How Much Weight Can You Lose From Water Fasting?

Water fasting has long been a method used to induce rapid weight loss and plenty of studies have shown it to be quite effective. But exactly how much weight can you lose from doing it?

Research from combat athletes, for instance, regularly show up to 20 pounds of weight loss in the days and weeks leading up to a fight where they rapidly cut down on food to “make weight”. Those not following a rigorous training regime as combat athletes have been shown to lose an average of 14 pounds in the first week of a water fast.

However, all of this has to be taken with a grain of salt since the amount of weight you can lose during a water fast depends on so many variables.

Try our Water Fasting Weight Loss Calculator as a guide for estimating the amount of weight you could lose by water fasting.

What Determines Weight Loss During a Water Fast?

As a general rule of thumb, the more you weigh the more water you’ll hold (more room for it) and the less you weigh the less you’ll hold (less room for it).

This means that people on a water fast who are “bigger” to start with will lose more weight when compared to those who are smaller.

Depending on which end of the spectrum you sit at the beginning of your water fast will ultimately have a major impact on your total weight loss.

This is not the only caveat though.

Understanding the other variables involved is key for determining total weight loss during a water fast.

As with most things regarding weight loss, the true answer has more shades of grey than black and white so how you approach water fasting shouldn’t be taken lightly. Let’s dive in and explain some of the nuances so you get the full picture.

1. Body fat percentages

Weight loss and fat loss are not the same thing.

When you pop on the scales, you’re weighing everything in your body (including muscle, food, water, body fat, your heart, liver, spleen, kidneys, bones, etc.) weighs a given amount. However only some percentage of your total body weight is body fat.

If we look at different athletes for example, lean athletes might only have 8-10% body fat. This means that only 8-10% of their total weight is fat. So, a 200-pound athlete with 10% body fat is carrying 20 pounds (200 * 0.10 = 20) of body fat.

The remaining 180 pounds of weight is muscle, bones, water, etc. We call this remainder lean body mass.

Big guy drinking water

In contrast, if a person who has the same “weight” is 50% body fat, only 100 pounds is lean body mass. As leaner muscle tissue holds more water than non-muscle tissue, the leaner individual would lose more weight if both of them started a water fasting.

The more muscle you have, the more storage site you have for water. From a health standpoint the leaner individual is also much healthier as they are carrying less body fat.

2. High carb diets

More carbs = more water retention and visa versa.

For every 1g or carbohydrates in the body you’ll typically store 3g of water. Think about when you cook pasta in water it absorbs the water into it. A similar process happens in the body.

As such, when you water fast, you’re cutting out carbohydrates. The knock-on effect of this is that water no longer has anything to stick to which in turn naturally limits your body’s water retention abilities.

The amount of carbohydrates your body stores typically comes down how much muscle mass you have, your DNA, and previous training.

We can make some rough estimates for adults which is around 400g based on previous research. So in a water fast we would be looking to drop 1.2 litres of water (400 * 3g) which would have otherwise been retained naturally by the body.

This also means that if someone has a typically higher carb diet and is of similar stature to another person who follows a lower carb diet, then the person on the higher carb diet will lose more “weight” albeit mostly water when they start water fasting.

Fruit and veg at the market

Other foods also have high water content so if these are part of your diet before the fast, you’ll likely get an extra pound or two drop from cutting these out too. Typically, we are talking about fruits and veggies such as:

  • green peppers
  • berries
  • cauliflower
  • watermelon
  • spinach
  • cucumbers
  • radishes
  • celery

3. Amount of water consumption

The more you drink the more you lose.

It sounds strange that the amount of water you drink during an actual water fast also has a direct impact on how much you lose.

For many years, a practice known as “water loading” has been used to manipulate weight loss. It’s commonly used by those in “weight cutting sports” like boxing, MMA, and wrestling.

In its simplest form, the more water you drink the more you pee and thus the more weight you will lose while water fasting. Scientifically speaking this happens by “tricking” the hormones in charge of fluid balance in the body. These hormones are:

  • Aldosterone
  • Anti-diuretic hormones (ADH)

When you drink excess water well beyond what you need to stay hydrated, the sodium content in the body decreases, and in turn, aldosterone is secreted to combat the low sodium and regulates sodium resorption.

ADH then directly supports water resorption. Because of these interactions, consuming excessive amounts of water can induce a higher amount of water loss and subsequently extra “weight loss”.

4. Water fast duration

The longer you fast the more weight you will lose.

This goes without saying, but it’s always as simple as it seems.

While rapid weight loss can be a big motivator, you can only go so long without food – which is somewhere in the region of two months. Anything after that would just be increasing your chances of seeing your maker, and that’s not a risk worth taking just to lose weight.

Say, hypothetically, that you actually make it to the two month mark of a water fast, the weight you’d be losing at that point wouldn’t be just be fat, it would also consist of muscle tissue and it’s a downward spiral from there.

After your glucose stores are used up, your body will begin to use protein stores to provide energy. This process will affect your muscles and can carry your body along for about three days until eventually you’re in starvation mode.

Even though your body could probably make it that long with only water, it’s highly advisable that you should not go beyond 4-5 days. Please see your doctor for advice first if you’re looking to water fast longer than this.

For an idea on how much weight you could potentially lose during a water fast, please try our Water Fasting Weight Loss Calculator.

Key takeaways

Water fasting is just one of the many methods that people use to induce rapid weight loss. However, as shown in points 1-4 above, your “weight” is made up of different components and because of internal and external factors, how much water/weight you lose can easily be manipulated.

To be on the safe side it’s best to seek a method that induces fat loss not just weight loss. However, as mentioned above, there are times and situations where a focus on weight loss predominantly from water loss is needed. Know the boundaries though.

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Mike Julom

Mike is an ACE Certified PT and a CrossFit Level 1 Trainer. He is an avid lover of all sports. Basketball, tennis, athletics, volleyball, soccer, squash, golf, table tennis, even darts, you name it! He's a very active CrossFit athlete and has been WOD'ing for over 7 years. With such an intense fitness regime, Mike has learned to take care of his body physically, nutritionally, and spiritually. Mike founded ThisIsWhyImFit as a way to share his vast knowledge of exercises, diets, and general fitness advice.

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