An eating disorder is a physiological and a psychological disorder.
Regarding an eating disorder as a purely psychological problem and treating it as such can prolong and perpetuate the disorder, condemning a patient to osteoporosis, a weakened heart, kidney failure, dental problems, poor vision, death.
Causation and affecting factors
(With thanks to ELT)
Judging the "seriousness" of a patient's eating disorder on physical appearance and weight is a nonsense. I know of anorexia patients who are mortally ill at a BMI of 19 - within the accepted "healthy" weight range as specified by the WHO. As each patient is unique (see above), so are their vital organs. A heart does not conform to the DSM IV ideas of what constitutes "anorexia nervosa". If a patient is underweight for their particular optimum function, the heart doesn't always hold on until the patient is "thin" enough to receive treatment.
The consequences of bingeing and purging are well known. Many BED and Bulimia patients are of normal weight. Judging the seriousness of their condition by just looking at them and weighing them could leave you with a dead patient.
If your patient presents at a normal weight, be thankful. This gives you a chance to intervene early. Early intervention produces better results in the majority of cases. Be brave. Just because the DSM says you have to be "thin" to have Anorexia Nervosa, use your gumption and start treatment before your patient is seriously malnourished.
BMI and the DSM
Prejudging what may have precipitated their descent into an eating disorder and labeling eating disorder patients or their caregivers does not help them recover. Trying to work out what "caused" their eating disorder and disregarding their physiological needs puts them at a greater risk of long term physical impairment and, in 15% of them, at risk of death.
Patients don't choose eating disorder: Parents don't cause eating disorders