"Let Them Eat Dirt Saving Your Child from an Oversanitized World (2016)" Summary, comments, opinion on the book. (May 2018)

Michael Harrop

Active member
Jul 6, 2023

My biases:​

I read this book as part of my research prior to a future in-depth post titled "do NOT eat dirt".

I think the "eat dirt" notion is dangerous misinformation.

I have been on raw diets including raw dairy & meat, including fermented meat. I probably wouldn't do it in the future, or recommend them.

I have no formal education beyond GED. I've been following the research daily for years and it seems that many doctors & researchers are not up to date with the research, and/or draw poor conclusions, often due to missing large pieces of the whole picture.

The authors are microbiologists. B. Brett Finlay, PhD, and Marie-Claire Arrieta, PhD.

General thoughts:​

This book has plenty of great information, but regarding the "eat dirt" stuff, it is in part clickbait, they contradict themselves, jump to conclusions based on low-quality correlations, and thus conclude/advise dangerous behaviors which are unsupported in the literature.

It could be that everything in this book turns out to be right (though already some has been proven false), but currently it is unsupported and dangerous.

Many people will just see the title and see that it's written by PhD holders, and take it at that. I've certainly observed people doing this.

The origins of the gut microbiome are currently not fully known. It is however very well known that many microbes cause disease, possibly even much more so than is currently acknowledged, due to current limitations in detection, sequencing, diagnostics, etc..

To start off they are working on the premise that the infant gut is sterile and is populated from the environment - ie: poor sanitation/hygiene. This is still being researched and may be false [1][2].

Starts off with the farm environment example (less asthma when growing up on farm) - being in that environment is not the same as eating dirt or practicing poor hygiene. Jumping to conclusions based on correlations.

Pretty much just misattributing missing microbes from antibiotic use, poor diet, lack of breastfeeding & possibly vaginal births, and unhealthy people having kids (generational compounding), to good hygiene.

Low-quality anecdotes regarding the farm kids being unsanitary - so how are we supposed to judge whether this helped or harmed them? We're presented with no information about their health.

Accurate statement regarding "good to use antibiotic for life-threatening bacterial infection, but bad idea to use antimicrobial hand sanitizers [such as triclosan] for daily home use".

Accurate statement regarding "have mostly been focused on killing harmful microbes without paying attention to the majority portion which are beneficial".

"Cooking good due to less infection; agriculture bad due to lowered diversity".
"Paleo diet theory isn't accurate".

Talks about FMT mouse study done at Cornell (Dr Ruth Ley) where the microbiota of a late-pregnancy female mouse was transferred to a germ-free mouse, the mouse gained weight without increasing food intake or being pregnant. Microbiota of 3rd trimester pregnant woman resembles that of an obese person.

Says "pregnancy is not the time to get dirty and eat dirt". Advocates for sanitary practices during pregnancy.

Compares antibiotics to cutting down a lush forest and only a few species making a comeback. But says the adult microbiome is stable and usually returns to normal (in non-pregnant woman). Concern during pregnancy is that the microbiota fluctuates and thus more susceptible to permanent damage.

Most recent national birth defects prevention study in the US (collecting data since 1997) showed that almost 30% of women receive at least 1 course of antibiotics during pregnancy. Same for British women. 42% of French and 27% of German women.

Mentions 2014 study (700 women in NY) that showed antibiotic use during pregnancy is associated with childhood obesity (85% increased risk by age 7). Controlled for other known associated variables.

Antibiotic use during pregnancy also associated with asthma, hay fever, and eczema in infants. Also IBD and diabetes. And affects immune function in children (mostly mouse studies).

States that risk factors for various diseases begin before we're born, according to the literature. Late stages of pregnancy are the most influential.

Acknowledges recent studies have shown that the womb is not sterile. And even if it is, bacterial metabolites could reach the baby and/or affect development.

"Dirt is a known source of pathogens, toxins, and even lead".

Says "it's not the c-section itself causing the disorders associated with it, but rather the lack of exposure to mother's vaginal & anal microbes". - more jumping to conclusions. See: https://www.the-scientist.com/?articles.view/articleNo/47600/title/Infant-Microbiome--Vaginal-Delivery-Versus-C-Section/ and https://medicalxpress.com/news/2018-05-swabbing-cesarean-born-babies-vaginal-fluids.html

"Approximately 6.2 million unnecessary C-sections are performed around the world, with Brazil, China, the United States, Mexico, and Iran accounting for 75 percent of them. Brazil and China have an outright C-section epidemic; many hospitals in those countries deliver more than 85 percent of their babies surgically."

Acknowledges environmental microbes are very different from mom's [host-native] microbes.

"Vaginal seeding makes sense and there's backing in the literature".

NEC (Necrotizing Enterocolitis) likely caused by gut dysbiosis.

Covers breast milk.

Comparison of children living a rual lifestyle in Burkina Faso, West Africa, to urban, city-dwelling kids in Italy. Diet vastly different. Gut microbiota of the Africans was more diverse. More likely to suffer severe infections and malnutrition, and have lower life expectancy. But have decreased risk of suffering from immune diseases.

Standard unsupported statements about fermented foods.

States that it appears that children who adopt an "adult-like" gut microbiome earlier have poorer health. This seems to contradict the "eat dirt".

Most antibiotics are/were derived from soil bacteria.

Possible permanent immune system shifts from antibiotics.

Admits that dogs can pass on diseases to their owners but says it's rare for dogs who are well looked after and receive regular veterinary care. "If the dog looks sick" could be dangerous.

Early exposure to dogs, but not cats, decreases risks of certain asthma and allergic diseases.

Lactobacillus johnsonii associated with improvement of asthma in mice.

The first hygienic measures took place through an organization known as the Sanitary Commission, which originated during the American Civil War. It was very successful in reducing infectious diseases and deaths by promoting washing the sick, along with their bed linens and their rooms. Back then doctors and scholars were just beginning to accept the concept that germs transmitted diseases.

Spectacular drop in childhood mortality from following hygienic practices.
Then goes on to contradict that by saying these hygiene practices are preventing children from being exposed to microbes essential for their development. Which is another jump to conclusions.

Handwashing is, without a doubt, the best hygienic practice that we can follow to prevent contracting and spreading infectious diseases. It’s been shown time and again that communities with good handwashing practices stay healthier, and no one should stop washing their hands just to promote more exposure to microbes.
Then goes on to, in my opinion, be contradicting, and make poor, unsupported statements.

Probably correct about antibacterial sanitizers not having everyday use.

"it pays to follow hygienic practices in order to reduce the risk of infection in heavily populated areas. This means that it’s a good idea to teach your children not to play on the floor in these places, nor to lick any surfaces, and to wash their hands (with regular soap and water) when they get home or before eating"

Possibly good advice to "get dirty and play in dirt" - but that's a big difference from eating it.

Warns against cat parasites transferable to humans via unsanitary practices.

More jumping to causation statements via correlation studies.

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 in contact with whoever eats them. The irrigation systems used to water many types of crops are known to contain dangerous pathogens, and washing fruits and vegetables is an effective way to significantly reduce the risk of foodborne diseases. The CDC estimates that about one in every six Americans get sick from food poisoning; each year 128,000 people are admitted to hospitals, and 3,000 people die from food poisoning. Thus, this is a serious risk that ought to be reduced by following hygienic practices.
Here's a recent example of this https://www.washingtonpost.com/news/to-your-health/wp/2018/06/02/five-dead-nearly-200-sick-in-e-coli-outbreak-from-lettuce-and-investigators-are-stumped/

"there’s no better way to influence the development of a diverse microbiota than through diet. Offering a healthy diet rich in vegetables and fiber is probably even more important than not being overly clean with babies and children."

Gut microbiome effect on body weight. With FMT, lean microbes win over obesity-inducing microbes. Antibiotics cause weight gain.

Historically it was assumed that malnutrition was the result of a lack of calories, and the solution was to simply provide more food. However, this solution often does not work (it has been tried many times by feeding children in impoverished areas without success). A study done a few years ago showed that if the children were treated with antibiotics first, then many more of them gained weight, hinting at the role of the microbiota. Experiments have been conducted in which feces were taken from Malawian twins, one of whom was extremely malnourished and the other not, and transferred into germ-free mice. Similar to the results of the obesity studies discussed earlier, it was found that this fecal transfer also transferred the malnourished characteristics to the mice, which strongly supports the idea that the microbiota has a large role in malnourishment.
Example of environmental microbes (dirt) causing a severe illness.

Metformin is a drug commonly used to treat T2D by lowering blood glucose levels. Despite being approved for human use and being used extensively, exactly how this drugs works is not known, but there are strong hints that it may act via gut microbes. If the drug is delivered directly into the blood (intravenously), therefore bypassing the gut, it doesn’t work. Also, the drug isn’t effective in mice treated with antibiotics. Metformin causes a profound shift in the gut microbe composition to a healthier profile, and at least one of these healthier microbes can be given directly to mice in order to decrease T2D.
I couldn't find the citation.

Leaky gut/intestinal permeability.

Colic - caused by gut dysbiosis in babies that starts at 1-2weeks of age, prior to colic starting. - Perhaps caused by poor sanitation, IE: environmental microbes/dirt? And/or perhaps caused by missing microbes and thus highlighting the importance of host-native microbes? Possibly unhealthy mother/dysbiosis in the mother since they gave an example of a breastfed baby's colic ending when switching to formula. Could be specific oligosaccharides in the breast milk feeding problematic microbes in the baby.


IBD - could very easily be caused by environmental microbes.

Talks about 2 IBD FMT studies. One doesn't work. 2nd one doesn't work until donor B shows up and then it's a success. - I couldn't find the study.

Hutterite vs Amish shows that it's not just as simple as "eat dirt" or "grow up in farm environment".

Don't rush out to the nearest farm if you’re a city dweller and are pregnant or have a young child and bed down with the cows. Some studies show that occasional visits to farms may actually exacerbate any pre-existing allergy tendencies

Good info on gut microbiome communication/effect on the brain.

BDNF, dopamine, GABA, G-CSF, serotonin, acetylcholine, and norepinephrine are all affected by the [gut] microbiota. All play major roles in our everyday brain and nervous system functions, and alterations in these neurotransmitters can lead to neurological problems and mental illnesses.

Good info on autism, but there is a parallel here to be drawn. The “eat dirt” thing seems similar to certain anti-vax sentiments, in that people have forgotten how prolific certain microbial illnesses used to be (and still are in many 3rd world countries due to poor sanitation).

Good info on probiotics & some info on prebiotics.

Good info on diet and person-to-person variation/uniqueness.

Original 31 May 2018.
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