There is a fat bar in the character attribute. Eating will increase it (eating too much will make you vomit, which is counterproductive), while not eating and exercising (going to the gym is the most effective) will reduce it.

If it is full, you will become fat. You lose weight when you are empty. If it is not replenished in time, it will consume the muscle attribute column, that is, muscle.

So just eat and exercise properly according to your own needs.