For someone struggling with an eating disorder food is not the problem. [Hold on, what?] I'm serious. Hang with me here. Food is the symptom of the problem.
Consider this analogy – let's say you have a fever. You take acetaminophen and the fever is reduced for the next few hours then returns. By treating the symptom (the fever) you didn't treat the underlying problem (the infection) that caused the fever, thus it returned.
With an eating disorder the thoughts and behaviors related to food are the symptoms of the illness. We can try to manage the symptoms, but this will only be temporarily effective if we don't also address the underlying issue(s).
What are the underlying issues? That's a complicated question. It's not as simple as identifying the strain of bacteria causing an infection and prescribing the appropriate antibiotics. The causes of eating disorders remain poorly understood. We believe that it is an illness caused by multiple factors, including genetics, psychology and social/environmental influences. The contributing factors are different for each individual, as is the course of treatment.
The complex nature of this biopsychosocial illness is the reason that treatment requires a multidisciplinary treatment team including a therapist, dietitian, medical doctor, and possibly a psychiatrist, family or group therapist, or other individualized adjunct interventions.
If only it were as simple as "fixing" the food issues…