatively moderate like a 5 day water fast—will be a complete shock to your system.

Therefore, you should practice with shorter fasting durations to help your body adjust to the physical and mental sensations of not getting to eat anything or having any calories coming in. 

A calendar.

#4: Build Your Toolkit

Part of the reason it is helpful to practice with shorter water fasting before taking on 21 day water fasting is to build your physical and mental skills to handle the discomfort of not eating.

You will need to discover ways to best support your body and mind when you are hungry and have low energy. Examples include journaling, meditating, napping, light walking, listening to music, to name a few.

#5: Work Out the Logistics

For many people, a 21 day water fast may mean manipulating your work schedule or finding ways to streamline your life so that you can focus on the fasting and not feel the negative effects of fasting infiltrating into your professional life and other aspects of your personal life.

Perhaps you have vacation time to use, or you are a teacher with summers off, but if possible, it is helpful to schedule your 3 week water fast at a time when you do not have a lot of other things going on.

Electrolyte powder.

#6: Start Hydrated

Once your 21 day water fast begins, make sure that you are staying well hydrated with noncaloric beverages such as water, herbal tea, hot or cold caffeinated tea, black coffee, and seltzer water.

You should also have electrolytes with your water. There are different options for unsweetened electrolyte tablets and powders.

You cannot eat any foods and cannot have caloric beverages.

#7: Break Your Fast

When most people try a water fasting protocol, such as a 24 hour water fast, 48 hour water fast, or something even more extreme such as a 5 day water fast, they focus primarily on getting through the actual water fasting duration without putting much forethought into how to break a water fast.

Whether you make it through the full 21 day water fast, or you have to stop early because you’re not feeling well, or you’ve decided that you are done water fasting, it is important to figure out how to break a fast.

Because water fasting for three weeks will change your digestive system’s ability to handle food, it is crucial to gradually ease back into eating when your 3 week water fasting is over.

For specific guidelines regarding the best foods to eat to break a fast, check out our guide here.

A person measuring their waist after weight loss.

Benefits of a 21 Day Water Fast

There are various health benefits of prolonged fasting. It is important to note that for most of these benefits, it is absolutely not necessary to water fast for 21 days to achieve them.

That said, here are some of the general benefits of water fasting:

If you are mainly considering water fasting for 21 days in order to support weight loss, consider a more sustainable and safe option.

Check out our guide to various intermittent fasting diet schedules here.

A clock in the middle of an empty plate.
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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