Age 18 weight 48.i eat a normal diet and had a complete body checkup and there is nothing wrong or heriditary. how do i gain weight ?

Blood & stool tests?: I am happy to hear your physical exam was normal. Has this always been your weight? how tall are you? do you get a lot of physical exercise? if you want to gain weight a combo of increased protein and some fats is helpful. Ice cream is a good choice. make sure your blood and stool tests are normal first, and especially if you have lost weight