Let me be clear that I'm not saying everyone who is under-eating is anorexic or suffering from an eating disorder, or that only under-eating causes high cortisol. This is just some interesting information I found.
High Cortisol Levels In Anorexia Nervosa - Cushing Syndrome
"Persistently high cortisol levels may lead to Cushing Syndrome, a condition where one of the causes is overactive adrenal glands. Other symptoms of this syndrome include fatigue, depression, moodiness or stretch marks on the abdomen.
In anorexia nervosa, excess cortisol is linked directly to starvation mode. One school of thought is that the body is under a significant amount of stress during starvation and malnutrition -- and stress produces more cortisol.
Some anorexics also engage in other behaviors that are known to contribute to elevated cortisol levels. These are: excessive exercise, and self cutting, or self injury."
Low Blood Pressure, Hypotension (more common in those with Anorexia and/or Bulimia) - cause by lowered body temperature, malnutrition and dehydration. Can cause heart arrhythmias, shock or myocardial infarction. http://www.something-fishy.org/dangers/dangers.php