Over the past few decades, chronic inflammatory disorders such as inflammatory bowel disease (IBD), allergies, and asthma have become significantly more common in industrialized countries.
There has been an increase in the incidence of IBD, particularly in children, with 20% to 30% of patients having an onset of symptoms before the age of eighteen (1) Rates for food allergies have been steadily rising, and seasonal allergies have more than doubled since the 1970s. (2, 3) There has also been a sharp rise in asthma rates for both children and adults in the past decade, with nearly one in ten children and one in twelve adults suffering from the disease. (4)
Even autoimmune conditions such as type 1 diabetes and multiple sclerosis have increased at rates too rapid to be accounted for by changes in our genetics. (5) Many healthcare professionals, research scientists, and epidemiologists are at a loss for why the rates of these inflammatory diseases are increasing on such a steep incline.
Could it be that we’re all just not eating enough dirt?
Our culture’s obsessive attention to cleanliness, sanitation, and hygiene may actually be having unintended consequences on our immune system. While a sanitary environment may be crucial in areas such as hospitals or food production, our general avoidance of dirt, bacteria, and other infectious agents may be causing our under-stimulated immune system to become overreactive to benign antigens.
The “hygiene hypothesis” or “old friends hypothesis” suggests that the increased prevalence of inflammatory disorders is the result of defective immune system regulation due to reduced exposure to an adequate variety of microorganisms. (6) Since our immune system evolved alongside a massive variety of different microbiota, both commensal and pathogenic, the recent changes in society and environmental exposures to germs may play an important role in the global increase of inflammatory diseases, particularly in urban settings. (7)
Evidence for the hygiene hypothesis of inflammatory disease has recently been demonstrated in controlled animal trials.
In a 2012 study, researchers examined the immune system of “germ-free” mice who had been bred to lack gut bacteria, and compared them to mice with normal exposure to microbes. (8) They found that the germ-free mice had significantly more inflammation in the lungs and colon, similar to that found in humans with asthma and colitis, due to hyperactivity of specific T cells that have been linked to these conditions in both mice and humans.
What is most interesting about these results is that if the germ-free mice were exposed to microbes during the first few weeks of life, they eventually developed a normalized immune system and avoided inflammatory disease. On the other hand, those germ-free mice exposed later as adults never recovered a fully functioning immune system. This demonstrates a crucial time period during early life where the immune system must be properly conditioned in order to function normally. Of course, this effect needs to be demonstrated in humans, but these preliminary results are promising for the study of inflammatory disease development.
So what does this mean for us humans? Should we all start chowing down on bacteria-filled dirt?
Not so fast. Don’t forget that not all organisms in the environment are “old friends”. Some of them can cause significant disease, and even death. However, the hygiene hypothesis does highlight the importance of the gut microbiota in regulating our immune system and overall health. Additionally, it further emphasizes the potential consequences of things that adversely impact our gut microbiota, starting with a cesarean section birth and formula feeding (instead of breastfeeding) and continuing later in life with overuse or misuse of antibiotics. While there are likely many different factors that play into the development of inflammatory and autoimmune conditions, the evidence of the hygiene hypothesis is robust enough that it should not be ignored.
One important take-home message of this research is the notion that certain crucial biological developments happen during the early years of infancy and childhood, and these developmental milestones cannot simply be recreated during later years of life. This means that children are especially vulnerable to environmental inputs, both positive and negative, that may significantly affect their health later in life. It also means – as I’ve pointed out several times before – that there’s more to health than food. A nutrient-dense, whole-foods diet is certainly one of the best steps we can take to prevent disease and improve our health. But it’s not a panacea, especially in the face of environmental and epigenetic alterations of the commensal gut microbiota.
While we cannot control every factor of our children’s future health and wellbeing, we can at least feel better about letting our kids splash in the mud, put toys in their mouth, or play with the neighbor’s dog. Beyond being a little less worried about protecting our children from germs, perhaps we should actually encourage them to get a little dirty now and then!
What do you all think about the hygiene hypothesis? Do you let your children play in the dirt? Tell me your thoughts!