• Samantha Schofield

Hygiene hypothesis: is it good I ate dirt as a kid?

The hygiene hypothesis has been a theory circulating since the 1980s, theorising that having an environment that is “too clean” is causing children to have compromised immune systems. So, is eating dirt as a kid actually good for your gut microbiome and immune system? And how are the immune systems of kids growing up in these crazy COVID times with germs being hard to get your hands on?


The immune system and your microbiome

The immune system is the body’s defensive force against nasty infections. The immune system is very complex and involves various organs, proteins and cells. One important part of the immune system is the body’s microbiome, specifically the gut microbiome. The gut microbiome is the name given to all the critters in your gut helping digestion, providing vitamins and minerals to the body and controlling the immune system. The gut contains an ecosystem of microbes, including bacteria, parasites, yeast organisms, fungi and viruses. Think about it like a rainforest, with different critters scuttling around interacting with each other. While one person might have a rainforesty gut, someone who grew up elsewhere might have a gut that looks more like a coral reef, with completely different critters. The immune system’s ability to defend the body relies on the environment a person grows up in and the knowledge of certain disease-causing nasties.


The gut microbiome is extremely dynamic when you are in the early stages of life and very susceptible to change. What you eat, where you grow up and even if you were a C-section versus a vaginal delivery completely changes what your gut microbiome is made up of. When you are born, you are exposed to your first ever germs, as the uterus is almost completely sterile from germs. A vaginal delivery gives newborns exposes the baby to germs found within the mother’s birth canal. Babies who are delivered C-section have been shown to have more similar microbiomes to the nursing staff, rather than their mothers. Which has been shown to miss some vital microbes. Eating a healthy diet, exercising and not using antibiotics unnecessarily are important to have a healthy, balanced gut. If you only eat high fat foods, this causes a shift in the gut’s diversity, causes an unbalanced gut and increased chance for problems, as the body has an increase of critters to deal with fat, but not much else. Various studies have looked into how the microbiome affects the body and found links to Alzheimer’s, diabetes and even anxiety and depression. But today, we will focus on the relationship with the immune system.


Hygiene hypothesis

The hygiene hypothesis is a theory created by Dr David Strachan and proposes that too much hygiene and a “too clean” environment is actually detrimental to kid’s immune systems, and can cause certain diseases. This hypothesis can be used to explain the rapid increase occurring for atopic (allergic) diseases in children and increased hygiene in the past 30 years. Having a more sterilised childhood, causes kids to not have the establishment of certain microbes. The first study found children who grew up in larger households (more microbes floating around) were less likely to develop an atopic disease. Several studies found early exposure to microorganisms significantly lower the risk of developing allergies and asthma. Having pets and playing outside in the dirt, particularly before the 1st year of life when the microbiome is the most dynamic, allow for the development of protection in the immune system. This type of environmental learning is particularly important for children who have genetic predisposition for development of atopic diseases, such as allergies.


One study was completed in the US, analysing Amish children (who had no electricity, drank unpasteurised milk and had a very old-school style of living) against suburban kids (who had more sanitisation practises). The results found 50% of suburban kids had a positive skin test result for allergies, compared to 7% of Amish children. Therefore, it was hypothesised that the Amish children had early exposure to allergens in which protected them later on in life from developing atopic diseases. For a healthy microbiome, you need diversity in the gut and living in an aseptic society decreased microbiome diversity significantly.


What about the kids growing up in these COVID clean times?

So, we know exposure to germs as a kid is important for developing a healthy immune system. This seems rather contradictory from hand sanitising 20 times a day when you’re going for a shop. Staying safe from COVID means staying sanitised and having high hygiene in public places. Well, what are the experts saying? How are kids who are growing up with everyone wearing masks, all gross surfaces being sanitised and not being able to go outside and mingle with their fun, germ infested friends? The COVID pandemic means there has been a loss in microbial diversity and the question is how bad is this for the kids? Experts from the CDC to Harvard Medical School suggest allowing a kid to play in the dirt, eat a healthy balanced diet and getting enough sleep whilst also sanitising when you go out in public will be enough to sustain a healthy gut microbiome. It’s a balance between practising good hygiene and allowing the exposure of different microbes. Importantly, this pandemic will allow researchers to investigate the interactions between hygiene practises and the gut microbiome all around the world.


So… when I ate half a cockroach as a 2-year-old (I wish I was joking), was I doing my body a favour? The answer is most likely, that delightful action probably exposed me to many gross germs and could be the reason I don’t have any allergies!


Sources and further reading

https://www.pnas.org/content/pnas/118/6/e2010217118.full.pdf?casa_token=aUEFq0oZzqQAAAAA:GerFpm3_0WECq6xZLTrvtWy_9Y6gn3EHRpBEqp57FHiBm8vkHfUnIaENT2c0-JyNY8mv6ACNISUy6Qwz

https://open.spotify.com/episode/2LFeqwmrfIKROPz7D5x9Aj?si=dcfb338cadb54822

https://www.youtube.com/watch?v=ofZTJoIVBg0

https://www.youtube.com/watch?v=B9RruLkAUm8

https://www.nature.com/articles/d41586-019-02807-x#:~:text=Distinct%20differences&text=The%20differences%20between%20their%20gut,community%20of%20vaginally%20delivered%20infants.

https://www.livescience.com/54078-hygiene-hypothesis.html#:~:text=The%20hygiene%20hypothesis%20says%20a,to%20develop%20resistance%20to%20diseases.%20(

https://www.fda.gov/vaccines-blood-biologics/consumers-biologics/asthma-hygiene-hypothesis

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1448690/