Your gut lining is one cell thick, and it's the only thing between a colon full of bacteria and your bloodstream. Keeping it whole comes down to three things: feeding the lining, feeding the right bacteria, and digesting food properly before it gets there.
The short list
Ranked by how much each one moves
Match the fiber to your gutFermentable fiber feeds butyrate makers. Psyllium is the one for IBS and constipation.why ↓
Eat fermented food every dayIn a Stanford trial it raised gut diversity and lowered inflammation markers.why ↓
Skip emulsifiers and gumsThey thin the mucus layer that keeps bacteria off the lining.why ↓
Go easy on NSAIDs and alcoholBoth punch holes in the lining directly.why ↓
Keep your stomach acidAcid kills swallowed bacteria. Long-term acid blockers let them through.why ↓
Drink bone brothGelatin and glycine kept the gut lining intact in mice.why ↓
Glutamine after a gut infectionTightened a leaky gut and calmed IBS that started after food poisoning.why ↓
Mastic gum for indigestionEased bloating and upper-gut discomfort in a blinded trial.why ↓
Strong human evidenceSome human dataMostly animal / cell
A gut week
A fermented food daily: kefir, plain yogurt, sauerkraut or kimchi
Beans or lentils, onion and garlic, oats, berries most days (if they agree with you)
Cooked-then-chilled potatoes or rice a few times
A cup or two of bone broth
Ice cream, sauces and dressings with short ingredient lists
Coffee and tea. Their polyphenols feed gut bacteria too.
Watch out
Blood in stool, weight loss, night-time symptoms or a change after 45 need a doctor, not a diet
Get tested for celiac before going gluten-free. The test needs gluten in your diet.
Add fiber over 2–3 weeks. Going fast causes the bloating people blame on fiber.
Don't stop a prescribed acid blocker suddenly. Acid rebounds. Taper with your doctor.
How it fits together
The gut lining is a single layer of cells, stitched together by tight junctions and covered by a mucus layer that keeps bacteria at arm's length. Those cells replace themselves every few days, burning butyrate and glutamine to do it. Butyrate comes from bacteria fermenting fiber in the colon. Upstream, stomach acid, bile and enzymes break food down and kill most of what you swallow. If any one of the three fails, bacteria and their products get closer to the lining than they should, and the immune system reacts.
Real food with short ingredient lists supports all three
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1 · Digestion
Acid, bile, enzymes
Stomach acid kills swallowed bacteria and starts digesting protein. Bile and enzymes finish the job so less undigested food reaches the bacteria downstream.
A mucus layer keeps bacteria off the cells, and tight junctions (green) seal the gaps. Emulsifiers thin the mucus, and NSAIDs and alcohol open the gaps.
Bloating, irregular stools and low-grade inflammation: bacteria too close to the lining
Gut symptoms and whole-body inflammation often share a cause: the barrier.
Match the fiber to your gut
Fiber isn't one thing. Fermentable fibers (beans, lentils, onion, garlic, oats, cooled starch) feed the bacteria that make butyrate, which fuels colon cells, tightens the barrier and calms immune cells. Gel-forming fiber, like psyllium, holds water in the stool and barely ferments, so it helps constipation and IBS without much gas. Wheat bran is the fiber most likely to make IBS worse.
If beans and onions bloat you, that's often a sign of IBS or too many bacteria in the small intestine, not proof fiber is bad. Start with psyllium, cut the high-FODMAP foods for a while, and reintroduce them slowly.
How: about 25–30 g of fiber a day, built up over 2–3 weeks. For constipation or IBS, take about 2 rounded tsp of psyllium husk (~10 g) a day, split between meals, each dose with a full glass of water. For resistant starch, cook potatoes or rice and chill them overnight.
In a 10-week Stanford trial, 36 adults were assigned to eat either lots of fermented food or lots of fiber. The fermented-food group built up to about six servings a day of yogurt, kefir, kimchi, sauerkraut and kombucha. Their gut microbes became more diverse, and 19 inflammatory proteins in the blood went down, including IL-6. The high-fiber group didn't gain diversity in 10 weeks. Fiber probably needs the right microbes present first, and fermented food may help bring them.
How: start with one serving a day and build up: plain kefir or yogurt, a forkful of raw sauerkraut or kimchi (refrigerated, not shelf-stable, so it's still alive). Watch the sugar in kombucha and flavoured yogurt.
Many processed foods use emulsifiers to hold fat and water together. Common ones are carboxymethylcellulose (cellulose gum), polysorbate 80 and carrageenan. In mice they thin the mucus layer, let bacteria reach the lining and cause low-grade inflammation. A controlled feeding study then tested cellulose gum in healthy people for 11 days. It reduced gut microbe diversity and short-chain fatty acids, and in some people bacteria moved into the mucus layer, where they shouldn't be.
How: read labels on ice cream, creamers, plant milks, dressings, sauces and gluten-free baked goods. Traditional recipes use egg yolk, mustard or nothing at all.
Ibuprofen, naproxen and aspirin block the prostaglandins that keep blood flowing to the gut lining and keep its mucus coming. Camera-capsule studies find small-intestine damage in most regular users, not just stomach ulcers. Alcohol loosens tight junctions directly and lets bacterial toxins through to the liver. That's part of how drinking damages the liver.
How: use NSAIDs for days, not months, and take them with food. Paracetamol (acetaminophen) spares the gut but is hard on the liver at high doses. Keep drinking modest, with food.
Stomach acid is the first barrier. It kills most of the bacteria you swallow, starts protein digestion and releases B12 and iron from food. Proton-pump inhibitors (omeprazole and similar) are often taken for years when they were meant for weeks. Long-term use is linked to more C. difficile infections, bacterial overgrowth in the small intestine, and lower B12, iron and magnesium. Some of that link is confounding, since sicker people take them, but the mechanism is straightforward.
How: if you've been on an acid blocker for months without a clear reason, ask about tapering. Fix the triggers first: big late meals, alcohol, smoking and extra weight around the middle. For indigestion, mastic is worth a try.
Gelatin is about one-third glycine, with lots of proline and some glutamine, the raw materials for rebuilding the gut lining and its collagen underneath. In mouse colitis, bone broth kept the lining intact and the mucus layer thick, and cut inflammatory signals. There are no human gut trials yet. Broth also restores salt and fluid when you're sick, and it's cheap.
How: 1–2 cups a day, homemade from joints, knuckles, feet or oxtail with about 4 tsp of vinegar per quart, simmered 8+ hours. Good broth sets like jelly in the fridge.
Gut-lining cells burn glutamine as a main fuel. Most people make and eat plenty, so extra does little for a healthy gut, and pooled trials on "leaky gut" show no effect overall. The exception is IBS that started after a gut infection. In that trial, 5 g three times a day for 8 weeks tightened the leaky gut and relieved symptoms in 80% of patients, against 6% on placebo.
How: for post-infection IBS, 1 level tsp (5 g) three times a day for 8 weeks. Treat it as a course with a goal, not a daily habit.
Mastic is a resin from a Greek tree. In a blinded trial for functional dyspepsia (bloating, fullness and upper-gut pain without a found cause), 77% improved on mastic against 40% on placebo. Its role against H. pylori is weaker than often claimed. If you have ulcer symptoms, get tested and treated properly.
How: 350 mg capsules of Chios mastic, three times a day. Reassess after 3–4 weeks.
Polyphenol foods. Coffee, tea, berries, pomegranate and dark chocolate are mostly broken down by gut bacteria, not you, and they feed helpful species. Pomegranate needs the right bacteria to make urolithins. Pomegranate · Pu-erh
Chew well and slow down. Bigger food pieces reach the colon less digested and ferment there. It's free.
Greens with mould-prone foods. Chlorophyll binds aflatoxin in the gut. Chlorophyll
Probiotic pills after antibioticsIn a human trial they slowed the microbiome's recovery compared with doing nothing. Fermented food and time are the better bet. Fiber
Stool tests that "diagnose" SIBO or leaky gutSIBO is diagnosed with a breath test or a sample from the small intestine. The common zonulin "leaky gut" test was found to measure a different protein.
Wheat bran for IBSThe fiber most likely to make it worse. Use psyllium.
Colon cleanses and detox teasMostly laxatives. They strip the mucus and microbes you're trying to keep.
Signs worth reading
Bloating within an hour of eatingPoints to the small intestine: overgrowth, low acid or FODMAP sensitivity.
Iron deficiency with no clear causeTest for celiac disease and for blood loss.
Low B12 or magnesiumLong-term acid blockers, or low stomach acid. B12
Diarrhoea that started after food poisoningPost-infection IBS. Glutamine and psyllium have trials.
Pale, greasy, floating stoolsFat isn't being absorbed. Think bile, pancreas or celiac.
Blood, weight loss, waking at night with symptomsNot a diet problem. See a doctor.