"Dirt is Good - Jack Gilbert and Rob Knight (2017)" Summary, comments, opinion on the book. (Jul 2018)

Michael Harrop

Active member
Jul 6, 2023
3rd and final book of my research into the "eat dirt" recommendations. This book was by far the most infuriating one. I now consider this chapter to be closed and debunked. This nonsense stems from all 5 authors clearly having cognitive deficits in the areas of judgment, evaluation, reasoning, problem solving, and likely cognitive flexibility. Being able to analyze, associate, compare, and reach rational conclusions.


I had previously seen Rob Knight give a talk with the same title and remember the "dirt" part being mostly clickbait, while the actual talk contained good information (think it was this one www.youtube.com/watch?v=mlTFbuVvMU0), so I was expecting the same from this book.

Unfortunately, they go even beyond the irresponsible and dangerous advice of "Let Them Eat Dirt Saving Your Child from an Oversanitized World (2016)" by B. Brett Finlay, PhD, and Marie-Claire Arrieta, PhD, and literally recommend that children eat dirt. The valid reasons to not eat dirt, that are given by the previous authors, are ignored.

For most of the book, dirt is only indirectly addressed, and only opinion, no facts. It gets addressed directly in the last chapters. It seems based on the idea of "germ-free mice". And in my opinion completely fails to differentiate between dirt/environmental microbes vs host-native microbes. And thus jumps to conclusions & advice that seem premature, dangerous, and wrongheaded. The lack of understanding of the differences between host-native vs environmental microbes is egregious and inexcusable for "top" microbiome researchers.

Plenty of good information on various topics but I disagree with some of their statements such as downplaying the benefits of breastfeeding & diet, and ignoring the impacts of unhealthy parents on the development of the child. See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4850918/ and https://humanmicrobiome.info/maternity/

Largely agrees with the info in my probiotic guide. But makes a statement about s.boulardii & antibiotic-associated diarrhea that is contrary to the evidence I've seen.

Mixes opinion with fact, and I disagree with much of the opinion. For instance when it comes to parents chewing their child's food, and lack of dirt/environmental microbes being the reasons for declines in human health.

Sometimes they do a good job of saying "this is only correlation" or "there is no evidence for this opinion of ours", and other times they do not.

Again ranking allergy and asthma higher than the plethora of diseases from infectious microbes.

Unsupported, and in my opinion, ignorant statement of "extremely unlikely to be any dangerous microbes in a public restroom". We know from FMT how easy it is to transfer pathogens (including many unidentified ones). Any unhealthy people who used the restroom could have (and likely did) left pathogens anywhere in the restroom via a variety of ways [1][2], including aerosolized particles, which are known to be spread via toilet flushing. Their reasoning is that a study found that fecal bacteria were mostly confined to the stall, and the toilet seat mostly contained skin bacteria. Neither of these provide enough support for their statement or refute my criticisms, and there exists contrary evidence.

What seems to be an irrational, non-evidence-based statement of "raw meat bad, animal poop bad, dirt good".

The Hadza for instance, do not follow these guidelines, and regularly consume gut microbes/poop from animals, and raw meat too: https://archive.ph/MFbyt#selection-2587.0-2587.1

Yet the Hadza lifestyle seems to be what they're promoting throughout the book.

"Don't worry about germs in the subway. Let your kid eat dirt."

I can't help but be severely angered by this. It seems they actually believe this rather than just trying to sell a book. Are they so ignorant/arrogant that they conclude we have a good understanding of the ratio of beneficial to harmful microbes in dirt? You can see here https://humanmicrobiome.info/testing/ that is definitely not the case. Additionally, it's looking more and more like infectious microbes play a major role in most human diseases which are currently beyond medical capabilities/knowledge: https://humanmicrobiome.info/intro/

I lived more or less according to their principles growing up. I remember biting my toenails as a kid. I have no allergies but I have many other health problems. Which microbes & behaviors caused what results? We virtually have no clue and it's extremely irresponsible for them to be pretending like they know it's not only safe but beneficial.

There is no backing for their general ethos of "farm/rural living people are all around healthier", especially not due to literally eating dirt. And the discussion section of this 2014 review paper "Rural and urban microbiota - To be or not to be?" provides contrary arguments: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4153773/

Where are all the studies identifying and comparing beneficial vs harmful microbes picked up from soil/poor hygiene? Where are all these studies showing eating handfuls of dirt is safe and equally effective to FMT???

Certainly not in this book, and from my knowledge, not existing at all. And likely not even possible at the moment.

Why are there no repercussions for degree holders who spread dangerous, non-evidence-based "information" like this?

The worst thing about all this might not even be the fact that they're spreading dangerous advice, but rather sidestepping the real sources of the problems and thus falsely assuring people that B can fix the problem, even though the real fix is A. Thus, A is continued to be ignored and the problem continues to worsen.

Rob Knight was even one of the authors of a rat study showing long-term damage from antibiotics that could not be reversed with FMT: https://genome.cshlp.org/content/20/10/1411.long. And his is not the only one: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5395657/. There is a TON of evidence in the literature for FMT being extremely beneficial, and I've seen virtually none for eating dirt. Yet somehow eating dirt is their solution/conclusion when not even FMT can fully restore the damage..... Incredible. https://en.wikipedia.org/wiki/Compartmentalization_(psychology)?

They admit in the book that the developmental period is extremely important, and that introducing the microbes later on cannot reverse the damage done in developmental phases. The problem, and how to avoid it is glaringly obvious, but it would require a major shift in the current ethos of childbearing, and this shift is disliked by most people (and also not recognized/understood by most), so it gets completely ignored.

What is it you may ask? The mentality of "I want a child", without regard to the severity and importance of this decision on not only the life of the human being you will be creating, but on the society you will be changing.

Are my body and mind really in peak condition, enough to create a high-functioning, healthy human being who will not suffer from physical or mental disease? Do I have a good enough understanding of human health and development to make this judgment? Am I able to grasp the systemic outcomes of a majority of the population being unhealthy and continuing to have children?

These are questions every parent should be asking and severely considering. Of course this should be included in high school health classes when people are becoming able to have kids.

I talked about the self-perpetuating nature of this problem here: https://archive.fo/gYR2p#selection-6484.0-6555.1

This was missing from that discussion, but was indirectly implied: Dunning-Kruger effect: the more incompetent someone is, the less likely they are to realize how incompetent they are: http://theness.com/neurologicablog/index.php/lessons-from-dunning-kruger/

Antibiotics are extremely damaging, and a variety of poor health markers in the parents (especially the mother) are known risk factors for antibiotic use and c-sections. The current evidence pinpoints antibiotics given during c-sections to be the major cause of poor health outcomes in the child associated with c-sections https://humanmicrobiome.info/maternity/. Optional c-sections should be banned worldwide. And microbiome researchers should be on the forefront of this push rather than telling people they can just eat dirt and everything will be fine.

Antibiotics have also been abused for decades without fully understanding the long-term damage being done. It's absolutely absurd and alarming that the vast majority of the population have taken multiple antibiotics. And doctors are not even using them on an evidence-based basis https://www.washingtonpost.com/national/health-science/did-his-artificial-hips-put-him-at-risk-of-infection-when-he-saw-the-dentist/2016/10/07/1a0d4b54-60a5-11e6-9d2f-b1a3564181a1_story.html.
Our current medical "system" is an unsystematic joke https://forum.humanmicrobiome.info/threads/doctors-are-not-systematically-updated-on-the-latest-literature-what-t.27/.

Majorly contradictory quotes:​

"hand washing before eating is a good routine to get your toddler into and will greatly reduce the risk of a range of food-borne, fecal-oral, and respiratory diseases. Indeed, according to the Centers for Disease Control and Prevention good hygiene can prevent about 30 percent of diarrhea-related illnesses and 20 percent of colds"
"we have no evidence as to whether the exposure to other people’s oral microbiome could improve or impact her [child] health"
"shared drinking vessels can transmit colds and flu as well as more serious pathogens such as tuberculosis. This is why a major campaign to replace “the common cup” with drinking fountains was conducted in the early twentieth century, with admirable public health benefits"
"We know of instances where infants became infected after drinking water visited by a snake. Of course, your chances of being infected are greater if you handle the animal directly. This is common sense. You should always wash your hands after handling any wild animal because they can harbor dangerous pathogens (think Ebola, salmonellosis, and influenza) that can be transmitted to humans"
Yet they conclude (without any evidence/citations) that those same harmful microbes don't get left behind in dirt??? God I feel like shouting in their face "ARE YOU BLIND".

Regarding obesity transfer:
"we have lots of evidence hinting that it might be possible. For example, if you have many overweight friends, you’re likely to be fatter yourself. If you’re heavier, your dog is likely to be chubby too, and we know that humans and dogs exchange microbes with each other all the time. you can transfer human microbes into mice to make them fatter. That’s right: mice that get microbes from an overweight person become heavier. This even works if instead of transferring the poop, you grow hundreds of strains of bacteria from a single person’s feces, then transfer that into a mouse. This proves that the bacteria are doing the job—not viruses, not chemicals transferred along with the poop, not antibodies or anything else."
So they are naive enough to think this type of transfer of problematic microbes only happens with obesity??? Especially with the TON of evidence https://humanmicrobiome.info/intro/ demonstrating otherwise? YOU FOOLS!!! Not to mention the complete lack of addressing whether those obesity-inducing microbes are found in dirt, or where they come from. Some researchers are looking at a possible transferable virus as the cause [1][2].

"combination of C-section and antibiotics in early life increases the risk of obesity later on. This effect can be partly counteracted by breast-feeding and a diet consisting of a diverse array of brightly colored plants"
Nothing to do with dirt!!

"risks of FMT are considerable and the benefits are at this point unclear"
True regarding risks. FMT is only as safe as the donor is healthy. The benefits seem fairly clear, but there is a TON more supporting evidence for safety and efficacy of FMT compared to eating dirt. Yet these two "top" scientists somehow conclude the opposite. And without any citations!!

It can make parents feel helpless to not have answers. And it can also make a scientist feel powerful. Maybe I have partially formed answers. Maybe I can help this mother. That feeling can be intoxicating, and for some, it can lead to a kind of addiction. Such researchers will start making claims that cannot be supported by currently available data. To put it simply, they start making things up.This abuse of authority lets down the whole science profession and can deal a major blow to public confidence in science and the scientists who are working hard to advance understanding. The origin of many of these erroneous claims comes from a place of frustration. The science is exciting and the early findings suggest a major opportunity to improve individual health. So it is very tempting to go just that one step further and say that the results support such and such a treatment.
Ironic much?!

Original 18 Jul 2018.