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Anorexia nervosa, bulimia - medical causes of eating disorders - symptoms, treatment, diagnosis
Starvation Response

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The human response to starvation is a fascinating phenomenon. Being aware of these adaptive responses to malnutrition is key to understanding the behaviors associated with anorexia nervosa.
 
Starvation induces many physiological and behavioral changes in the human body. As body mass drops, metabolism slows, to conserve energy and resources. Severely malnourished individuals may develop amenorrhea (in females), bradycardia, lanugo hair, hypothermia, reduced bowel motility, hypothyroidism, etc.
 
Many of the symptoms of malnutrition may be confused with the symptoms of medical illness, since there can be overlapping symptoms. In most eating disorder programs, bloating, constipation, early satiety, and abdominal distension are assumed to be due to effects of starvation. Starvation can also affect blood chemistry, which may complicate interpretation of test results. If the attending physician does not consider the possibility of underlying illness, these cues may be missed, assumed to be due to malnutrition.
 
In like fashion, the behavioral changes induced by malnutrition may be misinterpreted if the therapist does not acknowledge the effects of malnutrition on human behavior. The normal, adaptive responses to malnutrition are often interpreted as psychiatric disorders. 
 
Bingeing may be viewed by the therapist as maladaptive behavior, when in fact it may be a normal adaptive response to caloric restriction. Certainly, this behavior is seen in opportunistic social carnivores such as dogs, which "wolf" down their food in large quantities, oftentimes vomiting it up soon after. Caloric restriction, whether imposed by famine or self-imposed due to dieting, will tend to increase the liklihood of this behavior. In terms of evolutionary biology, this behavior would be a healthy adaptive response to a dwindling food supply, as it would not only maximize caloric uptake but also, once swallowed, exclude access for competitors. When resources are scarce, natural selection favors individuals who seize opportunities for resource acquisition, even if they do not have rightful possession. Thus, the urge to binge while experienceing caloric restriction is to be expected.
 
The phenomenon of impulsivity, irritability, and hyperactivity may be interpreted by the therapist as a manifestation of obsessive-compulsive disorder or depression, when in fact these behaviors are likely normal adaptive responses to starvation honed by millions of years of natural selection. Normal individuals facing severe dietary restriction develop an obsessive preoccupation with obtaining and consuming food, and this shapes impulsive behavior such as hoarding, bingeing, or stealing food. This healthy adaptive response would give a greater certainty of securing nutrition, and thus survival. Of course, for the individual who cannot eat due to medical illness, all these behaviors may be present, yet the consumption of sufficient food remains an impossibility. 
 
The phenomenon of excessive exercise, agitation, and restlessness observed in many anorexic patients may also be a normal adaptive response to caloric restriction. Most mammals become restless and agitated when experiencing hunger, and most increase foraging activity when food supplies dwindle. In the research lab, this restlessness is called "semi-starvation induced hyperactivity." Researchers working with rats on calorie-restricted diets noticed that those eating less daily ran between 2.8 and 5.3 miles more than the other rats. Some of the rats on restricted diets ran themselves to death. This response to starvation would increase the liklihood of survival in famine conditions where competition for resources would be intense. In the lab rat's situation, if no food is added to the cage, the increased activity does not result in acquisition of nutrition. In like fashion, for the anorexic patient with a digestive disease, the hyperactivity does not lead to food intake, since the underlying disorder has either suppressed the appetite or produces gastrointestinal distress paired with food intake (see page on classical conditioning).

Perhaps one of the most fascinating accounts of the physiological and psychological effects of self-imposed caloric restriction is one written by Emily Carlson, a science writer from the University of Wisconsin-Madison. In an article in Johns Hopkins magazine (click here for the article), she describes her experiences restricting her caloric intake for a period of several weeks. She describes the phenomenon of the semi-starvation hyperactivity response, which is experienced by many adherents of the Caloric Restriction movement.  The conventional wisdom regarding excessive exercise it that it is a manifestation of obsessive-compulsive disorder; however, one can see that the phenomenon occurs in the absence of any psychiatric disorder.

Another behavioral trait that caloric restriction can induce is a submissive response, which may be interpreted by the therapist as a manifestation of perfectionism. Quite commonly, anorexic patients are observed to be extremely sensitive to others needs, and eager to please. This may be partially due to existing character traits, but these traits may also be enhanced by malnutrition. This phenomenon has been exploited for centuries in prisoner-of-war camps, forced labor camps, and with slave labor. Although one might think  a well-fed prisoner or slave would be more productive, in actuality, a slightly malnourished, hungry slave will tend to be more submissive if their cooperation is rewarded with food, and their failure to cooperate is punished by withholding access to food. You can readily observe this phenomenon in domestic dogs, which are generally much more eager to please and obey their masters when they are hungry, than when their appetites are satiated. 

Starvation has been used as a form of torture to induce submission to authority for thousands of years. Natural selection would favor submissive behavior in such circumstances, since it would be a prerequisite for survival. Of course, in an anorexic patient with an undiagnosed digestive disorder, eating or absorbing sufficient nutrition may be an impossibility, so they may appear to be extremely compliant and eager to please in every area except regarding the ingestion of food. 

It is apparent that many of the behaviors associated with anorexia nervosa (excessive exercise, emotional problems, food preoccupation) are not due to mental illness, but are normal adaptive responses to starvation.