Weight gain is not easy as to reduce weight.
To get strong and more muscles you have to eat large amount of calories and vitamins that help to increase metabolism and increase weight.
You eat healthy foods like fish, chicken, meat, egg etc.
These are the best foods that boost weight...
If you are having trouble gaining weight and are eating enough calories, there are possibilities that you could have metabolic or gastrointestinal issues that could be preventing you from absorbing adequate calories or making you burn them up too quickly. I'd suggest seeing your doctor to go over these concerns and get checked out!