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Reference topic 12: The Gut That Expects Dirt

Gut microbiome health: fiber, dirt and what the evidence says

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.74 sources7 graded recommendationsTopic hub: Gut and immunity

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Khun Chang Khian is a rural Hmong village in northern Thailand, about an hour from Chiang Mai. Mae La is a camp for refugees from Myanmar, on the Thai side of the border. Researchers sampled the gut microbes of Hmong and Karen women there, in Minneapolis-St. Paul where many had resettled, and among US-born daughters of Hmong immigrants: more than five hundred women in all.1

The most revealing group was the smallest. Nineteen newly arrived Karen women were followed through their first six to nine months in Minnesota. All but one gained weight. They ate more protein and a narrower range of foods. Inside them, one bacterial genus (a group of related species) was pushing out another: Bacteroides displaced Prevotella, often with a tenfold jump in the ratio between the two.1 Elsewhere, Prevotella has been linked to carbohydrate-rich diets and Bacteroides to protein and animal fat.2

Mismatch card

The system

The gut and its roughly 38 trillion bacteria, which ferment what your own enzymes cannot digest.

What it expects

Tens of grams of fiber a day from many plants, a seasonal menu, and microbes from soil and food.

What it gets

About 17 g of fiber a day (the US adult average), a narrow and refined diet, courses of antibiotics, and sanitized surroundings.

What happens in the gap

Fewer species and less fermentation, with links to disease still being untangled.

The handrail

More fiber from more kinds of plant, fermented foods with live cultures, and hands in soil. (●●○)

Strength of evidence

●●○ The loss of microbial diversity with industrialization is well documented and fiber's health associations are strong, but proof that the lost microbes cause disease in people comes mostly from mice.

What this topic covers

  • ·Thirty-Eight Trillion Tenants
  • ·Fifteen Million Years of Company
  • ·The Most Diverse Guts Ever Measured
  • ·The Missing Food
  • ·Housekeeping Between Meals
  • ·A Gut That Changes in Days
  • ·Seed Oils and the Leaky Gut
  • ·Soil and the First Microbes

The practical version is in the book: Chapter 6, The Gut That Expects Dirt.

The handrail

Each recommendation carries the strength of the evidence behind it. How the grades work.

  1. Eat more fiber, from more kinds of plant

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Evidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism. for fiber·Evidence grade: Emerging. Early, small, animal-only, or actively contested. for the number 30

    Aim for at least 25 g a day. In meta-analyses the benefit was greatest at 25-29 g, and more may help.28 The US adult average is about 17 g.23 In the self-selected American Gut project, people eating more than 30 plant types a week had more of some fiber-fermenting microbes than those eating fewer than 10. But 30 was a comparison group, not a tested dose.70 Build up slowly to limit gas; with irritable bowel syndrome, inflammatory bowel disease or a past bowel obstruction, ask a clinician first.

  2. Add fermented foods with live cultures

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Trial participants worked up over four weeks to about six servings a day; start with one.36 Yogurt, kefir, most cheeses, miso and unheated fermented vegetables can carry live microbes; bread, vinegar pickles and heat-treated products do not.71 Expect some bloating at first.36 Pregnant people, adults 65 and older and anyone with a weakened immune system run a higher risk of Listeria infection and should avoid milk, yogurt and cheese made from unpasteurized (raw) milk.72

  3. Skip the probiotic aisle

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    unless a clinician recommends one for a specific reason.6260

  4. Take antibiotics when you need them

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Healthy microbiomes mostly, though not entirely, recover: a reason to avoid unnecessary courses, never to refuse a needed one (Topic 20).34

  5. Put your hands in soil

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Evidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. for gardening·Evidence grade: Emerging. Early, small, animal-only, or actively contested. for microbial effects

    Garden, grow food, let children dig, and wash hands before eating.5150

  6. Treat eating windows as optional

    EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.

    An overnight fast lets the migrating motor complex run. But in a 12-month trial of 139 adults, eating only between 8 a.m. and 4 p.m. added no significant weight loss to calorie restriction.73 Skip fasting if you take diabetes medication, are pregnant or have a history of eating disorders.

  7. Eat oily fish rather than fearing seed oils

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    The omega-3 index is the share of omega-3 fats in red blood cell membranes. In national data from 2021-2023, over half of Americans aged six and over had an index below 4%, a level the researchers called undesirable.74 Topic 40 builds the full food program.

The skeptic's corner

The case against this topic, from the research itself.

Microbiome science has a hype problem. Of 38 published studies that transplanted human gut microbes into rodents, 36 reported transferring a disease, a rate the reviewers called "implausible."55 Much of the case that microbes shape mood or weight rests on such animals.5657 Diversity is no simple health score: in healthy women, harder, slower stools went with more species.58 And consumer tests disagree. Given the same reference stool, the companies' results differed about as much as different donors' stools do.59 The Hadza are modern people, not ancestors. By one estimate, fewer than 200 still follow the traditional foraging life,18 and their diversity belongs to a whole way of life, hardships included, that few outsiders would choose (Topic 38).

Probiotic supplements have little to show in healthy people. A review of randomized trials found no effect on healthy adults' microbial diversity,60 and in a small study, probiotics taken after antibiotics delayed the gut's recovery.61 The American Gastroenterological Association backs specific probiotics only for a few narrow uses, such as warding off C. difficile, a gut infection, during a course of antibiotics.62 The clearest clinical proof that gut microbes matter is the fecal transplant. In a trial for recurrent C. difficile, one infusion of donor stool, after a short antibiotic course and a bowel wash, cured about four in five patients. A standard course of the antibiotic vancomycin cured fewer than one in three.63 The US Food and Drug Administration has since approved two fecal microbiota products to prevent the infection's return.64

Fiber's record is strong in cohorts but patchier in trials. In two large trials, a bran supplement and a low-fat, high-fiber diet did not stop colorectal polyps, small growths in the bowel lining, from returning over three to four years.6566 "Leaky gut" as sold online deserves the most doubt. A widely used commercial kit for measuring zonulin in blood does not detect zonulin, according to a study Fasano co-authored.67 Emulsifiers harm the gut in mice, but the human evidence is thin: in a small feeding study, one common emulsifier modestly reduced microbial diversity and caused mild abdominal discomfort.6869 The claim that seed oils damage the gut has not even that much human support (Topic 38 grades it).

Terms this topic defines

Related topics

Sources cited above

  1. 1.Vangay, P., Johnson, A.J., Ward, T.L., Al-Ghalith, G.A., et al. (2018). "US immigration westernizes the human gut microbiome." Cell, 175(4), 962-972.e10. https://doi.org/10.1016/j.cell.2018.10.029 (University of Minnesota and Chiang Mai University, samples collected in 2016 and 2017 in Khun Chang Khian, Chiang Mai province, Mae La camp, Tak province, and Minneapolis-St. Paul; 514 Hmong and Karen women plus 36 US-born European American women; 19 newly arrived Karen women followed for six to nine months)
  2. 2.Wu, G.D., Chen, J., Hoffmann, C., Bittinger, K., et al. (2011). "Linking long-term dietary patterns with gut microbial enterotypes." Science, 334(6052), 105-108. https://doi.org/10.1126/science.1208344
  3. 55.Walter, J., Armet, A.M., Finlay, B.B., and Shanahan, F. (2020). "Establishing or exaggerating causality for the gut microbiome: lessons from human microbiota-associated rodents." Cell, 180(2), 221-232. https://doi.org/10.1016/j.cell.2019.12.025
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  9. 60.Kristensen, N.B., Bryrup, T., Allin, K.H., Nielsen, T., et al. (2016). "Alterations in fecal microbiota composition by probiotic supplementation in healthy adults: a systematic review of randomized controlled trials." Genome Medicine, 8(1), 52. https://doi.org/10.1186/s13073-016-0300-5 (seven trials)
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  13. 64.US Food and Drug Administration (n.d.). REBYOTA (product page; approval letter dated 30 November 2022). https://www.fda.gov/vaccines-blood-biologics/vaccines/rebyota; US Food and Drug Administration (2023). FDA Approves First Orally Administered Fecal Microbiota Product for the Prevention of Recurrence of Clostridioides difficile Infection (Vowst, 26 April 2023). https://www.fda.gov/news-events/press-announcements/fda-approves-first-orally-administered-fecal-microbiota-product-prevention-recurrence-clostridioides
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  23. 71.Marco, M.L., Sanders, M.E., Gänzle, M., Arrieta, M.C., et al. (2021). "The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods." Nature Reviews Gastroenterology & Hepatology, 18(3), 196-208. https://doi.org/10.1038/s41575-020-00390-5
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  28. 73.Liu, D., Huang, Y., Huang, C., Yang, S., et al. (2022). "Calorie restriction with or without time-restricted eating in weight loss." New England Journal of Medicine, 386(16), 1495-1504. https://doi.org/10.1056/NEJMoa2114833
  29. 74.Powers, C.D., Sternberg, M.R., Mineva, E.M., Clifton, C.C., et al. (2026). "Over half of the United States population had an undesirably low omega-3 index based on erythrocyte membrane measurements: results from the cross-sectional NHANES from August 2021 to August 2023." Current Developments in Nutrition, 10(6), 107715. https://doi.org/10.1016/j.cdnut.2026.107715

This topic carries 74 notes in the research manuscript; the 29 above are those behind the material on this page. Full source register.