Definition of "gain muscle and lose fat at the same time": It is based on a time span of half a year or more. If you gain muscle and lose fat at the end of this time period, then I think it is very possible, especially in the mode of increasing first and then decreasing or decreasing first and then increasing. It is also possible to increase muscle mass and decrease muscle mass in a certain window during the day, but this amount is almost negligible.

Extended data

Reducing fat is the behavior that the fat in animals exceeds the normal range. Because of their health, they lose excess fat in various ways.

Energy will neither be produced out of thin air nor disappear out of thin air. It can only be converted from one form to another or from one object to another. When some form of energy decreases, there must be other forms of energy increase, and the decrease and increase must be equal. This is the law of conservation of energy in physics.

Fat is the most important energy storage substance in human body. According to the law of conservation of energy, the so-called fat reduction is not to reduce energy, but with the reduction of fat, the energy of fat decomposition is bound to be converted into another energy, such as providing mechanical energy for exercise.

Baidu encyclopedia: reducing fat