Why do you need to eat to lose fat?

The yo-yo effect explained

Many people start doing sports or regular exercise because of their weight. A lot of them try to eat as little as possible (or nothing at all) when starting regular training, saying that first they want to lose excess fat. But they realise within a short time that the effect is just the opposite: after a drastic diet their scales show they are heavier. This is very discouraging!

The reason for that is our system's survival mechanism. If consumption is reduced, the system switches over to energy saving. Judging from internal signs, your system draws the conclusion that it will get less nutrients in the future, so it reduces fat burning  to save more energy stored in fat. The level of metabolism will also drop. Your system will even use the protein of the muscles as a source of energy. Meanwhile it will keep sending messages to the brain: "There is danger, we must eat! We must eat high calorie content food! We must eat!" The brain will win over the body; and the final result will be eating more. As your muscle mass decreases, your system cannot burn nutrients with the same efficiency, so it stores even more fat. The final result is 5 kgs of excess fat placed in neat little packages on your body.

In order to lose weight efficiently, meaning burning excess fat reserves, you need to keep a net body weight and not lose any muscle mass. The only way to do this is by proper nutrition and regular exercise.