The Crash Diet – How it works – Part 1

There are many reasons why people go on a diet. Maybe for the upcoming holiday, general beach body season, a wedding or because they know that they could shed a few kilos. There is absolutely nothing wrong with that. It is rather the contrary case, losing excess fat will be beneficial for your health (1, 2).  The means to get there, on the other hand, aren’t always healthy nor are they sustainable. That what we will discuss in this article series: the crash diet.

Crash Diet - weight loss

Throughout this four-part series, we will go on a crash diet together, and loose easily 5kg. I’ll show you how it works, why it works and why it’s not a great idea to use a crash diet in the first place. Finally, how to lose weight the in a sustainable way and how to create your own weight loss protocol.

What you’ll learn at a glance throughout the article series:

What’s a crash diet?

Well, a crash diet is a dietary protocol, often combined with cardiovascular exercise, with the only goal to get the number on your scale lower in a (very) short amount of time (3).

In order to achieve this, any crash diet resorts to extreme modalities. This would be mainly restricting the calorie intake dramatically. They aim to create a huge negative energy balance. That is why they often add plenty of cardio time to the equation.

Crash Diet - plate

Cardio is a great tool to quickly burn a lot of energy. Combining both concepts will tip the energy balance even further. This sets you up for long-term failure. That being said, it gives you outstanding short-term results.

Why is the crash diet working in the first place?

The answer to that is simple but multi-faceted. To start with, being in a caloric deficit for a certain amount of time, will lead to fat loss eventually. So far so good. That’s what a crash diet does, often limiting caloric intake at 500 calories a day for periods of up to one week (4), sometimes longer.

Chances are, if you are tempted to crash diet, you might be on the Standard American Diet (SAD) or a similar Western type of diet, with a high daily carbohydrate intake.

That is where those short-term wins partially come from. It is very unlikely that you will find a fad crash diet that won’t cut or at least dramatically reduces carbohydrate intake. It is a very easy way to lose weight on the scale.

Full Glycogen Stores Make You “Heavy”

In order to understand the carb-cutting, we have to dive a little into human physiology. Unlike fatty acids and protein, carbohydrates need water for their metabolism (to put it simply). One “side effect” of this is, that glycogen always binds to water when it is stored in our muscle cells and the liver.

Crash Diet - carbs

One gram of glycogen can bind up to 3g of water. Depending on how much skeletal muscle tissue you have, your body can easily store up to 500g of glycogen (in an average 70kg human being).

Poof, 2kg gone, and your scale is happy but keep in mind: you haven’t lost any fat so far.

This initial weight loss is neat and might be extremely motivating. The second you re-introduce carbohydrates back into your lifestyle, those kilos will be back.


Sodium can cause water retention. Depending on how sensitive you are, your weight might be up somewhere between 1kg to up to 3kg of water.

Crash Diet - sodium

Let’s say on your crash diet this will cause you to drop 1,5 kg of water (5). Now we lost already 3,5 kg without even trying and it’s still not sustainable weight loss. But there’s more.

The Food You Eat

As I said initially, the crash diet might put you on a daily 500 calories intake. This is not only very little energy, it’s also literally only a very little amount of food.

In other words, your scale might show you a number even closer to where you want to be, simply because there is less food going through your digestive system to begin with. This number can vary significantly from person to person.

Did you find a diet that looks great but you aren’t sure if it’s a crash diet? Or maybe you want to create your own weight loss diet and don’t want to fall into the crash diet trap. Get my free “Crash Diet Worksheet” with 8 red flags to watch out for!

Let’s be moderate and say this will amount to half a kilo in the course of a couple of days. At this point in time, we might be 5-6 days into the crash diet, we have lost 4 unsustainable kg of body weight. Or in other words, you changed the scale weight but didn’t lose actual body fat.

Bowel movement

You read that right, bowel movements. Depending on how well you can handle your crash diet, you aren’t constipated (yet). This means every bowel movement will make you lighter, especially at the beginning of the diet.

Crash Diet - bowel movement

Usually, the food you eat daily and your bowel movements keep you more or less in balance. At least balanced enough to not see a major fluctuation on the scale. But now this isn’t the case any longer, as there is much less bulk in your digestive tract.

Research isn’t very clear on the average amount of a daily bowel movement. Studies (6) range it from 250g up to 2kg. I would say this depends highly on your lifestyle, body size and diet.

I add another half kilo to our weight loss. Now we already lost 4,5 kg – remember, all of this weight will come back once you go back to your normal diet.


To ramp up the weight loss even further crash diets suggest doing cardio. Often for several hours a week. The general thought is, burning calories will increase the deficit further and you’ll lose even more weight.

If you aren’t careful, you definitely will lose a lot of weight, in form of water. Cardiovascular exercise is very demanding for the body and the increase in body temperature will make you sweat. Depending on the circumstances, you can quickly lose much more than a litre in a session.

If you don’t rehydrate properly, you managed to lose more weight on the scale. But rehydrating after exercise is crucial for recovery and for your overall physical health.

Crash Diet - cardio

Minute by minute, cardiovascular training is a great way to burn calories, but it is the least effective and least sustainable way too. I wrote an extensive article about why doing cardio for weight loss sucks and what you should do instead of a while ago.

That being said, many people tend to overestimate how much water they drink compared to their needs. Let’s say you are doing more or less well with rehydrating, but because you’re chasing a number on the scale, you might hold back a little. That’s potentially another half kilo of weight. Now we lost 5 kg already – on the scale.

5 kg of unsustainable weight loss on our crash diet

Now we are done with our 7-day crash diet and lost 5 kg of weight. Unfortunately, this weight loss is very unsustainable. This means several things at once:

  1. these 5 kg will come back  the moment you go back on a maintenance diet
  2. you lost weight but not much fat during those 7 days (your body simply isn’t able to get rid of that much fat that quickly)
  3. you might have lost additional weight, in form of lean body mass. This might include real muscle mass

We are done with our crash diet and go back to our former lifestyle diet. While we bounced back to our initial weight after a few days of maintenance, we haven’t tackled the yo-yo effect yet, nor any potential harm we have done to ourselves.

These aspects of the crash diet will be discussed in part 2 of this series. Make sure to check back soon or get on my mailing list and don’t miss any new content.

Key takeaways:

  • crash diets aim for a high caloric deficit
  • use cardiovascular training to grow the caloric deficit
  • only work because of basic human physiology
  • focus on scale weight and not on changing body composition
  • potentially dangerous

If you found this article helpful or have friends who need to read this, share it with them on social media. Your help in getting the word out there is very important.

Here’s the next part in the Crash Diet series “Your Metabolism – Part 2

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