You are on a diet and the scales drop.

Well done! Now you may be wondering if you are losing fat, muscle or water.

How to lose only fat mass?
And is there a diet to lose fat only?

You’ve heard that many bad or bad diets result in a loss of water and lean body mass, and that’s partly true.

Losing fat with diet alone is impossible. There are no diets to lose only fat mass, and if you happen to hear such a thing, run away.

There is no food program, of any kind, no protocol, not even fasting, detox, or anything else that allows for such a thing. No special wording. Anything.

On the other hand , losing enough fat mass together with the inevitable loss of lean mass is desirable by combining physical activity and slightly reducing the daily calories (for example by 15%).
Liquids, on the other hand, recover.
So if you go on a diet and gain two kilos for a very small chunk, then that weight loss was mostly water.

But why, when you lose weight, do you lose water? Yup.

Why do you lose water when you lose weight?

This happens mainly because, with a low-calorie diet of any kind, we obviously introduce fewer calories than we consume. This leads the body to use glycogen reserves, primarily from the liver, as an energy source.

Glycogen is a type of sugar that the body stores in both the liver and other organs, such as muscle tissue. The liver stores this type of sugar in 24 hours to allow us, on the one hand, to have a stable blood sugar even when we are not eating, and on the other as a source of energy. When liver glycogen levels drop, we may feel hungry or fatigue, for example. When we eat, we restore our share of glycogen.

However, if we eat less, the body uses more glycogen for energy purposes.
However, glycogen is not simply sugar.
It is a kind of molecular compound, with a protein unit in the center surrounded by sugar molecules. To create glycogen, a portion of water is used. So the more glycogen we lose, the more water we lose.

This effect is greatest in low-carbohydrate diets.
This is why weight loss is greater when we eliminate carbohydrates. We are not just losing fat like on a normal low calorie diet. By the time we are not feeding carbohydrates to the body, the liver glycogen stores are quickly depleted. This leads to an initial loss of water.

For every gram of carbohydrates stored in the body as glycogen, the body retains 2 to 3 grams of water.
This means that if I ate 150 grams of net carbohydrates per day before the low carbohydrate diet (which correspond to two portions of fruit, one of bread and one of pasta or rice), I will lose 450 grams of water the next day. Clearly, if on a low calorie diet I have carbohydrates, the water loss will be much less, and therefore the weight lost will be a “real” weight, not masked by the water lost.

But even this weight will always be partly of lean mass and not just body fat.
In addition, every time we gain weight after a diet, apart from water, we mostly gain fat mass, as evidenced by a series of studies, like this one. 

HOW TO KNOW IF YOU ARE LOSING MORE FAT MASS?

Here I will try to give you a practical answer, which you can do in 4 ways. I have already told you that losing fat with diet alone is not possible. Okay, so some of the lean mass goes away all the time. Exactly. But we will also lose fat mass. That’s right too.
How much will we lose?
And how to understand if the diet is making us lose fat and not (only) lean mass?
What can we then do to minimize the loss of lean mass so that we can lose as much fat as possible?