Question

People who begin to lose weight often have a rapid weight loss in the first few days. Common knowledge says that this is "just" because of a loss of water from the body. Why might this be true?

Short answer

Rapid initial weight loss is due to water loss from depleted glycogen stores.

Step-by-step solution

- Understanding Initial Weight Loss

Recognize that the initial rapid weight loss when starting a diet or exercise program is often attributed to the depletion of glycogen stores. Glycogen is a form of carbohydrate stored in muscles and the liver.

- Glycogen and Water Binding

Understand that glycogen binds water. Roughly 1 gram of glycogen can bind about 3 to 4 grams of water. So, when the body uses up glycogen stores for energy, it also releases the water bound to it.

- Decrease in Glycogen and Water Weight

When the body depletes its glycogen stores during the initial phase of dieting or increased physical activity, a significant amount of water weight is lost. This results in the quick drop in weight observed.

- Conclusion

Therefore, the initial, rapid weight loss often seen in the first days of weight loss efforts is primarily due to the loss of water weight as glycogen stores are used up.

Key concepts

glycogen depletion

When embarking on a new diet or exercise regimen, one of the first steps the body takes is to tap into its glycogen reserves. Glycogen is a form of carbohydrate stored in the liver and muscles, and it serves as a quick energy source. During the initial phase of weight loss, your body will start to deplete these glycogen stores.

Glycogen is crucial because it provides readily available energy, especially when you're reducing calorie intake or increasing physical activity. The body uses up glycogen to maintain energy levels, leading to a rapid drop in these stores.

Why does this matter for weight loss? Well, using up glycogen directly affects another important factor - water binding.

water binding

Glycogen doesn't get stored alone; it binds to water molecules. Specifically, for every gram of glycogen, your body stores around 3 to 4 grams of water. This means that the depletion of glycogen has a cascading effect on water storage in the body.

When you start losing glycogen, your body also releases the water bound to it. This process leads to a significant amount of water weight loss.

Here's an illustration: If you lose 500 grams of glycogen, you are essentially shedding between 1.5 to 2 kilograms of water at the same time.

This mechanism explains why initial weight loss can seem so rapid and significant, but remember, a lot of it is water, not just burned fat.

rapid weight loss

The phenomenon of rapid weight loss during the first few days of a new weight loss program can be quite encouraging but also potentially misleading. The substantial drop you see on the scale is often due to the combined effect of glycogen depletion and water loss.

Since glycogen stores are quickly accessible, the body uses them up rapidly, especially during calorie deficit and increased physical activity. This process causes the attached water to be released, leading to a noticeable but temporary reduction in weight.

While rapid weight loss may boost your motivation, it's essential to understand its primary cause—water loss due to glycogen depletion. Such weight loss is not entirely indicative of fat loss, which occurs more slowly over time.

Expect initial weight loss to slow down after the first week or so, as the body will start focusing more on burning fat stores, leading to more sustainable weight reduction.