Animals like us crave fats and sugars because they offer densely-packed calories that keep us from starving. We crave water because we need it to survive. And sometimes we crave dirt.
Geophagy is the name given to the practice of eating dirt, apparently, its quite common in the human and animal world. But why?
Some geophagy is done for sound reasons. Many birds and bats spend much of the day eating fruits. The fruits contain alkaloids; chemical compounds that are poisonous to the birds and bats and can kill them if eaten in sufficient quantity. The minerals in dirt bind to these poisons, and are passed out of the body undigested. Scientists found that young bats, and bats who were pregnant or nursing their young, stopped most frequently sites of exposed dirt and clay eat.
These sites were also frequented by larger animals like wild pigs. The sites were called 'mineral licks' because the clay below was often rich in important minerals that the pigs needed to get proper nutrition. When scientists noted a 'craving' for dirt in people, especially pregnant women and young children, they thought that the people were imitating the larger mammals and trying to get nutrients like calcium and iron into their system. That idea was the accepted explanation for geophagy until scientists started looking into the kind of dirt that was eaten. After studying hundreds of cases of geophagy, the researchers found that the dirt eaten didn't have much iron, calcium, or zinc in it. No one was eating it for nutrition. Instead, they think it might be to take out the pathogens and parasites that invade the stomach. Dirt might take out what's already living there, and causing harm.
Just the opposite claim was made by scientists a few years ago. They found that people who ate dirt were more likely to suffer from intestinal parasites and digestive sickness, and concluded that that's expected when people eat dirt. When examining the actual dirt eaten, this new study found that it was dug up from far beneath the surface level, and was boiled thoroughly before eating. (That's right. People need to properly cook dirt.) There wasn't much evidence that any new bacteria were introduced into the system with the dirt. Instead, it kills off whatever harmful stuff is already there. It seems that humans, and some other mammals, eat dirt to clean themselves up.