Why cant i gain weight?

See Dr., then eat...: If one is truly underweight (based on height, family background, and body type), a Dr. can evaluate for a hormonal, digestive, or other disorder. If no medical cause is found, then one can add weight by eating more calories of nutritious foods, while doing light weight-training to improve muscles. Proteins: egg white, chicken breast, salmon,... Better fats: olive oil, peanuts, almonds, tofu,....