I was underweight for a lot of my life, most of it really. Some of it seemed to be just how I was (and I was born a little small), and I also never liked eating very much until I was... I don't know how long, but I know in elementary school I did not like to eat (in retrospect, I think it was physically painful and I was also heavily constipated). And when I did not get enough to eat in early adulthood I was very underweight. For me. I think I was still supposedly weight-wise within the bottom end of the "healthy" range, but I could feel my bones everywhere including just trying to sit down. I seem to have heavy bones or something, so those calculations by number don't work on me. I know I was underweight, and according to others I "looked anorexic", but I didn't weigh as underweight just by calculation.
Then after I finally got, not even enough to eat, but a little bit more to eat, suddenly I shot from underweight to fat, and I've been fat ever since (which, believe me, I prefer to being underweight). My staff all knew that I had been near starvation before I got services, so they always made sure to tell me how much better I was looking because it was a sign I was eating, even if I still wasn't eating enough, they were great about not being anti-fat.
Interestingly, at this point, like some other posters a lot smaller than me have said, my weight does not fluctuate by how much I eat. Sometimes I forget to eat for a long time. And when I was on Prednisone I gorged myself for months. And I have stayed exactly at 195 pounds the entire time. (I'm 5'2".) I'm currently in one of my periods where I forget to eat and dislike eating. (I'm not anorexic, I just find eating painful due to a bad jaw and severe reflux that makes it go into my lungs, as well as the ever-present constipation, and I'm no longer on Prednisone to give me an appetite; the moment I got off Prednisone I started completely forgetting to eat again.)
I'll now bow out of this discussion unless I find something else to say about my previous underweight, because I'm no longer underweight.
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams