"I wish we could derive the rest of the phenomena of Nature by the same kind of reasoning from mechanical principles. For I am induced by many reasons to suspect that they may all depend upon certain forces by which the particles of bodies, by some causes hitherto unknown, Philosophers have hitherto attempted the search of Nature in vain."
- Sir Isaac Newton, Philosophiae Naturalis Principia Mathematica
Anorexia, bulimia and ED-NOS share the common diagnostic criterion of over-evaluation of shape and weight. This is distinguished from the "normative discontent" of body shape dissatisfaction in the general population. Diagnostically and in treatment, "over-evaluation of shape and weight" and "body shape dissatisfaction" are seen as very different things. "Body dissatisfaction" is rampant in our culture. At what point does the "normative" set of thoughts transmogrify into the disordered thought pattern identifiable in the psychopathology of eating disordered patients?
The difference between the former and the latter is that the "normative" behavior doesn't unduly influence a person's sense of self or her evaluation of herself as a worthy/good/successful/etc. person. Part of this difference occurs because the preoccupation with body shape and weight is functioning as a stand-in for something else. Part of it is thanks to the higher level of obsessionality observed in ED patients (especially anorexics and underweight ED-NOS patients). (Those two "parts" together function as a sort of psychic ouroboros.)
Eating disorders, much like chemical addictions, are blithely self-sustaining. You discover you can quell [insert uncomfortable emotion here] by [insert disordered or addictive behavior here], and you experience (though you don't necessarily "discover") that [disordered or addictive behavior] actually leads to [uncomfortable emotion], so you use [disordered or addictive behavior] to quell [uncomfortable emotion], and then.... (Look. Another psychic ouroboros.) (And isn't it grand that an ouroboros is a serpent eating its own tail?)
This being the case, boy oh boy can eating disorders (and addictions, I'd imagine) feel really fecking futile. The eating disorder is a classic, if rather gross, example of inertia.