Revising the Hygiene Hypothesis: We Know Too Clean Isn't Always a Good Thing but That Isn't the Full Story.


In this era, we witnessed heightened rates of allergies and autoimmune disorders — an estimated 300% rise since the 1950s — mainly in developed and rapidly developing countries. This rise is also accompanied by a dramatic decrease in infectious diseases such as measles, mumps, and tuberculosis owing to improved antibacterials and hygiene.

Hygiene Hypothesis

David Strachan, an epidemiologist at the London School of Hygiene and Tropical Medicine, first proposed the hygiene hypothesis in 1989. He showed that incidences of eczema and hay fever (allergic rhinitis) decreased in households with more children due to higher chances of the spread of infectious diseases among siblings.

In other words, not-so-clean environments with more infections appear to suppress the development of allergies. Put it another way, environments that are too hygienic allow allergies to flourish.

While subsequent research continues to support the hygiene hypothesis to a certain extent, more recent advancements in epidemiological, animal, and molecular research data also discovered loopholes and contradictory findings to this hypothesis.

“We know an awful lot now about why our immune system’s regulation is not in terribly good shape, and it’s got absolutely nothing to do with hygiene,” Graham Rook, an emeritus professor of medical microbiology at the University College London, explained.

The hygiene hypothesis is a “dangerous misnomer which is misleading people away from finding the true causes of these rises in allergic disease,” Sally Bloomfield, chair of the International Scientific Forum on Home Hygiene and honorary professor, added. “The trouble is, as soon as you use the words ‘hygiene hypothesis,’ the word hygiene prejudges what the cause is. Relaxing hygiene standards would not reverse the trend but only serve to increase the risks of infectious disease.”

Biodiversity Hypothesis

This biodiversity hypothesis states that “contact with natural environments enriches the human microbiome, promotes immune balance and protects from allergy and inflammatory disorders,” explained Tari Haahtela, professor emeritus from the Helsinki University Hospital, Finland in The Journal of Allergy and Clinical Immunology in 2019. “It enlarges and binds together the hypotheses of hygiene, old friends, microbial diversity, and microbial deprivation.”

Professor Haahtela also asserts that we are in the age of biodiversity loss. Wildlife populations are estimated to have plummeted by 67% compared to 1970. Global urbanization has radically altered about three-fourths of the land surface and two-thirds of the oceans worldwide.

It is the friendly — “old friends” — microbes rather than infectious microbes that may be largely responsible for the increasing rates of allergies and autoimmune disorders. These friendly microbes around and within us — in the soil and in our gut —regulates our immune system to properly respond and not overreact in response to a harmless foreign entity.

“The problem comes when our immune system meets an allergen like pollen or peanuts and doesn’t know that it is harmless,” said Professor Bloomfield. “It’s not about just learning what to attack, but learning what to tolerate.”

Mounting epidemiological data support this theory. Growing up with dogs or farms or in green areas rich in soil drastically reduces the occurrences of allergies and autoimmune disorders.

Causality evidence between lack of ‘natural environment’ with allergies and autoimmune disorders has been demonstrated in animals. This is because our microbiome — be it in the gut, skin, respiratory tract, or body parts — is influenced by our interactions with the environment.

Indirect pieces of evidence also convince its truth. Factors such as cesarean section, early-life antibiotics usage, and western diet, which discourages gut microbial diversity, are also strongly linked to elevated rates of allergies and autoimmune disorders.

So, how do microbes train the immune system? Research on this mainly concerns the gut bacteria. Examples include Clostridium species and Bacteroides fragilis which can stimulate T-cells in the gut to release interleukin-10, an anti-inflammatory cytokine. Whereas gut Bifidobacterium species reduce the release of pro-inflammatory C-reactive protein from the liver.

On a larger scale, the genomic content of the gut microbiota is over 10 times larger than the human genome. This means that gut bacteria are metabolically active in synthesizing molecules that could affect the immune system. It is no coincidence that 70% of our immune system resides in the gut — it’s called the enteric nervous system or the “second brain.”

Nature Step to Health

The 12th General Meeting of the Global Alliance against Chronic Respiratory Diseases (GARD) was held in Helsinki, Finland, in August 2018. They decided to embark on the Nature Step to Respiratory Health which emphasizes engagement with nature for overall health. This involves outdoor physical activities, gardening, having pets, consumption of fresh greens and roots, and avoidance of alcohol, refined sugar, and tobacco across all ages.

“It is evident but poorly studied that everything we eat, drink, inhale and touch affects the composition and function of our microbiota and promotes a cross-talk of human DNA with the environmental metagenome,” explained Tari Haahtela, professor emeritus, pioneer of the biodiversity hypothesis, and leader of the Helsinki by Nature project.

The project’s aim is to reduce the disease burden of respiratory diseases and other related non-communicable diseases (including allergies and autoimmune disorders) by 30% in 2030. This is in accordance with the United Nations Sustainable Development Goals (UNSDGs) which acknowledges biodiversity as a cornerstone of human health, for the first time, in 2015.

Another decade awaits to witness the outcomes of the Helsinki by Nature project. Anecdotally, however, reverting back to nature has shown success. Urbanized Australian aborigines developed increasing rates of type-II diabetes. Returning them to their traditional places for just 7 weeks completely normalized their metabolic dysregulation.

“Revisiting asthma and allergy paradigms has led to actions relevant to society and healthcare as a whole,” Professor Haahtela said. The multidisciplinary approach starting from clinical allergology to microbiology and to ecology has “helped to recognize the slow and silent trends influencing health and disease in modern society,” the professor continued. “Reducing harmful exposures and strengthening immune tolerance could be promoted through a Nature Step, resetting the connection between humans and nature.”

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