Goji Vita
The common advice of gaining muscular weight has often been: eat lots and exercise less. Yes, that will put on (unhealthy) weight, but will not do much for your strength, stamina and overall health. Plus, the weird logic of, "Well, you just put on fat and then turn it into muscle," is also so much nonsense.
For an under weight person gaining muscular weight is quite a different process. Take my own story. I was a typical 98 pound weakling as a teenager. One of the running jokes was a bet to how many chin ups that I could do: namely none. I ate like a horse, exercised like a nut case and played sports. Later, I lifted weights and ran almost every day. I was spending my hard-earned money on every protein
powder or weight gain pill on the market. While I did experience a gradual gain up to about 116 pounds (53 kg.) in high school, I just could not seem to gain much more. Theses minor only happened after I started sleeping for more than 6 hours a night. Hanging out with friends until 10 p.m., getting up at 4:30 a.m. and delivering paper, on a bike several miles, 6 days a week, on an empty stomach did nothing for muscle gain or school studies for that matter. When I quit the route and got the extra sleep, THEN I was gaining muscle weight.