What is the Hygiene Hypothesis?
77Doctors and scientists have been noticing that it appears that asthma, allergies, and other auto-immune conditions are becoming more common in the industrialized countries. And there appears to be something more going on to drive this increase than due to better detection and diagnosis.
The "hygiene hypothesis" suggested by Dr. David Strachan in 1989 was one attempt to explain this increase.[1] This is not the only explanation that can explain this increase. And it is still called a "hypothesis" because it have not been definitely proven. Some scientists feel that the hygiene hypothesis is too simplistic to explain the complex interactions of the bodies immune system and the environment.[1] However, there is ample evidence for this hypothesis based on comparison of population in rural versus urban locations. And other scientists take the hygiene hypothesis seriously.
The hygiene hypothesis is the idea that our modern overly sanitized environment where our children developed are too clean and did not stress the developing immune system enough to make it strong. Just like muscles have to be stressed a little to make it stronger.
In a paper by Becker, it defines the hygiene hypothesis like this ...
"The hygiene hypothesis is a widely held theory of the etiology of asthma and atopic disorders which builds on observations of rural versus urban distribution of disease. It suggests that cleaner environmental conditions in westernized countries, as compared to developing countries, play a role in the increase of the prevalence of these disorders in western countries"[3]
The reason that it gives for the increase in asthma and other similar conditions is that ...
"Risk for asthma and atopy may be due to a lack of early immune challenge of the post-natal immune system by microbial or parasitic infection possibly including environmental saprophytes and gut commensal organisms, relative to the developing innate immune system"[3]
The book Celiac Disease writes that the hygiene hypothesis ...
"proposes that an exposure to infections and unhygienic conditions early in life somehow conveys protection against the development of allergies."[1]
What Hygiene Hypothesis Tries to Explain
The hygiene hypothesis have been considered in the explanation of increase of incidence of many auto-immune type diseases and conditions. These include allergies, asthma, hay fever, ezcema, celiac disease, and even autism. These conditions have the common thread that they are auto-immune conditions involving the immune system.
Hygiene Hypothesis an Interesting Idea to Ponder
In a sense, it is counter-intuitive that elimination of bacteria may cause disease condition. Obviously, we do not want to go back to pre-modern times when bad bacteria kills many people.
Dr. Marc McMorris says "We certainly should not step back in time and stop immunizing our children against deadly diseases."[6]
And of course vaccines and anti-bacteria soap are good and have their proper roles. This is fine. However, the hygiene hypothesis says that there is the possible side-effect of increase auto-immune conditions. It is an interesting idea to ponder.
Should we let kids play outside in the dirt? Perhaps, we don't have to worry so much about germs. Our bodies can handle it. And our kids may develop stronger immune systems with less allergy and asthma as a result -- according to the hygiene hypothesis.
Article on ScienceDaily.com says ...
"Ultimately, McMorris advises parents to just let kids be kids. Let them play outside and with friends, and don’t worry about them coming into contact with dirt and germs – but always be cautious with children with life-threatening food allergies, he cautions."[6]
Evidence for the Hygiene Hypothesis
Wikipedia says there is epidemiological evidence supporting the hygiene hypothesis...
"Studies have shown that various immunological and autoimmune diseases are much less common in the developing world than the industrialized world and that immigrants to the industrialized world from the developing world increasingly develop immunological disorders in relation to the length of time since arrival in the industrialized world."[2]
and draws the conclusion that ...
"One conclusion is that a clean environment, with lack of infections (like early life infections) increases the risk of an autoimmune disorder."[2]
Dr. Peter Green cites a study where the celiac disease, an auto-immune disease, has an occurrence of 1 in 496 in Russia, while it had a higher occurrence of 1 in 107 in Finland. Since the genetic predisposition and the wheat consumption were about the same in both, some researchers suggested that the more hygienic condition of Finland may be partly the cause of higher incident of celiac disease.[1]
Then there is a longitudinal study that looked at a developing country in Africa over time as the country becomes more affluent and presumably cleaner. Study showed that the country has increase incident of asthma and allergic diseases.[4]
Health researcher Erika Von Mutius compared the allergy and asthma rates between East and West Germany. She found that East Germany has lower rates because the children are more exposed to other children (such as in daycares) and hence had greater exposures to microbes. This greater exposures to microbes gave them increase tolerance to the types of irritants related to asthma.[5]
As reported in ScienceDaily.com, modern times are associated with smaller family size which lessen children interactions, which lessen the amount of germ exposure. [6]
On page 188 of The UltraMind Solution, Dr. Hyman writes ...
"We are becoming hypersensitive to our environments, perhaps because we live in an oversterilized environment and our immune systems don't mature properly. Or because we are eating hybridized and genetically modified (GMO) foods full of antibiotics, hormones, pesticides, and additives that were unknown to our immune systems just a generation or two ago. ... In the Third World countries where hygiene is poor and infections are common, allergy and autoimmunity are rare."
Hygiene Hypothesis and Autism?
In a paper by Becker, it notes some similarities between autism and asthma and suggests that there may be a "shared mechanism between these disease types".[3] And since the hygiene hypothesis has been considered as one cause to the increase in asthma, it should also be considered for the increase in autism. The paper says...
"shared observations between autism and inflammatory disorders are used in support of the development of a hypothesis for the apparent rise in the prevalence of autism using the framework of the immune hygiene hypotheses."[3]
Like asthma, autism is higher in urban than in rural environments.
Counter-arguments may say that this could be due to increase mercury in
urban environments, which may in fact be true and a possible cause. However, if you look at the Inuit people of northern Canada, they have high degree of mercury exposure in a rural environment; but they have low rates of autism.
The paper propose that ...
"the hygiene hypothesis, a viable theory in the etiology of asthma, should be considered in the etiology of autism. Underlying factors important in the hygiene hypothesis, whether they are truly related to hygiene practices or to overall microbial or parasitic load, thought to be relevant to the increase in asthma and atopy, may contribute to the rise in the incidence of autism as well."[3]
To be clear, no one is saying that a clean environment is the cause of autism. Although it is believed that autism is due to a combination of genetic and environmental factors, no one really knows for sure what causes autism. The paper simply is saying to look at immune deficiency as something to consider when researching autism. To see the various theories (and there are a lot of them) out there about autism, read the about.com article linked here.
Notes
[1] Celiac Disease: A Hidden Epidemic by Peter Green
[2] Hygiene Hypothesis - Wikipedia
[3] Becker KG, Autism, asthma, inflammation, and the hygiene hypothesis, Med Hypotheses (2007), doi:10.1016/j.mehy.2007.02.019
[4] Exercise-Induced Bronchospasm and Atopy in Ghana: Two Surveys Ten Years Apart
[5] Hygiene Hypothesis on PBS.org
[6] The Hygiene Hypothesis: Are Cleanlier Lifestyle Causing More Allergies for Kids?
Inspiration88 20 months ago
Hmm, interesting. By making our urbanized environment more and more sterile we actually make our natural immunity weaker and weaker.. of course, makes perfect sense. Never thought about this.