MIAMI (CBS4) -- Imagine having an irresistible urge to eat chunks of dirt.
The condition known as pica actually happens to some people, and doctors get especially concerned when it happens to pregnant women.
Pica can involve an intense craving to eat a variety of non-food items, such as dirt, clay, starch, chalk or even charcoal.
"I had a woman confess to me that she was eating toilet paper – many, many rolls of toilet paper in a day – and it was an insatiable appetite that she couldn't control," said Amy Levi, Ph.D., the director of the Nurse Midwifery Program at the University of California-San Francisco.
Nutritional anthropologist Dr. Sera Young, also of UCSF, has studied pica around the world. Her research evolved into a landmark study, called "Craving Earth."
"What you do see is pregnant women, more than anyone, are loving this stuff," Young said.
A recent study of pregnant women in U.S. public health clinics found a staggering 68-percent engaged in pica.
Dr. Young says women tell her, "'We're addicted to these things. It's like rum; it's like opium; it's like smoking; it's like crack.'"
When it comes earth, not any dirt will do. Women are seeking out clean dirt which can be bought by mail or over the internet and sent discreetly to homes.
If you think those who eat such substances must be crazy, Young strongly disagrees.
"To say, 'crazy,' is a pretty inappropriate label because these women are as sane as you or me," she said.
Dr. Young believes pica might have evolved to serve a biological purpose.
"Think of this as sort of a mud mask to coat your intestines," she said. "With this lining, it's harder for pathogens like bacteria and viruses or other harmful chemicals to permeate your gut."
But the problem is pica can be dangerous.
"It can interfere with the absorption of vitamins and minerals that your body really needs when you're pregnant," she said.
Dr. Levi says let your doctor know if your engaging in pica because experts can redirect patients' cravings to something that won't be harmful to them or the fetus.
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