By gaining weight slowly and steadily, you add less body fat and more

When you gain weight steadily and slowly, you add less body fat and more muscles. It is because if fat lessens, the one who will replace or takes place of the fat that has been lessen will be the muscles. That is why people who undergo to trainings in gym or having to conduct exercise will likely burn fat and these fats will be replaced by muscles.

It's cell theory. easy name i would think.