How to add body weight?

Build muscle: Best way to add body weight is to build muscle by strength and resistance training. Also, add, but not too much, protein to diet to help build muscle. Best sources of protein are plant proteins in nuts and beans. I am not a fan of whey protein contrary to the view of many because of reported issues with it.

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.

Strength training: Strength training using the hypertrophy formula or 80% maximum strength capacity for select multi joint exercises is the natural way to increase lean body mass by increasing the size of muscles and bone density. Working with a trainer to strength test and assign and progress loads is most safe. Working at 80% 1RMMax increases testosterone and growth hormone- anabolic hormones that make you hungry!