There’s a habit that’s had scientists puzzled: the practice of geophagy—eating dirt. People around the world munch on dirt, and not just when they’re hungry enough to eat anything. So is there any nutritional or health benefit? A meta-analysis published in the Quarterly Review of Biology may offer a clue. [Sera Young et al., "Why on Earth?: Evaluating Hypotheses about the Physiological Functions of Human Geophagy," in press]

Researchers collected more than 480 reports from missionaries, plantation doctors, explorers and anthropologists. These included who was eating dirt and under what circumstances. Seems that dirt doesn’t offer much in the way of nutrition—but it may protect against toxins, pathogens and parasites.

Dirt is most commonly eaten by women in early stages of pregnancy and preadolescent children. Both are particularly at risk from parasites and pathogens. Also, people tend to eat dirt when they’re suffering from gastrointestinal distress. The distress probably doesn’t come from the dirt, which is usually clay found deep in the ground and that doesn’t house pathogens. Plus people often boil the clay before eating.

Scientists say more research is needed to confirm the hypothesis that dirt has health benefits. But they hope this offers evidence that eating dirt isn't, well, as bizarre as it may seem.