Classical conditioning, or Pavlovian Conditioning, may play a significant role in the loss of appetite experienced by many anorexic patients. Classical conditioning is an important learning process in most species, and plays a vital role in survival.
Generally, classical conditioning involves the pairing of an unconditioned stimulus (example: meat) with a conditioned stimulus (example: a bell ringing). The normal, or unconditioned response of a dog to seeing meat is to salivate. If the bell is rung every time the meat is presented to the dog, the dog will eventually develop a conditioned response to the conditioned stimulus. In other words, the dog will start to salivate upon hearing the bell ring. The speed at which the dog learns these responses may depend on the timing of the pairing, the genetic traits of the dog, cognitive ability, and it's personality. Once acquired, this conditioned response may also diminish or extinguish, if the bell is repeatedly rung without any meat being presented.
The theory of classical conditioning can help us understand many human behaviors. For example, the amount a person likes another person may be associated with how much the other person has come to represent positive stimuli or gratification. People may develop certain fears due to negative stimuli that occurred at the same time as another event. These fears can develop very quickly, especially if they involve pain or trauma. How quickly these fears develop depends on multiple factors, including the magnitude of the pain or trauma, the timing of pairing, and the character traits of the individual.
Food selection can also be influenced by classical conditioning. Foods that cause gastrointestinal distress may elicit avoidance of that food. Certainly, most of us have specific foods that we just will not eat, usually due to some negative experience we've had with that food, or due to associating it with something repulsive (many people have difficulty eating exotic delicacies such as escargo, snake meat, frogs' legs, etc.)
Some therapists may feel that it is possible to develop a fear of becoming obese, to the point that it results in the cessation of eating. I doubt very much if this is possible, as in a healthy individual the consequence (i.e. obesity) is so far removed in time from the action (overeating) that it could not possibly induce such a strong fear response. Certainly, the threat of eternal damnation has not elicited a strong enough fear response in any of us to result in the cessation of sinning!
If food intake is paired with pain or intestinal distress, in some sensitive individuals, this may result in avoidance of that food type. As with the dog, the speed at which this response is acquired will depend on the magnitude of the trauma, as well as the genetic traits, cognitive ability, and character traits of the individual. The fact that many or most anorexic patients seem to be sensitive, intelligent, high achieving individuals could explain why this type of person develops avoidance behavior so quickly. This avoidance behavior could only be acquired due to pain or gastrointestinal distress due to illness, being paired with food intake. It is unlikely that an obsessive-compulsive personality trait would produce this sort of behavior. It is also unlikely that so many people would suddenly develop obsessive compulsive tendencies. Besides, the sensation of hunger is so closely tied to survival that the urge to eat would normally overpower any sort of belief or superstition.
This would not be the case, however, in a person with a gastrointestinal disorder, who is experiencing pain or trauma paired with eating. Pain is also prioritized by the human brain, for it's avoidance is closely tied to survival. The anorexic personality type (shy, sensitive, intelligent) may be more prone to develop avoidance behavior due to anxieties induced by pairing of trauma with food consumption. The bulimic individual, on the other hand, may not develop this avoidance response, and instead may continue normal feeding behavior, but due to a gastrointestinal disorder will experience nausea and vomiting. Again, the magnitude of the trauma, the timing of the pairing, as well as the genetic traits, cognitive ability, and character traits of the individual would determine the behaviors. Many intestinal disorders will also result in diminished appetite, although this may not be associated with any particular food group.
Imagine that you get a bad reaction to not just one food type, but many. You're not sure which ones, but eventually you will learn to avoid the ones that seem to cause the most problems. This may happen consciously, or subconsciously. If the trauma you experience is intense, and paired quite closely chronologically with the particular food you're consuming, you will likely learn to avoid that food very quickly. If the trauma occurs quite a while after, you may or may not develop this avoidance behavior. There are so many variables, that it is difficult to predict if you will develop avoidance behavior or not. How much pleasure you experience while consuming it will certainly be a factor. For the bulimic individual, hypoglycemia may also play a role in the urge to consume certain food types despite the resulting trauma.
It is the goal of the therapist for the patient to resume normal eating behavior. If the patient has a mild gastrointestinal disorder, cognitive behavioral therapy may be enough to effect weight gain. Relapse would be quite likely, however, if an underlying medical illness exists and is not addressed. It may take the full concentration of such a patient and a great deal of willpower and resolve to continue eating normally. Under times of stress, it may be difficult to maintain this resolve, and relapse would then occur.
If the patient has a serious digestive disorder, however, the trauma will still be paired with food intake, and it is unlikley that extinguishment of the maladaptive response can happen. When the digestive disorder is corrected, then the trauma should no longer be paired with the intake of food. With time, and enough positive eating experiences, the fears and anxieties related to eating should abate.
I am surprised that none of the best-selling books on eating disorders make any mention of classical conditioning, and it's role in eating behavior. Certainly, Pavlov was one of the founding fathers of modern psychiatry. Of course, none of these best-selling books even acknowledge the possibility of physical illness causing eating difficulties, so perhaps this omission is understandable.