Guess what: unlike what your mother told you, eating dirt actually has some remarkable good effects on your immune system.
With concerns about chemical contamination, bacteria, and nematodes, it's clear that consuming dirt isn't without risk. Yet it does not seem that eating soil is all risk and no benefit. Soil consumption in children appears to be driven by a biological need. Scientists have many speculations about this need, but little real evidence.
The hygiene hypothesis suggests that early exposure to bacteria reduces allergies and improves a child's resistance to disease. According to this theory, children who grow up in environments that are very clean develop allergies and other immune disorders because their immune systems haven't had enough experience with friendly visitors or dangerous invaders. Researchers who study the immune system are divided on the hygiene hypothesis, and there is substantial evidence both for and against it.
The largest part of our immune system shadows our intestines. This gut-associated lymphoid tissue, or GALT, keeps our normal gut bacteria in line and helps recognize and fight off unwelcome newcomers. But animals raised in a sterile environment without contact with bacteria do not develop normal GALT.
Eating dirt might be one way that animals, including people, can get some 'experience' with bacteria and learn to tell the harmless from the deadly. The bacterial count in dirt is large, but mostly harmless.
And there is more than just bacteria, fungi, and roundworms in that peck of dirt. Manufacturers add clay-like compounds to some vaccines to increase the immune system's response, making the inoculation more protective. It's possible that a child's mud pie may be a kind of primitive self-vaccination, letting the gut get used to a selection of common bacteria that rarely cause harm.
Most dirt is safe, despite the thousands of species of bacteria and other organisms it contains. Understanding the dangerous exceptions can make outdoor time safer for you and your children.
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