Individual differences |
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Family and twin studies have suggested that genetic factors contribute to about 50% of the variance for the development of an eating disorder and that anorexia shares a genetic risk with clinical depression. This evidence suggests that genes influencing both eating regulation, and personality and emotion, may be important contributing factors.
Several rodent models of anorexia have been developed which largely involve subjecting the animals to various environmental stressors or using gene knockout mice to test hypotheses about the effects of certain genes on related behaviour. These models have suggested that the hypothalamic-pituitary-adrenal axis may be a contributory factor, although the models have been criticised as food is being limited by the experimenter and not the animal, and these models cannot take into account the complex cultural factors known to affect the development of anorexia nervosa.
There are strong correlation (but not proven causation) between the neurotransmitter serotonin and various psychological symptom such as mood, sleep, emesis (vomiting), sexuality and appetite. A recent review of the scientific literature has suggested that anorexia is linked to a disturbed serotonin system, particularly to high levels at areas in the brain with the 5HT1A receptor - a system particularly linked to anxiety, mood and impulse control. Starvation has been hypothesised to be a response to these effects, as it is known to lower tryptophan and steroid hormone metabolism, which, in turn, might reduce serotonin levels at these critical sites and, hence, ward off anxiety. In contrast, studies of the 5HT2A serotonin receptor (linked to regulation of feeding, mood, and anxiety), suggest that serotonin activity is decreased at these sites. One difficulty with this work, however, is that it is sometimes difficult to separate cause and effect, in that these disturbances to brain neurochemistry may be as much the result of starvation, than continuously existing traits that might predispose someone to develop anorexia. There is evidence, however, that both personality characteristics (such as anxiety and perfectionism) and disturbances to the serotonin system are still apparent after patients have recovered from anorexia, suggesting that these disturbances are likely to be causal risk factors.
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