Healthy Happy Life Info: What is the hygiene hypothesis?

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The idea that we need to be exposed to germs and bacteria in early childhood is the subject of much debate, but what is the hygiene hypothesis?

It sounds counter-intuitive, but not being exposed to viruses, bacteria and parasites in your early years could have a negative effect on your health, or at least that's what the hygiene hypothesis suggests.

The idea, which is subject to much debate, suggests our obsession with cleanliness and sterile surroundings is causing the increase in allergic and autoimmune diseases we've seen in recent decades.

Gastroenterologist Professor Timothy Florin says the term 'hygiene hypothesis' has been bandied around a lot, but has generally come to refer to the effect of children not being exposed to enough pathogens at an early age.

"The basis is that our immune system has evolved over evolutionary time as a defence against bacteria and viruses – bacteria and viruses have evolved over time with humans, rather like domestic dogs and pets, and they're used to seeing each other," says Florin, from the University of Queensland and Brisbane's Mater Hospital.

"So we're used to, in our growth as a population, seeing certain viruses at certain times and if you don't see them at that time then it's a bit like missing out on a maths lesson at a critical stage in infancy – you're missing certain building blocks."

Emerging hypothesis

The idea was first put forward in the 1980s by epidemiologist David Strachan, after he studied a cohort of more than 17,000 British children in an attempt to understand the gradual increase in incidence of hayfever in post-war Britain. During his research he noticed a striking pattern emerging.

The more older siblings a child had, the less likely he or she was to develop hay fever or eczema by age 23. He found it was by far and away the most important risk factor in determining whether a child developed the allergic condition.

Strachan concluded there must be some protective factor spread from older siblings to younger siblings.

"[These observations] could, however, be explained if allergic diseases were prevented by infection in early childhood, transmitted by unhygienic contact with older siblings, or acquired prenatally from a mother infected by contact with her older children," he wrote in the British Medical Journal, on the 18 November, 1989.

Since then, other studies have found links between various aspects of Western living standards and a range of allergic diseases – including eczema, asthma, and food allergy – or autoimmune conditions such as type 1 diabetes.

Research also suggests children born to families migrating to western countries also experience an increase in the incidence allergic disease. And it's been found certain factors – such as attending childcare or having a pet – can help to reduce the risk of allergic disease.

It's in the gut

However, immunologist Professor Barbara Fazekas says the hygiene hypothesis has become something of a catch-all that is used to point the finger at a variety of 'modern evils'.

"Currently, the hygiene hypothesis seems to mean whatever you want it to mean – we're too clean, and we eat fast food and we do this and this," says Fazekas, Principal Research Fellow at the Centenary Institute of Cancer Medicine and Cell Biology, University of Sydney.

However she points to Strachan's research which showed differences in the risk of allergy between siblings of the same family. "None of those things that society is blamed for could possibly affect families in that way."

Fazekas, who describes herself as a 'hygiene hypothesis purist', believes the protective factor Strachan was reaching for has something to do with the gut microbiome – the microbiotic population of our gut – and its vital role in immune function.

"The gut has a huge surface area and most of our immune system is focused on the gut so that interaction is by far the biggest immune reaction that's going on in all of us all the time," Fazekas says.

Before birth, a baby's gut is sterile, but during birth and the first few days of life they begin to develop essential gut flora. Factors affecting the development of this gut flora include how a baby is delivered, early contact with parents and siblings, whether the baby or its mother are given antibiotics or other medications in hospital.

She believes as gut flora are established early in life and remain with us for life, they influence and educate the immune system everyday for our entire lives.

"I think that's much more likely to educate your cells all the time than something that happened in terms of a cough or a cold."

Healthy living and gut bacteria

So how can improve our children's chances of developing healthy gut flora, while maintaining adequate hygiene?

Fazekas says it's not as simple as eating dirt.

"If we're talking about things that live in the human gut, most of the things that live in the human gut are adapted to live in the human gut and you get them from other humans, you don't get them from dirt," she says.

In her view, older siblings can help, as the original hygiene hypothesis paper illustrated, or exposure to other sources of diverse gut flora at an early age, such as child-care. Research also suggests having pets and spending time on farms could help.

But the hygiene hypothesis shouldn't be an excuse for poor hygiene, says Florin.

"It's important to be very hygienic in situations where there's some invasive bug going around, but within your own home where there's no epidemic bug going around, I don't think it's important to be hygienic at all."