Kerry Cohen in "Sustenance" at Literary Mama - she has a memoir, Seeing Ezra, forthcoming in 2011 on the topic from Seal Press:
Ezra has pica, which is a childhood disorder characterized by compulsive and persistent cravings for nonfood items, such as mud, paper, and dirt. The word "pica" comes from the Latin word for magpie, a bird known for its indiscriminate appetite. Twenty-five to 30 percent of children have pica, and it is most common among those with developmental disabilities like autism, which Ezra has. Ezra expresses his autism in two primary ways: he struggles with language, especially conversation, and he doesn't eat.
There are all sorts of theories as to why children might crave things that are not food. One is that the craving grows from nutritional deficiency. Another is that it comes as a distorted response to neglect or abuse. When the child has autism, though, there is no obvious reason for the behavior. It is a neurological hiccup, a disturbance in the brain's reward circuitry. Our drive to eat is generated through the hypothalamus, which contains the pituitary gland and is located in the middle of the base of the brain. But if there is confusion among the neural pathways, or damage near the lateral hypothalamus, the organism will experience aversion to sources of sustenance.
All these words are so inadequate. Sometimes words seem utterly foreign to me, as I imagine they do for Ezra. These terms and concepts - pica, hypothalamus, autism - don't explain to me why my child won't engage in this basic survival instinct of eating food.