Let Them Eat Dirt: Saving Your Child from an Oversanitized World

It may be the hottest area in health research going these days—the microbiome and its relationship to disease. Our microbiome is considered an organ in and of itself and encompasses the myriad of microbes that inhabit us both inside the gut and outside our bodies on our skin. These cells outnumber those that make up our human tissue. Let Them Eat Dirt by R. Brett Finlay and Marie-Claire Arrieta is one of the latest books to approach the topic. The primary purpose of the book is to introduce parents to the gut microbiome. The authors cover the microbes we host, how babies accumulate the microbes which comprise their microbiome, and the role of human microbes in the development of chronic diseases. I read the book to gain a better understanding of  gut/diet interaction. What I learned was more about parenting and less about diets than I had anticipated. My review will focus primarily on the dietary part of the book as it relates to chronic disease.

Drs. Finlay and Arrieta are well qualified to write this book both as intestinal microbiologists and as parents who must take their understanding of basic research and apply it to real life. Let Them Eat Dirt is the third in a series of popular books I have read in trying to learn about the practical implications of recent research in this area. Unlike the authors of Missing Microbes and The Good Gut, Finlay and Arrieta do not consider an understanding of gut microbes to be the answer to all of our modern health problems. They readily admit that the science of the gut is in its infancy and that many of the claims made about the gut and human health are open to interpretation. These authors are enthused about recent research and point to many potential, long-term implications for treating chronic diseases. They fail, however, to caution their readers that such claims also provide an opportunity for hucksters, profiteers and food companies to exploit news in this area.


Much of the book is aimed at how parents can provide an environment for their children to cultivate a healthy microbiome with a diverse set of microbes. At the top of the list is choosing a vaginal birth instead of a C-section when possible. If a C-section is necessary, a swab of the mother’s vagina should be applied to the baby’s mouth shortly after birth. Having pets and letting a child play in the dirt are also recommended. As we might expect, the authors steer us away from eating the Western diet, which is characterized by too much fat, sugar and refined grains. Too many processed foods, which according to the book have become a problem only in the last 30 years, have led to a much less diverse microbiome. Incorporating more fiber in the form of whole grains, fruits and vegetables is recommended.

Fermented foods are rarely mentioned in the book. Products of choice are yogurt and kefir. Parents are told that they should stay away from food products high in sugar and any that contain artificial sweeteners. No references are provided on the dangers associated with artificial sweeteners. It is apparently just assumed. Other author recommendations differ from what I was taught in my food microbiology courses. An accurate, but very brief, primer on how to avoid food poisoning is provided, but they also caution us against eating decaying vegetative tissue. Apparently, the authors are unaware of the difference between food-spoilage microbes and human pathogens. Many fermented foods around the world are controlled or uncontrolled, decaying vegetative tissue. More on this topic in next week’s blog. The authors might be surprised at my assignment in a graduate food microbiology class where Dr. Oblinger told us that to isolate our assigned food pathogens we needed to look in either feces, water or soil, another name for dirt.

Throughout the book, Finlay and Arrieta promote the use of probiotics, presumably in the form of pills or potions. Is it that easy to colonize the gut? If so, shouldn’t probiotic foods be a good idea? The effects of probiotic supplementation, however, do not appear to be prolonged unless administered regularly and routinely.1 A recent study shows that the microbial flora within an individual can vary widely within the course of day2 indicating that it might be more difficult to properly colonize our gut than we have been led to believe. In the book we learn of dangers of an unhealthy microbiome, particularly one that is not diverse, in the rise of Clostridium difficile—a devastating microbe that can colonize a compromised gut. Food scientists like me obsess over the dangers of food pathogens, but estimates by the CDC attribute 15,000 deaths a year to C. difficile, which are much higher than the 3,000 deaths a year by microbes causing food poisoning. The issue is a serious one, but somewhat overstated in Let Them Eat Dirt as over 80% of the deaths from C. difficile are among the elderly particularly patients in nursing homes being treated with antibiotics.

Let Them Eat Dirt advocates the “Eat clean, live dirty” attitude of our times. It poses the question “How sanitized should we and our kids be?” What I liked most about the book was its objectivity and not proclaiming that the research on the microbiome provides an explanation for every disease known to mankind. What gave me some pause about the book was a lack of knowledge about microbes that inhabit food and how they contribute to the microbes that inhabit us. I am happy to recommend the book to anyone interested in learning about the microbes our bodies share with our own cells, particularly to parents with young children or young couples planning on children in the future. I have no doubt that research in this area will have profound implications for health in the coming decades. At present it may be to early to draw definite conclusions. After reading the book will our reaction change the next time a child in our care raises a lump of dirt to their lips? Should we intervene or let it be? Is the lump teeming with pathogens that could make them sick or a diversity of microbes that could protect them from chronic diseases—or both?



1 Kristensen, N.B., T. Bryrup, K.H. Allin, T. Nielsen, T.H. Hansen and O. Pedersen, 2016. Alterations in fecal microbiota composition by probiotic supplementation in healthy adults: a systematic review of randomized controlled trials. Genome Medicine 8:52-62.

2 Kaczmarek, J.L., S.M.A. Musaad, and H.D. Holscher, 2017. Time of day and eating behaviors are associated with the composition and function of the human gastrointestinal microbiota. American Journal of Clinical Nutrition 106:1220-1231.

Next week: What kinds of microbes inhabit the food that we eat?


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