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Water Fasting for Weight Loss - Does This Diet Work?

Michael Garrico
Published by Michael Garrico
Last updated: October 10, 2022

As a personal fitness coach with over ten years of experience, I have encountered some of the craziest ideas when it comes to weight loss diets.

When I first heard of water fasting, I thought it was just a method of extending intermittent fasting times. But it seems like some people have taken it to extreme levels.

So, to figure out if this is a good way to lose weight quickly or a dangerous idea, I teamed up with a dietitian and nutritionist to research scientific literature.

Here’s what we found.

Quick Summary

  • Water fasting has been advertised as a crash diet where you only consume water and no macronutrients at all.
  • While water can help with hunger cravings during extended overnight fasting times, there’s evidence that it can cause issues after more than a day.
  • Scientific studies have also revealed that it’s more effective and safe for weight loss efforts to use water fasting only for short periods of time.

What Is Water Fasting?

A person drinking water

Water fasting is an extreme restriction diet that eliminates everything except for water [1].

While it does allow for the intake of mineral and vitamin supplements, the aim is to stop eating all macros and bring your calorie intake to zero.

If it sounds like you’re getting your body into starvation mode, that’s because it is.

Proponents of water fasting tend to claim that the theory behind it is that once your body feels like it’s starving, it will be forced to burn fat.

They also point to studies saying that there is evidence that extended water fasting under medical supervision can help with lowering blood pressure and reducing the risk of heart disease [2].

These typically involve periods of one to three days, and studies have also shown that water fasting can support overall health benefits and improvements in brain functions [3].

One study did find that a test group was able to safely get through a week of water fasting but highlighted that anything longer could prove dangerous and counterproductive for body weight [4].

It’s important to note that this test group was under constant medical supervision to monitor both high and low blood pressure.

Can You Lose Weight With Water Fasting?

Measuring her weight

Yes, you can lose weight with water fasting.

Studies have shown that extended fasting periods tend to help with insulin sensitivity providing a positive impact on blood sugar levels [5].

And more stable blood sugar has a significant impact on weight gain and associated health risks.

What I would point out is that intermittent fasting that lasts less than 24 hours and is part of a balanced diet has also been shown to positively impact blood glucose in this way [6].

“Intermittent fasting (IF) and time-restricted feeding (TRF) are two ways people reduce calorie consumption over time as an alternative to traditional dieting. “

- Brett Goerl, Writer at

The question then is whether you have to go to such extremes of only drinking water will result in faster weight loss.

Unfortunately, there aren’t any comparative studies that involve water fasting, intermittent fasting, and a control group to confirm this.

However, my personal experience with shorter fasting periods has been very positive.


Can You Exercise When Water Fasting?

Yes, you can exercise when water fasting. And many people believe that it’s important to exercise, even when intermittent fasting, in order to make sure that you don’t lose muscle mass [7].

The thinking behind this approach is that if you don’t exercise while fasting, then your body might trigger some muscle breakdown to produce energy.

Since you’re not working your muscles hard enough, that’s a reasonable way to look at it.

Now, I personally don’t recommend that you go through extended periods of water fasting that last longer than 24 hours.

But even with shorter periods, it all comes down to ideally timing your exercises for fat reduction.

Here’s what I do during my cutting phases.

The last time I eat solid foods is at 7 pm, and I don’t have a morning meal until noon the next day.

My body’s metabolism will experience continuous energy restriction, and my blood glucose and carbohydrate reserves will run low.

I then exercise at about 9 am for an hour, which forces my body to target body fat. I then also postpone my food intake for another hour as the increased metabolism from exercising can last for several hours [8].

Dangers And Side Effects

Compressing stomach with measuring tape

While there are known benefits of water fasting, it’s important to understand that it might not be healthy weight loss.

There are certain risk factors that such extreme eating habits can trigger.


While doctors probably diagnose high blood pressure more often, orthostatic hypotension is a real risk with water fasting [9]. This is a sudden drop in blood pressure when you stand up and is not one of the most common medical conditions.

Eating Disorders

There is also scientific evidence that prolonged fasting can lead to bulimia [10]. This is a dangerous path to go down that can lead to far more serious health issues with an extremely low body mass index.

Chronic Diseases

A person having a headache

Research has also shown that water-only fasting can increase uric acid production, which has been directly tied to increased risks of gout [11].

Whether you're a performance athlete or just someone who needs to lose weight, that’s not a health problem you want to deal with for life.


I know this sounds crazy. How could you possibly dehydrate when you drink only water?

The problem is that about 30% of your daily water intake comes from food [12]. And if you don’t carefully replace those fluids, then the risks of water fasting can include severe dehydration.


How Long Can You Do Water Fasting?

You can do water fasting for up to seven days. Medically supervised water fasts have been done in trials for that long, but researchers have concluded that anything longer is risky [13]. It’s also not a good idea for pregnant women.

How Long Does It Take To Lose Weight With Water Fasting?

It may take as little as 24 hours to lose weight with water fasting. However, this very much depends on your activity levels, age, hormone balances, and genetics [14].

Try Intermittent Water Fasting Instead

Personally, I’m not a fan of extreme and crash diets. I have managed to achieve great success with clients who stayed with a calorie deficit healthy diet.

It can bring about many of the same health benefits and weight loss without risking some of the known side effects.

And to speed up the process of burning fat, you can always add a stimulant-free fat-burner to your supplement stack.

These boost your metabolic rate and can get you to your ideal weight considerably faster. Just don’t expect this to happen while you eat a box of donuts on the couch.


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