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The ‘Hygiene Hypothesis’ has been proposed as an explanation for the increase in asthma, allergies and autoimmunity. The idea is that our increased hygiene and cleanliness, especially through the use of antibacterial products, has prevented the immune systems of our children from developing properly. 

We have two arms to our immune system: Th1 and Th2 (actually it’s more complex, but I’m trying to simplify this!). When a baby is born, its immune system is Th2-dominant. A healthy functioning immune system needs to have the two arms in balance. This happens by exposing the immature immune system to bacteria and bugs, which stimulate the development of the Th1 arm to maturity until both sides are in balance. So this means letting your children play outside in the dirt, get their hands muddy, play with pets, share toys and contact other children. 

Of course we must teach them good hygiene too, to wash their hands etc, but if the child’s environment is excessively clean and they are not exposed to these germs, they will end up with a Th2-dominant immune system. This leads to diseases like asthma, hay fever and other allergies, autoimmune diseases and generally leaves them more susceptible to illness. Research has shown that children in developing countries are less likely to develop allergies and asthma compared with children in the developed world. 

Now I’ve explained the original concept, which has been around since the 80s, let me bring you up to date. It is no longer thought to be about excessive hygiene, but due to a lack of exposure to a diverse range of ‘friendly’ bacteria; i.e. the problem lies in our gut and its microbiome. Consequently it has been renamed the ‘old friends’ hypothesis, referring to the microbiota that have been with us throughout evolution. We need these microbes to develop our immune system and fight off infection, and they are found in the soil, air, water and in our food. Observed changes to the composition of our gut microbiota and loss of its diversity are now said to be the cause of our dysfunctioning immune systems, presenting as asthma, allergies or autoimmune disease. 

This is also why we are told it’s better to have a natural birth and breastfeed (if possible). Maternal microbes colonise the gut whilst the baby is in utero and again as the baby passes through the birth canal and during breastfeeding. 

What has caused these changes to our microbiota?

Basically changes to our diet and lifestyle: less contact with nature, antibiotics, caesarean birth and not breastfeeding, and less variety in our diets, particularly diets that are low in fibre, fruit and vegetables. 

Olivia Smart


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