Many people recovering from AN experience hypermetabolism, a common phenomenon that is often overlooked. I did some more research tonight into the biology and etiology of hypermetabolism. So let's start with a basic definition from Wikipedia:
Hypermetabolism is the physiological state of increased rate of metabolic activity. The impact of the hypermetabolic state on patient nutritional requirements is often understated or overlooked. Hypermetabolism typically occurs after significant insult to the body. In hospitals and institutions, the most common causes are infections, sepsis, burns, multiple trauma, fever, long-bone fractures, hyperthyroidism, prolonged steroid therapy, surgery and bone marrow transplants.
And, yes, eating disorders. Specifically anorexia. As someone falls deeper and deeper into anorexia, their metabolic rate slows dramatically. During the Minnesota Starvation Study, resting energy expenditure (also known as basal metabolic rate- the amount of calories needed just to keep your body functioning and doesn't include ANY physical activity) fell by about one-third. The book "Introduction to Clinical Nutrition" says that
Starvation involves metabolic alterations that enhance the chance of survival by increasing the use of body fat stores, by sparing the use of glucose, by minimizing nitrogen loss, and by decreasing energy expenditure.
But when the ill person begins eating again, their metabolism kicks into high gear. Body temperature rises. A person can experience night sweats, which may also be related to hormone function returning to normal. Why? It seems a remarkably inefficient use of resources. And indeed, hypermetabolism may not be entirely adaptive from an evolutionary standpoint as the body's use of food becomes remarkably inefficient. Even so, the body needs tremendous amounts of energy to replace lost fat and muscles stores, depleted organs, bone mass, hair, nails, you name it. No organ system is spared during an eating disorder.
The amount of calories needed for people with anorexia to return to a healthy weight can vary by illness severity (a lower BMI means more calories, as well as duration of illness), and by illness subtype. One study found that those with the binge-purge subtype of anorexia needed significantly fewer calories than those with the restricting subtype; another study found that calories were about the same. Other variants include your metabolic rate before illness onset. I've known several good friends curse their fast metabolisms quire vociferously during refeeing.
When looking at the phase after weight restoration, caloric needs between people with anorexia nervosa and those with bulimia nervosa turn out to be quite different. One study found that people with AN needed more calories per kilogram of body weight than normal controls, while those with BN needed fewer. It appears that people with a current diagnoses of BN but a history of AN require more calories than those with BN alone. Further, people with the restricting type of AN needed more calories than those with the binge-purge type, both of which were greater than patients with BN (some of the studies cited above show that, for weight maintenance, caloric needs are basically the same for any patient with a current diagnosis of AN).
Some of these differences may rest in premorbid differences in metabolic rate. It makes intuitive sense that a person who finds it easy to lose weight would have a faster innate metabolism. Other reasons calorie needs may remain unusually high for a person even after weight restoration is the sheer amount of rebuilding the body needs to perform. Bulimia is violently destructive to the body, and I would never say differently. However, some of the damage done by anorexia is slightly different, and the body must rebuild and repair essentially every organ in the body. My psychiatrist told me that the nerves continue to repair themselves for up to two years after weight restoration. This can hardly be the only organ system taking a long time to recover.
How long hypermetabolism lasts will probably vary from person to person, and depends on how long you were sick, how your body responded to the damage from your eating disorder, your activity levels, among others. There's no real way to be sure. If your caloric needs are unusually high--even for hypermetabolism--you can have body composition analysis and resting energy expenditure testing done. Personally, I think this is best left to extreme cases since so many sufferers have a tendency to fixate on numbers.
I hope this helps explain hypermetabolism just a little bit. I can't answer every question, as it's been a long time since my college biochemistry days, but I can always look things up.