The “Hygiene Hypothesis” – A Phrase Whose Time Has Passed

Dr. William Parker of Duke University Medical School, an eminent researcher into the effects of helminths on the immune system among other specialties, wrote a commentary in 2014 in the British Medical Journal entitled, The “hygiene hypothesis” for allergic disease is a misnomer.[i]  His point is that it is not excessive hygiene that has led to our current epidemic of  allergic, autoimmune and inflammatory disorders but biome depletion – the human body’s loss of commensal organisms with which we coevolved, and without whom our immune systems lack the necessary stimulation to function properly. As Dr. Parker states, “…a lack of handwashing often results in an increased incidence of flu and other acute infections, many of which exacerbate rather than mitigate allergy and autoimmunity. Furthermore, modern hygienic practices often alleviate, not increase, allergy—by reducing the levels of allergens produced by organisms such as dust mites, cockroaches, and mold. Thus, allergies and autoimmune diseases are not caused by ‘hygiene’ as people now think of it.”

That is, not washing your hands after using a shopping cart will set you up for a cold or the flu.  But this will do nothing to modulate a chronic, excessive inflammatory response caused by a hyper-reactive immune system.

Thus, Dr. Parker has been very vocal about discontinuing the use of the phrase “hygiene hypothesis,” and instead, strongly promotes using the term biome depletion, which is  not a hypothesis, but, as he states emphatically, a paradigm of modern medicine.

I am reminding you of this today for a very good reason. My regular readers already know that sensationalism in medical news makes me crazy.  (You can read more of my rantings here, for example.) This past week, it was an article from Men’s Health that set me off:  Why You Should Be Picking Your Nose and Eating Your Bogies to Boost Your Immune System? [ii] From the article:  “According to Dr. Meg Lemon, a dermatologist treating people with allergies and autoimmune disorders, you should be picking your nose and eating the bogies and whatever else you scoop out of there in order to boost your immune system. Seriously. Dr. Lemon also says that people should be eating food they drop on the floor in order to expose the body to a range of germs that could help boost the body’s natural defenses and make people more resilient to infections and allergies.”

“Seriously”?  This is what an issue with global health implications is reduced to? The cure is eating snot and dirty food?

This same story was also written up in Business Standard[iii] which (after giving the snot eating plenty of virtual ink), at least, went on to state, accurately:  “Thanks to all the powerful learning we’ve done as a species, we have minimized the regular interaction not just with parasites but even with friendly bacteria and parasites that helped to teach and hone the immune system — that ‘trained’ it. It doesn’t encounter as many bugs when we are babies. This is not just because our homes are cleaner, but also because our families are smaller (fewer older children are bringing home the germs), our foods and water cleaner, our milk sterilized. Some refer to the lack of interaction with all kinds of microbes we used to meet in nature as the ‘old friends mechanism.’”

In this vein, I just read an interesting little article describing a study that came out in early January wherein researchers looked into the mechanism by which exposure to commensal bacteria alleviates asthma:  Mechanism for impaired allergic inflammation in infants may explain hygiene hypothesis.[iv] They found, “…that infant mice need a higher exposure to a bacterial endotoxin [byproducts of many bacteria], compared to adult mice, to avoid developing asthma-like reactions to house dust mites.” That is, without higher levels of exposure to the byproducts of commensal bacteria (as compared to adult mice), the infant mice developed asthma after repeatedly being exposed to dust mites. And this phenomenon existed only during a certain window of development – the equivalent of human infants and toddlers.  Interestingly, if the exposure to dust mites occurred simultaneously with a dose of bacterial endotoxins, the mice did not develop asthma. Bearing in mind that about 10% of school-age children in the USA now have asthma, this is a pretty key bit of research.

I won’t go into detail about their immune system findings which explain the mechanism of action, except to say that in infant mice, certain chemicals were not upregulated when the exposure to the bacterial endotoxin was too low, and this led to the production of certain proinflammatory immune cells.  (In contrast, in adult mice, even a small amount of the bacterial endotoxin led to a much higher production of these chemicals, which prevented immune cells from differentiating into pathogenic, proinflammatory ones.)

They conclude, “Our data, therefore, provide a plausible mechanism underlying the higher susceptibility to allergic airway inflammation observed in children raised in uber-clean and sanitized environments.”

To sum up:  that there is a huge difference between acute-disease causing “germs,” (like cold and flu viruses) and the commensal organisms that are native to our bodies and a natural part of our ecosystem’s biome.  I’m all for biome enrichment but rest assured, I always wash my hands when I come home from the supermarket!

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[i] Parker, W. The “hygiene hypothesis” for allergic disease is a misnomer. British Medical Journal. 2014;349. doi.org/10.1136/bmj.g5267

[ii] https://www.menshealth.com/uk/health/a26808694/pick-your-nose-and-eat-your-bogies-to-boost-your-immune-system/

[iii] https://www.business-standard.com/article/health/environment-is-cleaner-your-immune-system-has-never-been-so-unprepared-119031700024_1.html

[iv] https://www.sciencedaily.com/releases/2019/01/190109110115.htm