Why Crash Diets Always Fail: The Science Explained

We've all seen those magazine headlines promising to drop a dress or trouser size in a week. It seems too good to be true, and unfortunately, it is. Understanding why this is a con is actually very important to understanding weight loss in general, how our bodies work and what happens when we restrict what we eat.

Whilst they all might seem different at first glance, all crash diets work in exactly the same way. Typically they involve a sudden restriction in calories. Suddenly restricting calories means that the body panics. Our bodies have evolved over thousands of years to try to conserve energy and protect us from starvation. As far as the body is concerned, fat is very important. It is stored energy and it wants to try to hold on to this for as long as possible in case of prolonged famine.

When confronted with a sudden drop in calories, especially from carbohydrates and fats as happens in a crash diet, the body starts using up emergency stores of something called glycogen. This is an easily accessed form of glucose, which can give a boost in energy whilst the body hopes this lack of food passes. Glycogen is stored in muscle and in the liver.

But here's the key thing: glycogen is stored with a lot of water. Glycogen binds to three to four times its weight in water. So, 100 grams of glycogen will have about 400 grams of water with it, a total of about half a kilogram. When the glycogen stores are used up, this water is also released from the liver, enters the blood stream and then leaves the body as urine. So, temporarily starving yourself simply releases a load of water that you then wee out. This is why in a week you can see a reassuring drop in your weight when you stand on the scales.

But this kind of diet isn't sustainable as you're basically starving yourself, so as soon as you return to eating again your body makes up the stores of glycogen again and takes on the extra water that it needs to do this. We've been told we're losing 'weight' but the only weight we're really losing is water.

It's also worth noting the impact these kinds of diets have on how we feel. They might give us a brief boost when we see the weight apparently drop off, but it was always destined to fail. When the inevitable regain in weight occurs, it can make us feel a failure. It also falsely reinforces the idea that weight gain is down to a lack of self control and willpower.

Understanding why crash diets fail is actually liberating. It means you can stop blaming yourself for past 'failures' that were never going to succeed in the first place. GLP-1 medications offer a completely different approach: gradual, sustainable weight loss that works with your body rather than against it, giving you time to develop new habits and a healthier relationship with food.