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Chapter 6 of the book

The Gut That Expects Dirt

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.3 things to start with4 popular claims graded77 works cited

How the chapter opens

On paper, the Amish and the Hutterites look like the same experiment run twice. Both sects arose in Reformation Europe and later crossed to America. Both have large families and high rates of childhood vaccination. Both eat diets rich in fat, salt and raw milk, and both keep pets out of the house.

The difference is in the barn. The Amish "live on single-family dairy farms, and use horses for fieldwork and transportation." The Hutterites "live on large, highly industrialized, communal farms." Earlier surveys had found asthma about four times as common among Hutterite schoolchildren as among Amish ones.

What the chapter covers

The argument, the studies and the stories are in the book. Get the book.

  • ·What Your Body Expects
  • ·What It Gets Instead: An Empty Colon
  • ·A Cleaner Childhood
  • ·The Peanut Reversal
  • ·Fever and the Old Enemies

Mismatch card

The system

The gut's microbes and the immune system's learned tolerance.

What it expects

Tens of grams of fiber a day from many plants; microbes from siblings, animals, soil and food; early tastes of lifelong foods; fever and care while sick.

What it gets

About 17 g of fiber a day (the US adult average), small families and indoor lives, needless antibiotics, delayed allergenic foods until recently, and fear of the thermometer.

What happens in the gap

Fewer gut species, more allergy, autoimmune disease diagnosed in about one person in ten in the UK over two decades (mostly women), resistant bacteria and returning measles.

The handrail

More fiber from more plants, a daily fermented food, early peanut and egg, antibiotics only when needed, vaccines on schedule. (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.)

Strength of evidence

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. Trials show food tolerance can be taught and vaccines work; fiber's links are strong but observational; proof that lost microbes cause disease in people rests largely on mice.

What you've heard

Popular claims this chapter tests, each with the book's verdict and grade.

The protocol

Each action carries the book's evidence grade. How the grades work.

Start here

  1. Eat more fiber, from many kinds of plant

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

    How: beans or lentils most days (Chapter 5's third of a cup counts), whole grains instead of white, and a fruit or vegetable at every meal; frozen vegetables and canned beans count. A medium apple carries about 4 grams, so together these moves take a typical day from about 17 grams toward 25 or more. Build up slowly to limit gas. Why: it feeds the colon's bacteria, and high intakes are consistently linked to lower death rates.

  2. Add one serving of fermented food with live cultures a day, and build up if you like it

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

    How: a normal portion, such as a pot of plain yogurt, a glass of kefir, or a forkful or two of kimchi or sauerkraut from the chilled aisle; miso and most cheeses count too. Bread, vinegar pickles and heat-treated products carry no live microbes. Expect some bloating at first. Why: one small trial is promising, and cheap, old food has little downside.21

  3. Parents of babies: introduce peanut and well-cooked egg early, and keep them in the diet

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.for peanut in high-risk babiesProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.for egg and other babies
    • How: the main US and Canadian allergy societies advise introducing peanut and egg to all infants around six months, not before four, with no testing needed first. Once peanut is in, keep it going with a few small servings each week, using smooth peanut butter thinned into puree or cereal.
    • If high-risk: if your baby has severe eczema or a known food allergy, plan the first peanut feeding with a clinician, who may test first.
    • Choking and reactions: never give whole nuts to children under five, or peanut butter from a spoon or in lumps to children under four. Stop and seek care at any sign of a reaction.22 Why: LEAP cut peanut allergy by about four-fifths, and the protection lasted.

Then, pick from this menu

  • **Keep your vaccines up to date (StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.).** US advice has been changing since 2025, so check with your clinician or pharmacist. As of mid-2026 the adult schedule calls for:

  • **Treat the person, not the thermometer (StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.).** How: use acetaminophen (paracetamol) or ibuprofen when a fever makes you or your child miserable, not to hit a number. For children, give one drug at a time, do not sponge them or bundle them up, and do not use fever reducers to try to prevent seizures. Skip them "just in case" at vaccination (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.). Adults: no more than 4,000 mg of acetaminophen a day from all sources, cold remedies included. No aspirin for anyone under 16 unless a doctor prescribes it. In dengue areas, use paracetamol, not ibuprofen or aspirin, until dengue is ruled out.

  • **Rest, drink and stay home while feverish (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.).** No trial shows rest shortens a fever, but staying home protects others. If staying home costs pay, keep away from babies, older relatives and anyone pregnant, and wash your hands often.

  • **Know the red flags (StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.).** Get prompt help for a baby under three months at 38°C (100.4°F) or more, or three to six months at 39°C (102.2°F) or more; a fit; a rash that does not fade when pressed; a fever lasting five days or more; or anyone who is getting worse, confused, breathless, has a racing heart or weak pulse, or clammy skin. Fever in pregnancy, and fever after travel to a malaria area, even months later, also need prompt care.24

  • **Take antibiotics when you need them, and not just in case (StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism. for avoiding needless courses; EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested. that it protects your microbes long term).** How: do not ask for them for colds or flu, and never take leftover or borrowed pills. Experts are rethinking the rule to always finish the course, so ask your prescriber how long you need.25 Never refuse or stop an antibiotic a clinician says you need.

  • **Get outdoors, garden, and let children dig and live with animals (ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. for dogs and farm animals in a baby's first year; EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested. for soil and microbes).** How: for babies, a dog or farm animals in the first year if the family wants them; for everyone, a garden plot, a window box or a park. Why: early animal contact is linked to less asthma, and in adults a community-garden trial found a little more fiber and less stress. Wash hands before eating and after handling animals. If someone at home is already allergic to animals, do not get a pet just to prevent allergy.

  • **Keep washing your hands, with plain soap (StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.).** Handwashing prevents diarrheal illness. US regulators barred the common antibacterial ingredients from consumer washes after makers failed to show they were safe for daily use or better than plain soap.26

COVID-19 advice is in dispute; ask, especially at 65 or older or if your immune system is weakened. Outside the US, follow your national schedule. Why: about one American in three gets shingles, and in trials the shingles vaccine was about 97% effective in people over fifty.

If this is you

Over 50

move vaccines to the top of Start here: book the shingles series and a pneumococcal shot at your next pharmacy or clinic visit, then work on fiber. With stiff knees, a raised bed you can reach standing, or a walk in a park, does the outdoor step.

Tight budget

plain yogurt, oats, canned beans and frozen vegetables cover both food steps. Skip probiotic pills and greens powders.

Pregnant, or with a weakened immune system

ask about live vaccines, and call your clinician about any fever in pregnancy.

Short on time

the food steps need no extra time, only a different shopping list: yogurt or kefir at breakfast, a can of beans into dinner.

Irritable bowel syndrome, inflammatory bowel disease or a past bowel obstruction

raise fiber only with a clinician's advice.

Safety

Never stop a prescribed medicine or refuse a needed antibiotic without a clinician. People who are pregnant, over 65 or immunocompromised run a higher risk of Listeria from unpasteurized dairy and should avoid raw milk and cheese made from it.

The skeptic's box

What could overturn this chapter, from the chapter itself.

Microbiome science has a hype problem. Of 38 published studies that transplanted human gut microbes into rodents, 36 reported transferring a disease, a success rate the reviewers called "implausible." Much of the case that microbes shape mood or weight rests on such animals. The farm and pet studies are observational, and farm families differ in many ways. Bottling the old environment has failed so far: worm treatments for bowel disease faded in larger placebo-controlled trials, and a bacterial extract did not prevent wheezing illness in high-risk young children. Fiber's record is strongest in cohorts; two large trials of bran or a high-fiber diet did not stop bowel polyps from returning.27 And lowering a fever has not changed survival in human trials either way. The firmest ground in this chapter is the peanut trials, the vaccines, and antibiotic stewardship. What would change the picture: a randomized trial in which a microbial exposure, not a food, prevented allergy or autoimmune disease in people.

Go deeper in the reference library

The research this chapter was built from, as longer reference entries: each with its own mismatch card, graded recommendations and the case against it.

See also in the book

Sources behind this page

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Every work this chapter cites

The chapter's 27 notes cite 77 works, listed by note number. Each is also in the full source register.

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