Too,thin cannot gain wait?
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 also doing weight-training to increase muscle.
Proteins: egg white, chicken breast, salmon,... Better fats: olive oil, peanuts, almonds, tofu,...
EGG WHITE ALLERGY