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Is there clear evidence for the hygiene hypothesis?
Whether it is to nuts, animals or pollen, most of us have or know someone with an allergy. I personally suffer from hay fever, which is becoming a nuisance now in the British springtime. I’m not alone – its estimated that 10-30% of the world’s population has hay fever. Food allergies can be even more problematic, and in the US the number of children who have a food allergy has risen 50% from 1997 to 2011.
Genetics explains only around 10-40% of a person’s risk to develop allergies, so the remaining percentage lies with our environment. The hygiene hypothesis states that childhood’s lack of exposure to certain microbes could increase their chances of developing allergies. To find out more about this, we asked 5 experts in immunology, allergology and medicine, ‘Is there clear evidence for the hygiene hypothesis?’, this is what they said…
Is there clear evidence for the hygiene hypothesis?
What is the hygiene hypothesis?
Professor Marie-Claire Arrieta, an expert in microbiology and immunology from Calgary University in the USA, says “The Hygiene Hypothesis was born from work made by Dr. David Strachan, who observed that younger siblings were less susceptible to eczema and asthma, and proposed that this was a result of increased transmission of infectious agents via unhygienic practices within a household.”
Dr Cosby Stone, an expert in immunology and allergology from Vanderbilt University in the USA, says “The central tenet of the hygiene hypothesis is that we have gone a bit too far and inadvertently killed off our good bacteria along with the bad. As our society progressed from one that was chronically burdened with infectious diseases caused by poor sanitation, the thinking goes, we reduced our exposures to the things that gave our immune system an appropriate training and tolerance. Historically, our totally rational fear of dying from a cholera epidemic led to sewage and water management, but may have kicked off the allergy epidemic.”
What conditions are linked to the hygiene hypothesis?
The most well-known condition linked to the hygiene hypothesis are allergies. Allergies are when the body reacts to something that is normally harmless. Apart from allergies, the hygiene hypothesis is also said to be linked to autoimmune diseases.
Professor Graham Rook, an expert in immunology from University College London in the UK, says “What we see in modern, particularly urban, communities is an increase in disorders where the immune system’s control mechanisms are failing so that it attacks things it should ignore. For example, in autoimmune diseases like multiple sclerosis our immune systems are attacking our own tissues. Similarly, our immune systems attack harmless molecules in the air, or in food (allergies) or they attack the contents of our guts which contributes to inflammatory bowel diseases.”
How can being ‘too clean’ negatively affect our immune system?
Professor Rook says “The vertebrate immune system has co-evolved, over about 500 million years, with an increasingly complex gut microbiota (the community of mostly symbiotic organisms that live in and on us, particularly in our guts). The microbiota plays an essential role in driving the development and function of essentially all our organs (including the brain), but particularly the development of the immune system, and of the mechanisms that regulate it.
Similarly, our airways need exposure in early life, to microbial components from the natural environment that provide signals involved in setting up the appropriate background level of activity of the immune system, and above all, appropriate activity of anti-inflammatory regulatory mechanisms in those sites… In summary, the vertebrate immune system is a learning system like the brain, and without various microbial inputs that provide essential data it cannot function correctly.”
What is the evidence for the hygiene hypothesis?
Professor Robert Clark, an expert in immunology from Connecticut University in the USA, says “studies have demonstrated increased asthma in genetically similar populations in which one population farms in an old fashioned and ‘dirtier’ environment while the other population uses modern and ‘cleaner’ farming methods. The cleaner population had a significantly increased frequency of asthma, suggesting that exposure to bacterial products in the dirtier environment ‘educates’ and ‘acclimates’ and essentially calms the immune system to environmental and self-stimuli, - so there is less of an over-reponse to both environmental and self stimuli (and less asthma). In the ‘cleaner’ and more asthmatic environment, direct evidence was documented showing that the immune system over-reacts to such stimuli.”
Dr Stone adds that “Studies in mice have shown that inhaling certain molecules from soil-dwelling bacteria can set off a beneficial cascade promoting an immune system which focuses more on threats rather than nonthreats, such as allergens.”
Does this mean we should stop washing our hands?
Professor Rook says “The problem lies in the misleading term ‘Hygiene Hypothesis’. It is certain that our immune systems are malfunctioning because of diminished microbial exposures and diminished ‘microbial data input’, but it is increasingly clear that hygiene does not play a major role in causing this diminished exposure…. I emphasise that hygiene, particularly hand-washing, is entirely beneficial”
Professor Rook explains the major areas where the hygiene hypothesis comes into play, saying “caesarean deliveries and some inappropriate hygiene might inhibit transfer of maternal microbiota to the infant… The second most important factor is the overuse of antibiotics, particularly during pregnancy and in early life. Repeated courses of antibiotics increase the incidence of allergies, other inflammatory disorders, and obesity. The third factor is the bad Western diet. The biodiversity of the gut microbiota is maintained by a varied diet, and by fibre (plant cell walls) and plant polyphenols that nourish crucial beneficial microbial species, several of which help to drive the immunoregulatory pathways. A fourth factor is lack of exposure to the natural environment. Keeping a dog in the home partially compensates, because the dog brings the environment into the home. But walking and exercising in green spaces, and gardening are the best solutions.”
Professor Rook concludes that “Major lifestyle changes (diet, antibiotics, lack of exposure to the natural environment) are leading to defective education of our immune systems, and to disorders attributable to failing immunoregulation. We clearly need to rename the hygiene hypothesis. Recent suggestions include ‘Biodiversity hypothesis’ and ‘Old Friends Hypothesis’.”
Dr Stone adds that “Vaccinations appear to be a crucial exception to the rule of the hygiene hypothesis. They confer protection against diseases without any associated increase in the risk of allergic disease, likely because they, unlike antibiotics, are very specifically targeting only the worst disease-causing organisms.
Increased allergy and autoimmune disease risk is linked to overuse of antibiotics, poor diet and lack of contact to natural environments, because these can prevent your body’s exposure to ‘good’ or benign bugs. Importantly, allergies are not linked to hygiene and health practices such as hand washing and vaccinations, and these are important to protect against infections.
May the facts be with you!
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