One becomes amnesiarexic only if one is not anorexic in the first place i.e. the act of eating needs to already be part of your daily routine and thus to forget to do so makes one "amnesiarexic".
The most common symptoms of an amnisiarexic person include:
Indulgence in "Brunch"
Chewing of gum, pens or pencil top erasers
Stomach roaring sounds