How to gain weight naturally?

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. Add protein: egg white, chicken breast, salmon,... Better fats: olive oil, peanuts, almonds, tofu,...

Eat, eat, eat.: You can gain calories by just eating more food. People tend to gain weight with meat because it's more calorie-dense than other foods, but you'd gain more weight by just eating more of less calorie-dense foods. Just keep it healthy - there's a difference between gaining muscle weight and just getting fat. And exercise as well, so you gain lean mass and not just get fat.