Anorexia Nervosa is a mental disorder in which the person refuses to eat , or cuts back on so much food that they are 15% under the average body weight(aka bmi of 17.5 or lower) It's not that anoretics do not eat, it is the fact that they have rules as to what they can eat, when they can eat, how much of each substance they can eat. Anoretics are highly concerned about their weight even when they are below weight.
Iyou have naturally a high-metabolism and are naturally at a bmi of 17.5 then you are not Anorexic. As well, if you have amenorhea(sp?) meaning you have not gotten your period over a few months then that is another sign of Anorexia. If you do count calories, or obsess about your weight almost 24/7, and your fear of gaining weight interrupts your life then that is what Anorexia Nervosa is.
Signs that someone suffers from Anorexia Nervosa:
15% below average body weight for their height
refuses to eat
extreme fear of gaining weight
loss of menstration over 4 months
becomes distant-does not enjoy socializing
hope it helps...
P.S these are just from my knowledge through my counselors and such- if you want more information google it.