(NEW YORK) -- Sophie started starving herself in kindergarten, giving up sweets at first, then taking smaller and smaller portions of food. She exercised compulsively on the monkey bars.
But her parents had no idea she was developing anorexia nervosa because the active girl's height and weight looked normal on the pediatrician's growth chart.
"She was slim, but not skeletal," said her mother Anne, a college professor from Washington State, who did not want to use real names to protect their privacy.
Sophie complained of being dizzy, having "itchy skin" and constipation, all symptoms of malnutrition. She later confessed that she had been throwing out her school snacks and lunches.
One night when her mother was tucking her into bed, she blurted out, "Mommy, I have a problem …I am hungry all the time and I can't eat," remembers Anne. "A voice in my head is telling me not to eat."
When Sophie was finally diagnosed in first grade, she hadn't gained a pound for 10 months and had dropped from the 60th to the 19th percentile on the weight charts.
Anorexia nervosa is a relatively rare and chronic brain disorder with no known causes. Eating disorders have the highest mortality rate -- about 10 percent -- of any psychiatric illness, according to the National Institute of Mental Health.
Anorexia is rare among young children, but the number of hospitalizations is on the rise. According to the Agency for Healthcare Research and Quality, the rate jumped 119 percent between 1999 and 2006, the last year for which there are statistics.
Highly inheritable, it is estimated that 56 to 70 percent of those who are anorexic have a family member with an eating disorder or a co-morbidity like anxiety, according to the Kartini Clinic, a Portland, Ore., facility that exclusively treats children and young adults with eating disorders.
Sophie was adopted, so there was no family medical history to turn to.
"No one knows what triggers it," said Dr. Julie O'Toole, founder and medical director of the Kartini Clinic. "The science isn't there yet."
"But it's not caused by the media or by pressure to be thin, though people like to blame that," she said. "Parents don't cause eating disorders and children don't choose to have them."
"You can't cause it even if you wanted to," said O'Toole. "It has nothing to do with fashion magazines. We see farm kids, religiously-raised kids who are homeschooled and have no access to television ... who developed anorexia nervosa."
The disorder affects girls 10 times more often than boys in all age groups, but the true numbers may not be known because boys "conceal their illness better," said O'Toole, who is a pediatrician.
Early onset anorexia, under the age of 12, can look different from the adult illness.
"In the classic adult form, they are afraid of getting fat and believe themselves to be fat and quit eating on that basis," she said. "But there are some children 10 and under who refuse to eat and can't tell you why. And it's not kids who never did eat much or picky eaters -- that's a whole different field."
Weight restoration is the key to treatment. "If you do not do this, you do nothing," said O'Toole.
Every organ is affected by starvation, including the brain. Children who are anorexic show slow cognition.
Copyright 2013 ABC News Radio