anorexic people ARE abnormally sensitive about the preceived appearnce. That's a 'given' for any sufferer.
One will normally be able to see someone else as anorexic after their weight has degraded until they are just too thin. Even if someone starts off grossly obese the condition which makes them not wish to eat will, eventually result in the skeletal appearance that is so much a part of anorexia.
When a person starts with this terrible affliction there well may be a need to lose weight, but the anorexic doesn't, unlike the rest of us, see the genuine weight loss and continues avoiding food until, sometimes, they die.
Apart from the drastic weight loss the sufferer also suffers from a lack of nutrition (because they don't eat) and their general health level decreases accordinly.
It is, pricipally a mental state, not a physical disease, but the effects are terrible for the sufferer and their families.
My cousin, Bonnie, died a few years ago, weighing just 47lb, she was 5' 9" tall and six months older than I. She wasn't a teenager, but a grown woman of 56 years. She was anorexic.