Let them eat dirt!

Many epidemiological studies have led to the hygiene hypothesis. The hygiene hypothesis proposes that exposure to microbes in early childhood can decrease susceptibility to allergies and diseases such as asthma, irritable bowel disease (IBD) and autoimmune diseases. It’s believed that this early exposure to microbes helps the immune system function properly, which prevents the “overactivation” of an immune response that occurs in the case of allergies, asthma, etc. When it “overactivates” the immune system sees something that is not dangerous (ex. dust, pollen, peanuts) as harmful, and “freaks out”, releasing all sorts of inflammatory mediators that cause symptoms from a runny nose to full-blown anaphylactic shock. In most cases, this would be annoying or inconvenient, but in some cases it can be deadly. Although a lot is known about WHAT happens when the allergen or disease is triggered, it is not known WHY. Several studies have produced observations that suggest this is all related to hygiene, specifically that exposure to microbes in early childhood lowers the chance someone will develop allergies, asthma, IBD, etc. It has been suggested that this could be related to how “clean” we are now, particularly with the use of antibacterial soaps. It has been observed that more people in developed countries have allergies/asthma/IBD/autoimmune diseases. And, within developed countries, children that are from rural areas have lower rates of allergies and asthma compared to their counterparts in cities. It wasn’t really known why this was the case, but a recent paper in Science (http://www.sciencemag.org/content/336/6080/489.full) has provided some evidence about the immune system cells that are involved.

It has been hypothesized that invariant natural killer T (iNKT) cells play an important role in the pathogenesis of ulcerative colitis and asthma. These cells recognize lipid antigens and release large amounts of proinflammatory factors (called cytokines) when they are activated. Olszak et al investigated the age-dependent regulation of iNKT cells by use of microbes in mouse models of IBD and asthma.

They worked with germ-free (GF) and specific-pathogen-free (SPF) mice. GF mice are born without the bacteria that are normally present, and are protected from any bacterial exposure over their life. SPF mice are free of certain bacteria that would cause disease, but the normal intestinal bacteria are present. They found that the numbers of iNKT cells were increased in GF mice and appeared to be stable for life. When they induced colitis in the mice they found that the GF mice were more sensitive to colitis, with more severe weight loss, pathology, and higher mortality rates compared to SPF mice. If they re-established the microbiota (all the bacteria that should be present) in the GF mice, they found that the iNKT levels and severity of colitis was not reduced. If they took pups that had a GF mother and raised them in SPF conditions, they found a complete normalization of iNKT cells and reduced susceptibility to colitis. This indicated that the microbiota present at birth has life-long effects on health. They repeated these studies with a mouse model of asthma and had very similar observations.

This study fits with the epidemiological studies that led to the hygiene hypothesis, showing that early life exposure to microbes decreases susceptibility to diseases such as IBD and asthma, where as absence of microbial exposure may have the opposite effect. This is one of the first studies that provides a mechanism to explain the epidemiological observations that compose the hygiene hypothesis.

So, I’m not saying that you should feed your kids dirt, or stop them from washing their hands before eating, but let them play in the dirt and don’t freak out if they lick their shovel – it just might lower their risk of inflammatory diseases ;p

Christina