How can you gain muscle when you're underweight?

Two ways!: You have the ability toe amounts of sugar and consume calories. They should be primarily protein. Do not worry about the fat, unless you consume large amounts of starch and sugar. You need to engage in weight lifting or machine based resistance exercise 4-5 x per week, 45m per session.

Dr Friedlander agreed

Resistance training: This sounds like a great idea. Eat when you're hungry, plenty of protein, avoid the very sugar, greasy or salty stuff. Hit the gym every day, lift as much as you can, increase weight when you can do three sets of ten reps, work different groups on different days. This actually works; you just need to make it a habit. Be very polite and work hard and other gym folks will befriend you.

Dr Altman agreed