The Hygiene Hypothesis: How Modern Environments May Interact with Our Genes

THE HYGIENE HYPOTHESIS

Get sick, take antibiotics. Put dirty dishes in the dishwasher. Walk in shoes, on pavement, to and from work. Sit inside all day. Visit a farm once a year, maybe. Wash hands with antibacterial soap consistently throughout the day. Eat food from the grocery store. Never willingly touch dirt. Wear bug spray. Kill bugs in the yard with mosquito barrier traps.

Our modern environment is clean, isn’t it?

The hygiene hypothesis is a burgeoning theory in the health sciences that postulates that the development of new hygienic conditions may be a contributing factor to the appearance of autoimmunity in “westernized” nations. Several epidemiological and immunogenetic studies have investigated the concept by looking at rates of autoimmune disorders and changes in hygiene in a given population.

A particularly fascinating study researched the rates of multiple sclerosis (MS) in Sardinia, Italy after the eradication of malaria on the island. Historically, Sardinia was plagued by malaria, a mosquito-borne disease caused by a parasite that is transmitted through mosquito saliva. This Plasmodium then enters the bloodstream and infects the host. It can take weeks to get over such an illness, and it may even be fatal in certain instances. Needless to say, it’s a terrible ordeal, and thus prompted the movement to eradicate malaria in Sardinia. This mosquito-control project occurred almost fifty years ago and successfully altered the insect environment on the island. However, ever since then the population has been plagued with soaring rates of multiple sclerosis–a disease in which the immune system inappropriately attacks one’s own central nervous system. This occurrence required much investigation, and after many deep-dives into genetic and cellular activity within the population, the phenomenon ultimately has contributed to the hygiene hypothesis (I).

While the immune system is understood quite well in many regards, it remains to be a mysterious and dynamic element of human beings–ever-changing, adapting, and responding as we evolve. If we think about our immunity as a species, we must consider the places from which this magnificent system came. Over the centuries, humans have evolved to live in a symbiotic relationship with a population of microorganisms in our gastrointestinal tract–often known as our “microbiome.” The balance of this population plays an essential role in the development, training, and function of the host immune system. This microbiome was trained through interactions with other animals, dirt, organ meats, raw proteins, fermented foods, fresh foods, raw vegetables and fruits, insects, parasites, diverse water sources–and as we evolved–herding, planting, digging, harvesting, and management of animals. Needless to say, humans had a lot of exposure to a vast array of bacteria and other microorganisms. This exposure led to a balanced and hearty population of bacteria in our GI tract. Given that approximately 70-80% of our immune system is in our GI, this immune system–microbiota alliance made humans a robust species that was well-equipped to manage unfriendly invaders.

However, since the industrial revolution, we humans have separated ourselves from the environments in which we develop and maintain a strong relationship with microorganisms. The antibacterial soap, the separation from agriculture, the antibiotics, the rare visits to farms–even our cleaning products and dishwashers–are all contributing to far less interactions between our environment and our microbiome. Due to this separation, it’s possible that our microbiota are weakened, often out of balance, low in population, and unable to mediate appropriate immune responses. In this regard, the hygienic nature of our modern environments are a gene-environment mismatch. With all of this in mind, the hygiene hypothesis is an interesting lens to view the phenomena in Sardinia. The native DNA that regulates immune responses had probably evolved over centuries to learn to cope with strong parasites, like those that can cause malaria. Once that environmental pressure was removed, what did the immune system have to fight? Not much. If we think of the immune system as an army, then the immune system in autoimmune disorders could be thought of as a very powerful, untrained army. When this army has nothing to fight, what does it do? Well, it seems that it cannot regulate itself when it is placed in such an unfamiliar situation.

It goes without saying that modern medicine and hygiene is incredibly beneficial. Less disease and illness gives us longer lifespans and lower mortality rates. It gives us clean environments, and ways to fight parasites. Those are wonderful things. However, the environments of the old world weren’t necessarily bad, either. Neither one is “bad” or “good.” They each come with trade-offs that we must recognize. We’ve traded longer lifespans and more comfortable environments for more incidences of autoimmune disorders and allergies. Our historic environments produced more robust humans, but had higher mortality rates and shorter lifespans. We interact far less with the natural world, and it’s lowered the quality of our nutrition, animal proteins, and our physical health. However, if we can develop an awareness of these issues, we can help mediate them.

This research, among other similar studies, gives the hygiene hypothesis a lot of weight to at least be considered as a resource for learning. If we think about developing a healthy immune system in our modern environment, we can use this research to help guide some of our strategies. Namely, we could grow a garden in our own yard, we can visit farms frequently, we can use a sponge instead of a dishwasher (one study found that children in homes that used a sponge instead of a dishwasher had lower incidences of allergies, most likely from more bacteria in their environment (II)), we can buy local foods, we can embrace a little dirt and spending plenty of time outdoors, and we can try to avoid antibiotics at the first sign of a runny nose.

It is a challenge to consider what the human genome expects and what the modern world is providing. But research is increasingly showing us that there are more than one gene-environment mismatches occurring in industrialized nations. Some of these mismatches are affecting the alliances that we once had as a species with resources from our environment. If we can embrace this fascinating research, and work to stay connected to the natural world that empowers our body’s most ancient systems, we can find ways to function more optimally in our concrete jungles.  

I. Sotgiu, Angius, Embry, Rosati, Musumeci. 2008. Hygiene hypothesis: innate immunity, malaria and multiple sclerosis. PMID: 17889443. https://www.ncbi.nlm.nih.gov/pubmed/17889443

II. Hesselmar, Hicke-Roberts, Wennergren. 2015. Allergy in children in hand versus machine dishwashing. PMID: 25713281. https://www.ncbi.nlm.nih.gov/pubmed/25713281

THE HYGIENE HYPOTHESIS

Get sick, take antibiotics. Put dirty dishes in the dishwasher. Walk in shoes, on pavement, to and from work. Sit inside all day. Visit a farm once a year, maybe. Wash hands with antibacterial soap consistently throughout the day. Eat food from the grocery store. Never willingly touch dirt. Wear bug spray. Kill bugs in the yard with mosquito barrier traps.

Our modern environment is clean, isn’t it?

The hygiene hypothesis is a burgeoning theory in the health sciences that postulates that the development of new hygienic conditions may be a contributing factor to the appearance of autoimmunity in “westernized” nations. Several epidemiological and immunogenetic studies have investigated the concept by looking at rates of autoimmune disorders and changes in hygiene in a given population.

A particularly fascinating study researched the rates of multiple sclerosis (MS) in Sardinia, Italy after the eradication of malaria on the island. Historically, Sardinia was plagued by malaria, a mosquito-borne disease caused by a parasite that is transmitted through mosquito saliva. This Plasmodium then enters the bloodstream and infects the host. It can take weeks to get over such an illness, and it may even be fatal in certain instances. Needless to say, it’s a terrible ordeal, and thus prompted the movement to eradicate malaria in Sardinia. This mosquito-control project occurred almost fifty years ago and successfully altered the insect environment on the island. However, ever since then the population has been plagued with soaring rates of multiple sclerosis–a disease in which the immune system inappropriately attacks one’s own central nervous system. This occurrence required much investigation, and after many deep-dives into genetic and cellular activity within the population, the phenomenon ultimately has contributed to the hygiene hypothesis (I).

While the immune system is understood quite well in many regards, it remains to be a mysterious and dynamic element of human beings–ever-changing, adapting, and responding as we evolve. If we think about our immunity as a species, we must consider the places from which this magnificent system came. Over the centuries, humans have evolved to live in a symbiotic relationship with a population of microorganisms in our gastrointestinal tract–often known as our “microbiome.” The balance of this population plays an essential role in the development, training, and function of the host immune system. This microbiome was trained through interactions with other animals, dirt, organ meats, raw proteins, fermented foods, fresh foods, raw vegetables and fruits, insects, parasites, diverse water sources–and as we evolved–herding, planting, digging, harvesting, and management of animals. Needless to say, humans had a lot of exposure to a vast array of bacteria and other microorganisms. This exposure led to a balanced and hearty population of bacteria in our GI tract. Given that approximately 70-80% of our immune system is in our GI, this immune system–microbiota alliance made humans a robust species that was well-equipped to manage unfriendly invaders.

However, since the industrial revolution, we humans have separated ourselves from the environments in which we develop and maintain a strong relationship with microorganisms. The antibacterial soap, the separation from agriculture, the antibiotics, the rare visits to farms–even our cleaning products and dishwashers–are all contributing to far less interactions between our environment and our microbiome. Due to this separation, it’s possible that our microbiota are weakened, often out of balance, low in population, and unable to mediate appropriate immune responses. In this regard, the hygienic nature of our modern environments are a gene-environment mismatch. With all of this in mind, the hygiene hypothesis is an interesting lens to view the phenomena in Sardinia. The native DNA that regulates immune responses had probably evolved over centuries to learn to cope with strong parasites, like those that can cause malaria. Once that environmental pressure was removed, what did the immune system have to fight? Not much. If we think of the immune system as an army, then the immune system in autoimmune disorders could be thought of as a very powerful, untrained army. When this army has nothing to fight, what does it do? Well, it seems that it cannot regulate itself when it is placed in such an unfamiliar situation.

It goes without saying that modern medicine and hygiene is incredibly beneficial. Less disease and illness gives us longer lifespans and lower mortality rates. It gives us clean environments, and ways to fight parasites. Those are wonderful things. However, the environments of the old world weren’t necessarily bad, either. Neither one is “bad” or “good.” They each come with trade-offs that we must recognize. We’ve traded longer lifespans and more comfortable environments for more incidences of autoimmune disorders and allergies. Our historic environments produced more robust humans, but had higher mortality rates and shorter lifespans. We interact far less with the natural world, and it’s lowered the quality of our nutrition, animal proteins, and our physical health. However, if we can develop an awareness of these issues, we can help mediate them.

This research, among other similar studies, gives the hygiene hypothesis a lot of weight to at least be considered as a resource for learning. If we think about developing a healthy immune system in our modern environment, we can use this research to help guide some of our strategies. Namely, we could grow a garden in our own yard, we can visit farms frequently, we can use a sponge instead of a dishwasher (one study found that children in homes that used a sponge instead of a dishwasher had lower incidences of allergies, most likely from more bacteria in their environment (II)), we can buy local foods, we can embrace a little dirt and spending plenty of time outdoors, and we can try to avoid antibiotics at the first sign of a runny nose.

It is a challenge to consider what the human genome expects and what the modern world is providing. But research is increasingly showing us that there are more than one gene-environment mismatches occurring in industrialized nations. Some of these mismatches are affecting the alliances that we once had as a species with resources from our environment. If we can embrace this fascinating research, and work to stay connected to the natural world that empowers our body’s most ancient systems, we can find ways to function more optimally in our concrete jungles.  

I. Sotgiu, Angius, Embry, Rosati, Musumeci. 2008. Hygiene hypothesis: innate immunity, malaria and multiple sclerosis. PMID: 17889443. https://www.ncbi.nlm.nih.gov/pubmed/17889443

II. Hesselmar, Hicke-Roberts, Wennergren. 2015. Allergy in children in hand versus machine dishwashing. PMID: 25713281. https://www.ncbi.nlm.nih.gov/pubmed/25713281

2018-07-31T15:18:49+00:00

About the Author:

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Erin is a graduate student working toward a Masters of Science in Nutrition and Health Promotion, as well as completing the Didactic Program in Dietetics to become a Registered Dietitian. She’s also a Precision Nutrition Certified Nutrition Coach and Certified Sports Nutritionist.