Sheila was a child in Cameroon when she first got hooked on kaolin.  

"I was in primary school," she says. "My aunt would eat it, and it was often me who had to go and buy it for her." Sheila is currently studying at university in France. Many people back home, she says, continue to consume this substance every day. Some even become dependent on it.

Kaolin isn't exactly hard to come by - you can purchase it from most Cameroonian markets - but it's not something that appears on any lists of banned substances. Kaolin isn't a new street drug. It's dirt.

Eating dirt, or geophagy, has a long history in Cameroon. Colonial era texts concerning the region describe the behaviour in detail. "I am told that all of [the children] eat it," writes one perplexed author in Notes on the People of Batanga. "Even those belonging to the mission, who are [...] strangers to the sensation of hunger."

According to Sera Young, resident geophagy expert at Cornell University in the US, it has a long history around the world.

Young has spent nearly two decades getting her head round this behaviour, and in a comprehensive study  analysing nearly 500 historical and contemporary accounts from around the world, she and her fellow researchers documented its global prevalence.

Geophagy has been reported in countries as diverse as Argentina, Iran and Namibia, and certain trends keep appearing in  the team's analysis. Consumption seems to be higher in the tropics, and two groups tend to gravitate towards it in particular: children (predictably, perhaps) and pregnant women.

These non-food cravings happen a lot, and they happen right under our noses - Sera Young

Of course, the lower rates seen in other countries could well be a result of underreporting owing to cultural taboos.

"These non-food cravings happen a lot, and they happen right under our noses," says Young, citing a case she heard of a renowned opera singer in New York whose dark secret was the desire to eat dirt during pregnancy. Young's own interest in geophagy was piqued while undertaking field work in rural Tanzania. "I was conducting interviews with pregnant women about iron deficiency anaemia," she says. "I was sitting on the floor of this woman's house, and I asked her what she liked to eat during pregnancy, and she said: 'Twice a day, I take earth from the wall of my house and I eat it.'"

Understandably, Young was shocked. "Eating earth goes against everything we are trained to do," she says.

Indeed, Western medicine has traditionally regarded geophagy as pathological, classing it as a form of pica, the condition also attributed to those who intentionally ingest such harmful substances as glass or bleach.

And yet clearly in Cameroon there is no taboo surrounding the practice. Similarly, Young describes her surprise when, working in Kenya, she found she could buy packets of earth in a range of flavours, including black pepper and cardamom. In the US, the state of Georgia is renowned for the quality of its white dirt (there's even a website). While packets are labelled 'Novelty Item: Not Intended for Human Consumption', everyone knows their true purpose.

The grit sucks all of the water from my tongue, forming a paste that sticks to the roof of my mouth like peanut butter

Young asks me if there are any African grocery stores near where I live, in South London. I say yes.

"Just go into one and ask for pregnancy clay. They'll definitely have it."

Half an hour later, I walk out of a shop selling "African groceries" carrying a stone that had cost me 99p. I gingerly put a small piece in my mouth. The grit sucks all of the water from my tongue, forming a paste that sticks to the roof of my mouth like peanut butter.

I briefly entertain the thought that it tasted a little bit like smoked meat, before deciding that no, actually it tasted much more like dirt.

I wondered what it is that gets so many people hooked.

"Everyone has their reasons," says Monique, another Cameroonian student. "Simple desire is one, or else to treat nausea and stomach pain. The clay calms the pain by acting as a gastric dressing."

Could this be it? Instead of an illness, is geophagy a treatment?

In fact, three key explanations have been proposed for why people eat dirt, and Monique's answer touches on one of them.

Not all dirt is created equal. Kaolin belongs to a specific group of clay minerals, and these seem to be the most popular when people crave a mouthful of earth.

Clay could provide nutrients that are not present in conventional food items

Clay is very good at binding to things, so when Monique talks about it calming gastric pains, it could be doing just that by binding with or blocking harmful toxins and pathogens in the digestive system.

Experiments with rats and observations of monkeys indicate that other animals may seek non-food substances to combat ingested poisons, and various traditional food preparation practices involve mixing food with clays to extract toxins and make it palatable. Acorns are generally unpleasant to eat, for example, but the traditional production of acorn bread in both California and Sardinia involves grinding the nuts up with clay that seems to reduce the concentration of unpalatable tannic acid they contain.

The second hypothesis is perhaps more intuitive: clay could provide nutrients that are not present in conventional food items. Anaemia is often associated with geophagy, so perhaps eating iron-rich soil is an instinctive attempt to remedy iron deficiency.

There's also a suggestion that geophagy is a response to extreme hunger, or micronutrient deficiencies that make non-food items attractive. This hypothesis is non-adaptive, meaning it fits with the idea that eating earth is a negative behaviour with no benefits.

The first two hypotheses, on the other hand, suggest adaptive reasons for geophagy, and they go some way to explaining its distribution, too.

"We predicted that it would happen most in the tropics, because that's where there is the greatest density of pathogens," says Young. Furthermore, children and pregnant women are two groups that might need extra nutrients or protection against disease, as their immune responses are weaker.

Just because someone craves something doesn't mean it's good for them

That being said, there is a tendency to ascribe a special significance to pregnancy cravings. "Women see pregnancy as a time to indulge," says Julia Hormes, assistant professor of psychology at the University of Albany in the US. "There are a lot of myths around 'eating for two' and giving the foetus what it needs, but really there's not a lot of support for those ideas."

Hormes says in reality these cravings have a whole lot to do with culture, and not a lot to do with biology.

If there are culture-specific beliefs around eating dirt, then women in Cameroon are as likely to crave it as women in Europe or North America might feel cravings for chocolate or ice cream. Just because someone craves something doesn't mean it's good for them.

Still, the inclination towards geophagy clearly arises from somewhere, seeing as it's found even in cultures for which it has no such significance. Studies of animals suggest an adaptive, biological explanation could be at least part of the story.

When elephants, primates, cattle, parrots and bats engage in geophagy, for example, it is generally considered to be serving a useful purpose. Despite this, some of the same scientists who consider geophagy normal in animals still see it as abnormal in humans.

Undoubtedly some cases of dirt eating do indeed involve psychiatric problems, but drawing a line is difficult. In 2000, the US Agency for Toxic Substances and Disease Registry concluded that pathological levels of soil consumption constituted more than 500 mg per day, but even they conceded that this was an arbitrary measurement.

Eating dirt can even become an addiction, an impulsive act hidden from others

"Because geophagy has been so extensively documented as a culturally-based phenomenon, I'm not inclined to view it as 'abnormal' behaviour," says Ranit Mishori, a professor of family medicine and doctor at Georgetown University Medical Center. "If, however, this behaviour is associated with any clinical abnormalities, I would have a conversation about how to avoid it."

Clearly there are downsides to consuming dirt. The presence of soil-borne diseases and toxic substances in the clay is a major issue, as is the possibility that the very deficiencies supposedly cured by the practice might even be caused by them.

Eating dirt can even become an addiction, an impulsive act hidden from others. "With geophagy, the language of substance abuse is really common," says Young.

It's easy to dismiss geophagy as a disgusting habit of children, a wacky pregnancy craving, or an exotic behaviour from far-away lands, but none of these approaches really do it justice. Moreover, such characterisations risk alienating people who find it difficult to explain their 'unnatural' desires.

To fully grasp this phenomenon, and understand whether its effects are positive, negative or a subtle mix of the two, researchers need to undertake hypothesis-driven tests that take both biomedical and cultural factors into account.

"I'm not saying 'everyone should be eating three spoonfuls of earth a day'", says Young. "But we certainly don't know enough yet to write this behaviour off entirely."

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