Yes, BMI may be part of the issue and I think that below a certain level it definitely is. In your case I think its part of the equation since the body needs a certain amount of mass to generate the energy to function properly. In my case though, I am 5 10 and (yippie) have gained about 7 pounds to bring my weight to 150 in the last
six months. That puts me smack in the middle of "normal" range.