B. Brett Finlay and Marie-Claire Arrieta

We spoke to the Dr. B. Brett Finlay and Dr. Marie-Claire Arrieta, authors of Let Them Eat Dirt, about how to give your childs immune system the best start in life

The title of your book, Let Them Eat Dirt, is provocative — particularly in light of today’s parenting culture where we often sanitize everything from our kids’ bottles to their dummies.  Explain what this title means and why eating dirt is important for kids.

What we are trying to convey is that in our quest to clean up our world and get rid of infectious diseases, we now realize we have become too clean and we need to rethink our quest for cleanliness. We don’t directly advocate 'eating dirt' but we now realize kids, especially early in life, depend on abundant microbial exposure that is needed to develop normally.   Without this exposure, they are at a much increased risk for the 'Western' diseases such as allergies, asthma, IBD, obesity, diabetes, etc. later in life.


You talk about how children raised on farms have certain advantages over children raised in cities.  Talk about these advantages — and what city parents can do to help our kids get those perks.

We know that kids raised on farms are exposed to multitudes of microbes. Other studies suggest that these exposures are actually good for decreasing the incidence of asthma and allergies, among other diseases. While the odd field trip to a farm will probably not do a lot, decreasing 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 is not necessarily bad — this is how human children evolved, and by living in an extremely clean environment is not how we have evolved as a species.


Why do you think there has been a jump in the number of children with food allergies — and what can parents do to prevent allergies?

There are a lot of hypotheses about this, but the leading one is that children are 'microbially deprived' so their immune system doesn’t develop normally, and is shifted to allergies. It is also thought that the western diet increases gut permeability, exposing children more to allergens. Similar reasons go for increased rates of asthma and allergies.


Is it necessary to sterilize milk bottles and if so, until what age?

The American Academy of Paediatrics no longer recommends sterilizing bottles used for babies of any age. It recommends only a stove top method (cleaning them in boiling water) or a dishwasher with a hot cycle when using the bottles for the first time or when the water used at home is deemed not safe to drink. The same goes for any utensil or plate used to feed babies solid foods and for dummies and teethers.

One recent study from the University of Gothenburg analysed data from over 1000 children and found that children from homes that washed their dishes by hand were less likely to develop eczema by school age suggesting that exposure to microbes early in life may protect against the development of allergies and asthma. Doing the dishes manually on occasion rather than always using a dishwasher is recommended.


What’s your advice on washing kids’ hands?  How often and under what circumstances should we make our kids wash their hands? 

Hand washing is, without a doubt, the best hygienic practice that we can follow to prevent infectious diseases. It has been shown time and again that communities with better hand washing practices stay healthier and no one should stop washing their hands to promote more exposure to microbes. That said, children do not need to wash their hands all day long. Hand washing should occur before eating, after using the toilet, after being in contact with someone sick, or if the child is sick, after touching garbage or food that is suspected to be decomposing, after touching animal waste, or after being in places frequented by many people (subways, malls, etc.). Children do not need to wash their hands after playing outside (unless they are about to eat), immediately after they walk into the house, or after playing with other children (unless they are sick with an infection). Children should be outside and should be allowed to be barefoot and to get dirty, and hand washing does not necessarily need to immediately follow these activities. This list is certainly not exhaustive, but it is aimed to give an idea of the type of exposures associated with risk of infection from the ones that are not, which should guide hand-washing principles.



How often should parents clean kids’ toys?

This question was a popular one among the parents we interviewed for our book and this question often came accompanied by suggested answers, such as 'every day or after every use?'or 'with regular disinfectant or bleach?' Our answer is not to do it until they visibly look dirty or after a sick child has played with them. As for what to use, soap and water is more than enough. Harsh chemicals such as the chlorine in bleach or disinfectants are not necessary for this type of cleaning or to clean the surfaces where children play either.


Is the sandbox as dirty as many parents think it is?  And should we let our kids play in it?

Kids love sandboxes and it is no surprise to find a dozen children playing in one at once, making them a popular playground spot that undoubtedly has a higher concentration of microbes than other playground features (there are a couple of studies showing this). This means that a child has a higher risk of contracting an infection in the sandbox than on the slide. Does this mean that they should not go in the sandbox? Absolutely not, sandboxes are great fun and the risk of disease is still quite low. However, it does mean that parents and carers should follow hygienic practices, like hand washing, after using the sandbox. The other possible source of infection in a sandbox comes from the fact that a sandbox looks like a giant litter box to many animals (read cats) and they would use it accordingly given the chance. For private sandboxes covering the sandbox after use can easily prevent this, whereas in public ones it would pay off to inspect the sandbox before letting a child use it. If some animal waste is visible definitely scoop it out along with a good amount of the sand surrounding it, but if the sandbox looks like it has been used by all the neighbourhood cats refrain from letting a child using it and contact the local authorities to have its sand replaced. (Cat faeces can contain parasites, which can then infect humans.)


You say that many times it’s OK to let our kids put something in their mouths that has dropped on the floor.  Talk about when it’s OK to do this, and when we should wash the item before letting them eat or suck on it?

In general terms, putting something back in your mouth that has fallen on the ground is OK. However, not all ground surfaces are equal and common sense applies. If a child’s toy falls on the subway or mall bathroom floor, it is a good idea to give it a rinse with soap and water, but it if falls on someone’s home or while out hiking, simply remove the visible dirt (and hair) and give it back to your kid. In fact, a recent study by a Swedish research group suggests that the best way to clean a pacifier that has been dropped is to put it in your mouth first! In this study, 184 families were enrolled and interviewed their babies were 6 months–old. Parents were asked the question: Does your child use a dummy, and if so, does it get sterilized, rinsed in tap water or by parents sucking on it? Surprisingly, they found that the 65 babies raised by parents that cleaned their dummies by mouth had a significantly lower risk of developing allergy at 18 and 36 months of age. This small study remains to be replicated but it seems that the by sharing mouth microbes with a child, a parent is strengthening their child’s immune system and preventing the development or allergies. So, instead of following the 5–second rule to pick something off the ground, perhaps we need to follow a 5 second rule in mum or dad’s mouth rule before giving your child his teether/dummy back. We should point out that there may be a concern with parents passing cavity–making microbes to their children, but this appears to be an issue with parents that are prone to tooth decay only, which can be hereditary.


What should we be feeding our children and why?

This is a good one to answer because there is no better way to influence the development of a healthy microbiota than through diet. Offering a healthy diet, rich in vegetables and fibre is as important as not being overly clean with babies and children. When babies start eating solid foods they should be given a diet varied in vegetables, fibre and fermented foods. A child can be exposed to many good sources of microbes while playing and interacting with people, but if these microbes are not fed the right foods, they will not flourish in a child’s gut. If a child’s diet is mainly based on refined carbohydrates (white flours and sugar) and high fats, her or his digestive system will be able to digest and absorb most or all of the nutrients in the upper part of the digestive tract, leaving little for the vast numbers of microbes inhabiting the large intestine further down. The microbiota in the large intestine feed on fibres and foods that are somewhat resistant to digestion in the upper part of the digestive tract and if none of that makes it down they will starve.         


The dangers of antibiotics have made a lot of news lately.  Talk about your take on them.

Antibiotics are wonder drugs and they are one of the greatest discoveries in modern medicine. They very effectively treat most infections that only 100 years ago could kill someone. The problem — and it is a big problem — is that they are designed to indiscriminately kill microbes, both good ones and bad ones. Instead of precisely killing the bad bug causing trouble, antibiotics act like a grenade in the gut. The resulting collateral damage, especially when antibiotics are given in the first few years of life, is that they can wipe out the good microbes in charge of training our immune system and other aspects of our metabolism. Not surprisingly, taking antibiotics during infancy is associated with an increase of asthma, allergies and obesity, so they truly are double-edged sword. Our message is to use antibiotics only when necessary — during a bacterial infection. Our advice, based on paediatric societies guidelines, is to use the 'watchful waiting approach', especially if the child is older than 6 months, is otherwise healthy, and has mild symptoms. These guidelines also recommend that the parents have good access to painkillers for the kid to help ride things out. Generally they suggest waiting 48-72 hours, and then following up if the symptoms has not resolved. We also advise eating fermented foods (yogurt, kefir) and to use paediatric probiotics during and after the antibiotic treatment. Please consult your health practitioner regarding effective probiotics that have undergone tests in human trials (many have not).


Should I allow my child to eat unwashed fruits and vegetables?

In most cases you should wash produce. Fruits and vegetables are often consumed raw, which means that any contamination that occurred during farming or storage may come into contact with whoever eats them. Another good reason to rinse fruits and vegetables is to wash out pesticide residues. The only type of situation in which we would consider it ok to allow a child to eat unwashed fruit or vegetables is if they are grown in their garden, watered by rain or a hose and free from pesticides. This does not apply to store-bought organic produce – organic farming does not decrease the risk of food poisoning, although it does significantly reduce the levels of pesticides in crops.

Find out more about the author

Let Them Eat Dirt

B. Finlay (and others)

'This book might change your perspective on real cleanliness . . . and along the way help you to raise healthier kids. ' Giulia Enders, author of Gut

'A must-read for parents ... Let Them Eat Dirt takes you inside the inside tract of a child's gut, and shows you how to give kids the best immune start early in life.' William Sears, MD, co-author of The Baby Book

We all want what is best for our kids, but for years we’ve believed that microbes cause infectious diseases and have battled to keep them under control. Our modern lifestyle, with its emphasis on hyper-cleanliness, is having a negative effect on our children’s lifelong health.

In Let Them Eat Dirt, microbiologists B. Brett Finlay and Marie-Claire Arrieta explain how the trillions of microbes that live in and on our bodies influence childhood development and why an imbalance in those microbes can lead to obesity, diabetes and asthma, among other chronic conditions. With practical advice from conception through to pregnancy and beyond, this invaluable guide will help you to nurture stronger, more resilient and healthier children.

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