Early exposure to microbes will give children the best immune start to life, insist two microbiologists who have joined forces to pen the thought-provoking new book, Let Them Eat Dirt.

Modern lifestyles with an emphasis on hyper-cleanliness are having a negative effect on children’s lifelong health. That’s the claim of microbiologists Professor Brett Finlay and Dr Marie-Claire Arrieta, who insist there is now undeniable evidence that early exposure to microbes is beneficial to our children’s wellbeing.

The scientists have written the new book, Let Them Eat Dirt, to explain their conviction that microbiota (the microbes that live in and on humans) are great for our health.

“In our quest to clean up our world and get rid of infectious diseases, we have become too clean and we need to rethink our quest for cleanliness,” stresses Prof Finlay.

“We don’t directly advocate ‘eating dirt’, but we now realise kids, especially early in life, depend on abundant microbial exposure that’s needed to develop normally. Without this exposure, they are at a much increased risk of the ‘Western’ diseases such as allergies, asthma, inflammatory bowel disease, obesity, diabetes, etc. later in life.”

While microbes - bacteria, viruses and other microscopic organisms - can cause infections, some of them potentially deadly, Prof Finlay and Dr Arrieta point out that of the thousands of microbe species, only about 100 are known to cause human disease.

“The vast majority do not cause any problems and, in fact, seem to come with serious benefits,” they say.

One well-known theory, the ‘hygiene hypothesis’, suggests a lack of exposure to bacteria and parasites during childhood may be the cause of a rapid increase in allergies, as it prevents proper development of the immune system.

Prof Finlay and Dr Arrieta believe the huge rise in the number of children with food allergies is because youngsters are “microbially deprived”, so their immune system doesn’t develop normally and shifts to allergies.

And they say there is now plenty of solid evidence suggesting the hygiene hypothesis helps explain the development of many diseases as well as allergies.

The pair point out that as well as increased cleanliness reducing childhood exposure to microbes, there are other reasons for the reduction - primarily the “use, overuse and abuse” of antibiotics.

Dr Arrieta stresses that although antibiotics are “wonder drugs”, they should only be used when necessary for a bacterial infection, because when given in the first few years of life, they can wipe out the good microbes in charge of training the immune system and other aspects of metabolism.

“Taking antibiotics during infancy is associated with an increase of asthma, allergies and obesity, so they truly are a double-edged sword,” she says.

Many parents worry about kids putting things in their mouth after they’ve been dropped on the floor, but Dr Arrieta says it’s generally okay - although she points out that “not all ground surfaces are equal” and parents should use common sense. So if a child’s toy falls on a shopping centre toilet floor, for example, it’s a good idea to rinse it with soap and water. But it if falls in someone’s home, simply remove the visible dirt and give it back to your child, she says.

A recent Swedish study suggested the best way to clean a dummy that’s been dropped on the floor is for a parent to put it in their mouth first, says Finlay. The research found the 65 babies whose parents cleaned their dummies by mouth had a significantly lower risk of developing allergy.

“It seems that by sharing mouth microbes with a child, a parent is strengthening their child’s immune system and preventing the development or allergies,” he comments.

You don’t get much muckier than on a farm, but the book explains that children raised on farms have certain immune advantages over children brought up in cities. However, if you’re not planning to move to the country to improve your children’s microbe exposure, Dr Arrieta says there are many things city parents can do to help their kids get ‘farm perks’.

“Kids raised on farms are exposed to multitudes of microbes, and studies suggest these exposures are actually good for decreasing the incidence of asthma and allergies, among other diseases,” she says.

“While the odd field trip to a farm will probably not do a lot, unnecessary use of antibiotics, excess use of antibacterial hand cleansers, not having a pet, etc. are all ways of decreasing microbial exposures that can be changed.

“Letting a kid play in the dirt isn’t necessarily bad - this is how human children evolved, and living in an extremely clean environment is not how we have evolved as a species.”