Updating the Hygiene Hypothesis

In August of 2014, Dr. William Parker, of Duke University, wrote an opinion piece[i] for the British Medical Journal in which he points out that the term “hygiene hypothesis” is an incorrect description for what we now know is actually going on.  That is, the dramatic rise in the incidence of immune-mediated illness in the industrialized world is not because we are too “clean.”  It is the result of the lack of exposure to commensal organisms:  “…the biome depletion paradigm forms a fundamental principle of modern immunology and can no longer be considered a hypothesis.”

With this in mind, I was interested in seeing what the article, “Cleaning up the hygiene hypothesis,”[ii] had to say when it appeared last week in the Proceedings of the National Academy of Science.

Essentially, the paper makes the same point.  The modern day epidemic of immune illnesses does not stem from too much hand washing or floor mopping.  On the contrary, many of these infectious diseases, like measles and many respiratory illnesses, can increase the risk of allergy.  More than that, the original concept did not immunologically add up. The original thinking was that since bacterial and protozoan infections stimulate Th1 cytokines, the lack of infection in industrialized nations (i.e. lack of Th1 stimulation) caused a compensatory increase in Th2, which is associated with allergy.  However, helminths stimulate Th2 – and even with all that Th2 around, infected individuals have LESS allergy and autoimmunity. More than that, autoimmune diseases like IBD show increased Th1 activity.

The explanation lay in the subsequent discovery of the regulatory system (T(reg)): those chemicals which turn off our inflammatory response when it is no longer needed.  Scientists realized that exposure to nonpathogenic organisms, both micro and macrobiotic, stimulate and develop this system.  Without exposure to commensals, our bodies do not learn what we should actually tolerate versus not tolerate.  With this underdeveloped off switch, we hyper-react to everything: germs, allergens and even our own cells.

It is not any single factor that has made the difference.  “It’s diet, sanitation, antibiotic use, [lack of] parasites and more…We’ve altered all of those simultaneously and overwhelmed the host’s ability to modulate the immune system.” (The article points out how our Western diets severely lack plant fiber and other foods that nourish commensal species, further disrupting our microbiota. See my post about this for more information.)

The article concludes by pointing out the many, many things we do not know, like the optimal way to reduce rates of allergic and autoimmune disease.  This is always the point at which scientists and I part ways.  That is, I fully understand, as the article points out, that we do not yet know how to optimally use helminths, diet or probiotics. And while we have some indications that there are steps we can actively take, in truth, we don’t yet have enough data to make definitive recommendations.  I get that.  For example, the results are mixed in terms of swabbing C-section babies’ mouths with vaginal secretions to give them those types of bacteria which otherwise they would be missing at birth.

Without a doubt, there is a staggering amount we don’t know.

On the other hand – or actually, on the bright side – we do know the down side of these kinds of things.  For example, since it can’t HURT to populate a C-section baby with a vaginal swab, why not do it?   Had I known this when my younger son was born via C-section, I absolutely would have insisted upon it.

No…this is not science.  But while we wait for science to catch up, there’s no reason to not make these changes now.  I stand by my mantra, “If it can’t hurt and it could help, do it.”


[i] Parker, W. The “hygiene hypothesis for allergic disease is a misnormer. BMJ. 2014;249:g5267.

[ii] Scudellari, Megan. Cleaning up the hygiene hypothesis. PNAS. 2017:114(7);1433-1436.



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