Monday, September 26, 2016

“Let Them Eat Dirt” Book Review

I recently finished a book that just came out, called Let Them Eat Dirt: Saving Your Child from an Oversanitized World, by microbiology researchers Drs. B. Brett Finlay and Marie-Claire Arrieta. The publisher was kind enough to share an advance copy with me, and also put me in contact with Dr. Finlay to discuss the book. The book focuses on the gut microbe community, especially as it relates to pregnancy and childhood, so it fits right into the scope of BacterioFiles.

In many ways, Let Them Eat Dirt is similar to a book I read a couple years ago, Missing Microbes by Martin Blaser, and Dr. Finlay confirmed that that book was an inspiration behind this one, which was intended to be less academic and more accessible to non-biologists (and I would say it accomplished that goal).

This book is split into three main sections. The first discusses how important microbes can be and how it is practically impossible to avoid coming in contact with them, but that’s okay because the vast majority are harmless or even beneficial to health. It also gives some history of microbiology and of the science of human-microbe symbiosis.

The second section presents various ways that microbes affect babies, and vice versa. I learned some things in this section, and it was quite interesting. Starting with pregnancy, it discusses how the microbial community in the mother’s gut changes over the course of pregnancy, and actually comes to resemble microbe communities associated with obesity. Various things can affect the community composition even more, or disrupt it in harmful ways, including unhealthy diet, stress, antibiotics, and infection. This disruption can be harmful to the mother, but can also affect the baby before birth, and even throughout life after birth, since the mother’s microbes are the seed for the baby’s gut community. So it’s important for expecting mothers to take care of their microbes, and the authors offer suggestions for doing this, like getting a flu shot.

Then the book discusses the transfer of microbes from mother to infant during and after birth, another very interesting topic. This transfer can be disrupted in various ways, including by antibiotics or caesarian section birth, but is important, and the authors discuss the issues well and give advice on how to make good decisions. The next few chapters focus on how the gut community develops after birth: the importance of breastfeeding and the amazing properties of breastmilk, but also the difficulties that many encounter trying to breastfeed; the transition from milk to solid food and how best to introduce new foods to support a varied diet and avoid food allergies; and other related topics like when to use or avoid antibiotics, the effects of pets on microbes, and how to keep children clean but not too clean. I was pretty on board for most of this section.

The third section discusses the associations that scientists have been finding between microbes and various chronic diseases/conditions. There are chapters here discussing the connections between gut microbes and obesity, diabetes (types 1 and 2), intestinal diseases such as Crohn’s and irritable bowel syndrome, allergies and asthma, and even neurological conditions such as depression, autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD). As you might expect, the book links each of these conditions to microbes in the body, and discusses the research that has been done with each, then makes suggestions for how to approach and think about each of these conditions, especially as it relates to microbes.

The last two chapters are a little different: the penultimate is a strong message in favor of vaccines for preventable diseases, emphasizing their importance as a public health strategy and coming against the discredited link between vaccines and autism. This chapter also discusses recent research into how gut microbes can modify the effectiveness of certain vaccines, such as the flu shot; mice without gut microbes have much less response to it (as I’ve discussed on BacterioFiles in episode 196).

The final chapter wraps things up with speculation about how microbial communities could be an important consideration in healthcare of various kinds, from doctors prescribing specific diets to patients for the sake of their microbes, or modifying microbial communities directly with probiotics or antimicrobials (in even more targeted and effective ways than is done today), or developing more effective vaccines by taking microbes into account. It emphasizes that many aspects of research into the microbiota are in very early stages, so it’s premature to put too much faith in things like probiotics and fecal transplants to cure any and all diseases (and definitely don’t try fecal transplants at home!). But there’s a lot of promise for the future.

My Impressions
This book was entertaining and enjoyable to read, especially for a microbiologist. It seemed to explain microbiological concepts well, in a way that is accessible to scientists and non-scientists. And in addition to all the science in each chapter, there are anecdotes to give some personal touches, and at the end of each chapter is a section with advice for how to apply the concepts from the chapter.

For example, at the end of chapter 9, there’s a story about how one of the authors told her 3-year-old daughter about the “little bugs in her tummy” that were happy and friendly and colorful, and how they helped keep her healthy as long as she made sure to feed them with tasty foods like vegetables. What a great way to get children interested in taking care of their microbes!

I also really appreciated the way the authors repudiated various different forms of pseudoscience, like the Paleo Diet (the idea that we can know, and also follow the diet that our prehistoric ancestors ate, and this will result in optimal health for modern humans). They also mention the idea, most often encountered among anti-vaccine advocates, that infectious disease helps develop the immune system (and therefore should not be prevented by vaccines). In fact, while exposure to microbes is important in this respect, infection is often more harmful than helpful even beyond the short-term symptoms of the disease. And then there’s the whole chapter about vaccines that was right on target.

Broadly speaking, the book seemed to cover the science accurately and support it well, convincingly showing that our microbes are important for many aspects and that modern lifestyles in developed countries avoid microbes too much in many ways.

I did have a couple of complaints about the content though. For one, a lot of the research findings presented here are somewhat preliminary, much of it discovered in the past 5 or 10 years. Similarly, much of the research has been done in animals and not humans yet, so whether or not the results can be extrapolated to humans remains to be seen. Of the rest, much is observational and therefore shows only correlation, not causation—that is, it’s not known whether microbe changes cause health conditions, or health conditions cause changes in microbe communities, or a third issue causes both.

So I expect a lot of the findings are likely to be discredited as new studies are done in the next decade. So it’s important to be cautious with this research field and not put too much weight on a given study or finding. To the authors’ credit, though, Dr. Finlay mentioned that it was their intent that each claim they made should be supported by at least one published study, so the book is currently a good summary of the science on the issue, however much may become obsolete in the future. And the book does advise readers to stay abreast of the research, rather than assuming it is all settled.

Otherwise, the biggest problem I had with the book was the chapter on how microbes might affect neurological conditions such as ASD and ADHD, especially the section on ASD. It mentions some small studies and anecdotes of autism symptoms improving with treatments of antibiotics, fecal transplants, probiotics, and modified diets. It did try to make clear that the evidence for these treatments is not very good, and that it is not clear where causation lies regarding microbial changes and developmental changes, but overall it struck me as far too close to a vast number of pseudomedical treatments for autism, collectively known as “autism biomed.”

This term refers to treatment of children on the autistic spectrum using “supplements, pharmaceuticals, all-natural remedies, homeopathy, chelation, HBOT and special diets,” as one website describes it; overall, treatments with very little evidence of benefit and some with much potential for harm, to which parents, often desperate for any solution, subject their children. As you can tell, it sounds uncomfortably similar to what is mentioned in the book. My impression from speaking with the author is that this similarity was not intentional, and that the authors weren’t really aware of autism biomed or how this section of the book could be taken as supporting pseudomedical treatments. However, the similarity is there.

So I’d like to mention here some things that perhaps were not adequately emphasized in the book. There is no “cure” for autism; if you have children on the spectrum, accept them for who they are, rather than trying all sorts of unproven treatments to try to make them be “normal.” Taking care of their microbes is a good idea, the same as you would for any child, and if their behavior suddenly changes, consider the possibility that gut problems may be causing them discomfort and reducing their ability to cope with life.

But be aware that there are many unscrupulous healthcare providers that will claim to have all sorts of revolutionary treatments for autism outside the mainstream, so be skeptical of such claims and check whether they have good evidence to support them. Also, don’t go see a naturopath; their treatments are often not supported by evidence, aren’t helpful, and are potentially harmful. Check out Naturopathic Diaries, run by a former naturopath, or Science-Based Medicine for more info. (Update: Especially relevant is this article.)

So in sum, this book was well written, and for the most part a good presentation of the science of gut microbes and health. It gives plenty of good advice to consider, including the suggestion to keep up to date with research in the field, and offers a lot of hope for advances to be made in the future. So if it is a subject that interests you, and especially if you’re planning to have children (or even already have some), read it and consider the advice, but also consider that the science and the recommendations that follow from it may change significantly in the future.


  1. This good book, i need to read it next time, hehe

  2. This is a good book is useful for anyone who ignores the invisible life around us. I studied this book when I was pregnant, it was very important to me in this period of life. I loved to write essays for students I advise everyone, it is a high quality service that will make your essay at the highest level.

  3. Eating right is still the best thing to do if you want to stay healthy and in your correct weight range. Although the author did say some very interesting thing that provided plausible answers to questions on diet and its effects on the body. My fiancee was able to lose weight when he learned about the military diet that made a lot of people lose weight. Although the diet has both pros and cons, he stuck to it since it's just for a short duration. He was pleased that he was able to lose weight and keep it off. For more information on the diet, see this resource page: