By Anne Estes, PhD

Whether it's diet, medicines, or daily practices, our habits influence which microbes are in and on us. Our health is intricately intertwined with the health of our microbiomes. The first few years of life are particularly important, as the human and microbial "self" of a person is developing. Our medical and lifestyle choices can influence our microbiome composition and perhaps long-term health by influencing the developing immune system. It is critical that parents are well informed about the potential consequences of different choices so they can make more informed decisions. I was extremely excited to see that two fabulous microbiome scientists, Dr. Brett Finlay and Dr. Marie-Claire Arrieta wrote "Let Them Eat Dirt" focused on the importance of the microbiome in the first few years of life. "Let Them Eat Dirt" talks to parents and parents-to-be in a friendly, often entertaining, non-judgemental way about medical and lifestyle choices that may influence their child's microbiome. The authors wisely acknowledge throughout the book that microbiome research is still in its "infancy", but suggest that some findings can be used to make lifestyle choices.

The book begins with introducing the reader to the idea that not all bacteria are pathogens and the importance of the human microbiome. The authors then tackle topics ranging from birth and first foods to antibiotic use and the importance of playing in nature. While "Let Them Eat Dirt" covers topics typically covered in pregnancy/first year of life books, it addresses these topics in the context of the microbiome. The last few chapters focus on non-communicable diseases, obesity, diabetes, allergies, autism, and intestinal diseases, and the influence of the microbiome.

While I think this book is a good introductory book for anyone concerned about the first few years of a child's life, I worry a bit about some of the suggestions. In particular, my concerns focus on the importance placed on probiotics and some of the discussion in the section on mood disorders. See my blog post for more of those thoughts. I do believe that the microbiome field and probiotics research are making great strides. However currently, without clinical trials and more information about an individual person's microbiome, many of the probiotics are ineffective and can't replace healthy eating and more natural practices like vaginal birth and breastfeeding if possible. "Let Them Eat Dirt" stresses the importance of healthy eating throughout pregnancy, but especially during the last trimester in preparation for birth.

For childbirth educators, the chapters on pregnancy, birth, and first foods are perhaps most important. Overall, the information is solid. I appreciated that the authors specifically mentioned their and others' skepticism about the research on the placental microbiome. At this time, the evidence for a healthy baby's microbiome being first established in utero is weak. Instead, microbiome establishment is thought to be at birth. "Let Them Eat Dirt" is very realistic about the fact that even the best-laid birth plans go awry, so the authors discuss vaginal swabbing in the event of a cesarean section. They balance the discussion on seeding with the importance of Group B Strep testing as part of a "seeding plan". I'm uncertain about the information they present on vaginal probiotics to prevent GBS. While a probiotic vaginal suppository containing Lactobacillus crispatus is in phase 2 clinical trials for bacterial vaginosis, it has not been tested for GBS prevention. However, given that not all women are colonized by Lactobacillus crispatus, I think more work needs to be done before this is a routine suggestion. Overall, the authors are adamant that pregnant people follow their health care provider's recommendations concerning antibiotic use if there is risk of infection of GBS or other pathogens.

In "Let Them Eat Dirt", the authors discuss the importance of breastmilk for nourishing infants. I was surprised that human milk banks weren't mentioned as an option, especially for premature infants. While premature infants given some probiotics have decreased instances of necrotizing enterocolitis (NEC), there have been problems with probiotics contaminated with pathogenic fungi. In contrast, donations to the National Milk Bank are meticulously checked for contaminants and donors are extensively screened. While I agree, that breastfeeding isn't always easy and that formula supplementation may be needed or desired, the diversity and complexity of sugars present in today's formula pale in comparison to what is found in human breastmilk. Increasingly, research finds that it is the breast milk sugars that influence the bacterial community. When human milk sugars aren't present, then the bacteria, like Bifidobacterium infantis, that are thought to properly train the immune system, won't grow. While probiotic supplementation might help bring in probiotic strains that have been found to be beneficial, if those bacteria are not fed the proper sugars they die.

Overall, "Let Them Eat Dirt" is a great introduction to the importance of microbes for early childhood. The overall take-home messages are - let your kid get dirty, don't overuse antibiotics, and eat healthy. The authors relay the information in an interesting and approachable way for parents, childbirth educators, and others. The authors are overall cautious about much of the research and suggest that patients talk to their health care providers. While I personally feel like the emphasis on probiotics gets ahead of the current available products, I'm certain that the probiotics field will get there. However, there is still a great deal of research to be done.

Have you read "Let Them Eat Dirt"? Would you recommend it to the families that you work with? Let us know in the comments section of this post.

About Anne M. Estes Anne M. Estes, PhD is a postdoctoral fellow at the Institute for Genome Sciences in Baltimore, MD and a freelance science writer. She is interested in how microbes and their host organisms work together throughout host development. Anne blogs about the importance of microbes, especially during pregnancy, birth, first foods, and early childhood at Mostly Microbes.

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