Well, the idea is probably not to gain weight in the first place. Once you gain weight it is very hard to lose it. Many people manage naturally to keep perfect

Well, the idea is probably not to gain weight in the first place. Once you gain weight it is very hard to lose it. Many people manage naturally to keep perfect BMI by eating healthy diet and having enough exercise or by just moving around. Exercise will have more health benefits than just keeping your body weight low.