Symptoms versus causes
It is important to distinguish between the symptom and the cause of an eating disorder. Practically, too little or too much food will result in a change in weight. This input-output formula is HOW a person gets thin or fat. The anorexic and the bulimic either exercises excessively eat too much or too little food or purge their food. These are usually the practical aspects, or symptoms of an eating disorder. The symptom is the behavioural, observable manifestation such as the weight problem, the purging, the fasting, the excessive exercise, the laxative abuse, etc. The symptom is the level where most intervention starts. However, these factors are merely manifestations of a deeper problem and I believe that one cannot change the symptom without dealing with the underlying psychological and/or historical causes.
Psychological "causes", refer to an individual's personal dynamics, which contribute to how s/he uses food. Please click on the eating disorder in which you are interested. You will link up with the main page for that eating disorder and will find the link to its causes. (Anorexia nervosa, bulimia nervosa, obesity, overweight). Historical causes refer to practical events in a person's life, such as the loss of a parent, the birth of a sibling, changes in schools, sexual abuse, etc.