Water fasting — everything you need to know

Written by Trey Reck

Medically reviewed by Josie Porter, RD

Published on December 12, 2022 | Updated on June 10, 2024

Going 24–72 hours drinking only water and going completely without food — can that be good for you?

This is what’s known as water fasting, and while it might sound kind of bonkers, it’s a real thing.

There’s lots of contradictory information on the interweb about water fasting, which can leave you feeling pretty confused. 

• People often use it as a quick way to lose weight, but does it work?
• It’s talked about as a way to recycle old cells and make them “new” again. Is that true? 
• You’ve heard a water fasting “diet” can improve your health. If you tried it, what might happen?
• How is it possible to function for so long without food?
• Is it safe?

Certainly, intermittent fasting — where only water (and other calorie-free drinks) are consumed for a period of time, alternated with eating a nutritious diet — can get results. 

But whether water fasting is an effective weight loss or health improvement method is less clear. 

And, even if it is effective, is it worth the risk?

We’re going to answer all those questions and more. Come with us as we take a balanced look at how water fasting works, what it does, and whether it could be right for you. 

What is water fasting?

If you’re a newbie to this practice, you’re probably wondering:

What is a water fast? 

A water fast — also known as a water diet — is where you consume only water for 24–72 hours. 

When we say “only water” here, we mean it. No tea, no coffee, no zero-cal soda, no nothing except the clear, hydrating stuff. Mineral water is OK (extra micronutrients are always welcome).

Ideally, you’ll drink between 2 to 3 liters (8–12 cups) of water each day, and that’s all the sustenance that’ll enter your body for the duration of your water fast. 

Consuming plenty of water will keep you hydrated. That’s important. Your body can go without food for a while, but it can’t go more than a few days without water. 

One to three days without any food sounds pretty challenging, doesn’t it? 

Yet, if you’ve rocked up here to learn about how to do a water fast, you’re thinking perhaps it could help you somehow. 

Take a moment to think about that before we get into the details of how this all fits together. 

What would you hope to gain from water fasting?

Although we don’t recommend this fasting approach, the most important thing to remember when it comes to any fasting schedule is that it should suit you: your lifestyle, your body, your goals, and your needs. Fasting also isn’t right or safe for everyone, so you should always involve your healthcare provider in brainstorming what approach(es) might work for you (especially if you’re considering fasting for 18+ hours). We can also help with that! Take our Simple quiz to tell us about you and your goals, and we can make personalized fasting recommendations that support you and your health and well-being.

How does water fasting work?

When you start a water fast, do you just launch yourself in, lock your fridge and cupboards, shred your takeout menus (the less to tempt you), and muscle your way through? 

Is there a water fast schedule you can follow to make it easier on yourself and avoid any intermittent fasting mistakes?

How do you close it out and return to normal operating procedures? 

Let’s look at water fasting before, during, and after.

Before the fast

First, choose your fasting period. 

When does your body not require as much energy? That would be a good time to do this type of fast. 

For instance, water fasting before you hit up a HIIT class at the gym? Not a smart move. But how about during a day when you’re chilling in your PJs and doing nothing more strenuous than watching some Netflix? Much better. 

Then, get ready by spending 3–4 days preparing your body to go without food, either by eating smaller portions or by fasting for part of the day. 

During this time, eat foods that make your body feel well supported — whole grains, fruits and veggies, lean proteins, healthy fats — so you enter the fasting period feeling at ease in yourself (i.e., not bloated, windy, or hyped up on sugar and caffeine).

During the fast

During your specified fasting period (24–72 hours), drink only water. 

Help yourself find ways to hack hunger while fasting, like meditating, using breathing exercises, distracting yourself with fun activities, etc., and keep your energy demands low by resting up and avoiding intense physical movement. 

After the fast 

After your water fast ends, eat smaller meals for about the same length of time you fasted for (i.e., if you fast for two days, eat smaller meals post-fast for two days). 

Reason #1: Big meals could make you feel uncomfortable. Gradually reintroducing food will be a lot more pleasant. 

Reason #2: Eating big meals after a fast can leave you vulnerable to refeeding syndrome, a potentially fatal condition caused by sudden shifts in the electrolytes that help your body metabolize food after going without.

Post-fast, go steady. Much as you may feel super keen to dive in and eat All The Food, keep yourself safe by easing back into eating normally. 

Water fasting and weight loss

Back in the day, when we were hunter-gatherers, we often went without food. When there was a drought, an unsuccessful hunt, and/or foraging drew a blank, there’d be nothing to eat. Fasting was simply a part of life. 

Luckily, our bodies are designed to use our body fat stores for fuel when food is scarce, and this is how we lose weight. 

One of the things you’ll find with water fasting is that the weight you lose won’t necessarily be fat — it’ll predominantly be water. That means that the two-pound loss you see on the scale after 24 hours isn’t yours to keep. 

Here’s why.

When you don’t eat, your glycogen levels drop, shedding with it the water it’s stored (every 1 gram of glycogen carries 3 grams of water). When you go back to eating and drinking as normal, you’ll most likely gain most of that weight back. 

So, although water fasting can help you lose weight, it’s not effective at helping you lose weight because the weight loss you experience isn’t sustainable. 

Combine that with the increased risks, and water fasting doesn’t sound all that appealing for safety or successful results, does it? That’s why we recommend trying something else in the intermittent fasting family, like the less intense time-restricted eating. You can take our Simple quiz to get started.

Health benefits and risks of water fasting

Let’s be clear here: 

The benefits of fasting are not unique to water fasting. 

Research shows that many types of fasting, when done well, may offer several health benefits — and they don’t necessarily compound with more extreme or restrictive fasts. 

These benefits include:

Anti-aging and autophagy (maybe)

Fasting might — and it’s a big “might” — stimulate autophagy, a process that clears out old cells and regenerates new ones.[1]

It may activate metabolic pathways that protect your body from oxidative damage and aging. 

But, truthfully, researchers need to study this much more before any real conclusions can be drawn, as it’s mostly animal studies that suggest water fasting benefits our health like this. 

Lower blood pressure

Longer, medically supervised water fasts may help people with high blood pressure lower their blood pressure, though again, more research is needed to evaluate this connection.[2]

Improved insulin and leptin sensitivity

Water fasting could make your body more sensitive to leptin and insulin, reducing your risk of overeating (as leptin helps your body feel fullness signals) and diabetes (as insulin helps your body process blood sugar).[3,4] 

Lower risk of chronic diseases

Water fasting may lower the risk of chronic diseases like diabetes and heart disease,[5] as well as the risk factors of metabolic syndrome,[6] and improve overall metabolic and cardiovascular health.[7]

Let’s look now at the risks. 

Dehydration

Believe it or not, you can get dehydrated from water fasting. 

That sounds like it shouldn’t be possible, huh! 

But, because a significant chunk of your daily water intake comes from what you eat, it is. 

To make your water fast results positive, increase your water intake to between 2 and 3 liters to avoid dehydration-related dizziness, nausea, headaches, constipation, and brain fog.

Blood pressure changes

Hypotension is low blood pressure, and it can be caused by excessive water intake or increased water plus decreased salt intake — both potential outcomes of a water fast. Orthostatic hypotension is when your blood pressure drops when you change positions, like standing up after crouching down. It can make you dizzy and unsteady on your feet and even cause you to faint. Sometimes, a water fast results in this kind of hypotension, too, this time due to insufficient water intake and dehydration.

Worsening of health conditions

Water fasting can aggravate: 

• gout, due to the increase in uric acid production
• chronic kidney disease, as fasting may worsen kidney function [8]
• diabetes, by increasing the risk of hypoglycemia (low blood sugar)
• eating disorders, by triggering disordered eating patterns
• heartburn or gastroesophageal reflux disorder (GERD), as your stomach may keep making stomach acid despite not eating, which can worsen symptoms

Undesirable side effects 

Fasting prevents the body from getting the fuel it needs to function. You’ll probably feel irritable and tired, and you may also experience side effects like:

• dizziness
• headaches
• weakness
• nausea
• brain fog
• fainting (due to drops in blood sugar and blood pressure). 

Nutrient deficiencies

You might experience nutrient deficiencies and even muscle loss, especially if you fast for too long or don’t follow a proper protocol. 

By restricting calories, you’re limiting your intake of macro- and micronutrients, things your body needs to function and thrive.

Refeeding syndrome

When you eat again after a water fast, you may feel some digestive discomfort and nausea — or worse, run the risk of refeeding syndrome. 

It’s not super common, but reintroducing calories too quickly after an extended fast can cause this potentially fatal condition (which happens due to sudden shifts in the electrolytes that help your body metabolize food after not eating).

May lead to overeating

Restricting your food intake can often have a rebound effect where, when you’re able to eat again, you swing too hard in the other direction and eat too much when your fast is over. 

We’re going to put this out there in very clear terms:

While it’s normal to feel irritable or tired when fasting, if you try water fasting and start to feel disoriented or confused, or strongly experience any symptoms in this list of risks, stop fasting and eat something.

Is water fasting safe?

As we’ve seen, there are risks involved with water fasting for everyone, and we don’t recommend it (or any period of fasting or limited eating that lasts 18+ hours). While everyone should check with their healthcare provider before trying it out, there are some groups of people who should 100% not water fast without medical supervision. 

That includes people who:

• have certain health conditions, like:
  • gout
  • diabetes (type 1 or 2)
  • chronic kidney disease
  • heartburn or gastroesophageal reflux disorder (GERD)
• have an eating disorder or a history of one
• are pregnant, breastfeeding, or trying to conceive
• have a BMI of less than 18.5
• are extremely active
• are prescribed medication (like hypertensives or any medications that need to be taken with food) 

• are 80 years old or older
• are under 18

Although water fasting seems easy and straightforward, bear in mind that when it takes place in a medical setting, people are monitored for signs that things are not OK, like a bad taste in the mouth, back pain, headaches, vomiting, and so on. 

That’s not possible at home, and without that safety net of having trained medical professionals on hand, the risk goes up significantly.

Pros and cons of water fasting

Pros 

A water fast is undoubtedly Simple to do. For 1–3 days, you’ll consume only water, so your intake of sugar, processed food, alcohol, etc., naturally decreases. 

That, and the act of fasting itself, can lead to some health benefits like lower blood pressure, and you’ll likely lose some weight. 

Cons 

Water fasting involves several health risks, and it’s certainly something that’s not safe for everyone to try. 

While it can help you lose weight, any weight loss is more likely to be water than fat, and it could also deprive your body of the nutrition it needs to function and feel its best. 

Is water fasting right for you?

Is water fasting right for you?

You’ve read all about it. 

What do you think? Is water fasting right for you? 

Bring on the water fast!

If you:

• have your doctor’s blessing
• are crystal clear on the risks and benefits of water fasting
• feel safe and secure going 24 hours without food
• are well supported by those around you 

perhaps water fasting could be for you. 

We still recommend you only do it with medical supervision, though. 

Gonna pass on the water fast, thanks

But, if you:

• think the risks outweigh the benefits
• don’t feel you could safely perform a water fast
• have medical or non-medical reasons why water fasting isn’t suitable for you

that’s OK — there are plenty of other ways you can reach whatever health goals you have. 

Maybe another intermittent fasting schedule would suit you better. 

Water fasting vs. intermittent fasting 16:8

With intermittent fasting 16:8, you get to eat (YESSSSSS). Consume all your calories over eight hours and fast for the remaining 16. The fasting window is shorter than with water fasting, and this approach has far fewer risks and more benefits. 

That’s why it — or other time-restricted eating schedules like 12-hour intermittent fasting or 14:10 intermittent fasting — are our most recommended plans.  

That said, a time-restricted fast is something you do every day, and that can cause more disruption to your life than intermittent water fasting. 

The next three fasting schedules don’t involve fasting daily, but they do all still involve 18+ hours of either no eating or limited calorie intake. So, they may be less intense than water fasting, but they’re still pretty restrictive and can be tricky to manage safely and sustainably. That means — you guessed it — we don’t recommend them as alternatives unless your healthcare team is on board.

But let’s see how they stack up to water fasting. 

Water fasting vs. 5:2 fasting

5:2 fasting means following a modified fast (i.e., eating 500/600 calories) two days a week and eating as you usually would on the other five. 

As you’re not going completely without food, you’re on safer ground, less likely to hit significant side effects, and more likely to improve your health. 

Water fasting vs. alternate-day fasting (ADF)

With alternate-day fasting, you do a modified fast (i.e., eating 500/600 calories) every other day and eat as you like on the days between. 

Again, it’s a schedule you need to follow regularly to see the benefits, but the research suggests it’s safer and more effective than water fasting.

Water fasting vs. the Warrior Diet

The Warrior Diet is a 20-hour fast followed by a four-hour eating window. This time-restricted approach isn’t easy as the eating window is short, but fasting this way may feel more manageable. 

Not sure which of these intermittent fasting schedules to choose? We gotcha!  Take our Simple quiz, and we can help you figure out the best plan for your unique body, preferences, and lifestyle.

Water fasting — it’s not for everyone

As we’ve talked about, water fasting puts extra stress on your body, and the risks of a fast like this may outweigh any potential benefits. 

If you are someone who:

• has certain health conditions, like:
  • gout
  • diabetes (type 1 or 2)
  • chronic kidney disease
  • heartburn or gastroesophageal reflux disorder (GERD)
• has an eating disorder or a history of one
• is extremely active
• has a BMI of less than 18.5
• is prescribed medication (like hypertensives or any medications that need to be taken with food)

• is pregnant, breastfeeding, or trying to conceive
• is under 18
• is 80 years or older

keep yourself safe, and don’t try it. 

Instead, talk to your medical team (and those who support and care about you) about any health goals you have and work together to figure out the right approach for you. 

Written by Trey Reck

Medically reviewed by Josie Porter, RD

Published on | Updated on June 10, 2024