• Repair is a phase, not a supplement. It works because the earlier three steps removed what was damaging the lining in the first place.
• The gut lining renews every few days. Research puts full renewal of the small intestinal surface at roughly two to six days.
• You are ready to repair when digestion has calmed, bowel habit has settled, and food reactions have eased.
• Glutamine has mixed evidence. A 2024 review found no overall effect on gut permeability, though one trial in post-infectious IBS was strongly positive.
• Fibre and resistant starch provide what’s needed for your own bacteria make butyrate.
• Vitamin D supports the barrier through the vitamin D receptor in the gut lining. Test rather than guess.
• Demulcent and vulnerary herbs have a long clinical history here: marshmallow root, chamomile, calendula, liquorice.
• Repair stalls if the irritation returns. Going back to step one is often more useful than adding another supplement.

This is the last post in the 4R series on gut health. The four steps run in order: remove the triggers, replace what digestion is missing, reinoculate with helpful bacteria, then repair.

Repair is the step people want to start with. It is also the step that works the least well on its own.

In my years of experience in clinical practice, I’ve noticed that the people who see the most from this last phase are the ones who did the first three first, properly.

What follows is education rather than a prescription.

The gut lining is a single layer of cells, held together by protein seals called tight junctions. A soft mucous layer sits on top of it. Below it sits a large population of immune cells.

When that lining is irritated, the seals loosen. More material passes through than should. This is what people mean by a leaky gut.

Leaky gut is not something your doctor will describe, as it’s not a medical diagnosis. But, the underlying process, increased intestinal permeability, is well described in research. 

Repair means three things happening together. The intestinal cells renew. The seals between them tighten. And, the mucous layer rebuilds.

None of that is something you do to the gut. It is what the gut does, once the internal conditions allow.

This is the part most people find encouraging.

The intestinal lining has the fastest turnover of any tissue in the body. New cells are made in small pits called crypts, then travel upwards to the surface. Researchers put almost complete renewal of the small intestinal surface at around two to six days.

So the cells themselves are replaced within a week.

The wider picture takes longer. Rebuilding the mucous layer, calming the immune tissue underneath, and restoring the bacteria that support it all move more slowly. In clinic I usually think in terms of six to twelve weeks for the whole phase.

That gap matters. It explains why people feel some change quickly, then plateau. The cells turned over. Everything around them is still catching up.

Moving to this step too early is the most common mistake I see. Here is what suggests the ground is ready.

Digestion feels calmer after meals. There is less bloating, less urgency, less discomfort an hour or two after eating. Bowel habit has become more predictable. Foods that caused trouble are causing less.

If none of that has changed, the irritation is still there. Adding repair nutrients on top of ongoing damage is like re-plastering a wall while the roof still leaks.

Go back to remove first. Common culprits are alcohol, frequent anti-inflammatory painkillers, unmanaged stress, a very low fibre diet, and untreated gut infections.

Nothing in this phase replaces what you eat every day.

The lining is built from what you give it. Protein supplies the raw material for new cells. Colourful food across the week brings the vitamins and plant compounds involved in repair. Fibre feeds the bacteria that support the whole structure.

A simple pattern works: half the plate vegetables, a quarter protein, a quarter whole grains or starchy vegetables. Oily fish twice a week.

I have written separately about the micronutrients involved, with meal ideas, in Heal Your Gut Lining Naturally. 

Glutamine is the amino acid most associated with gut repair. It is the main fuel for the cells lining the small intestine, and demand for it rises during illness and injury.

The clinical picture is more mixed than it is usually presented. A 2024 review pooling ten trials found no overall effect of glutamine supplements on intestinal permeability. A subgroup analysis suggested higher doses over short periods may do more, though the trials pooled were small and varied.

Against that, a randomised trial in people with post-infectious IBS found a large benefit. Participants took 5g three times daily, for eight weeks. Permeability normalised in the glutamine group and not in the placebo group. Bowel frequency and stool form both improved.

This suggests that glutamine suits some situations better than others. It looks most useful where the gut lining has taken a specific hit, such as after an infection. It looks less useful as a general tonic.

Practical note: trials in this area have used 5g three times daily. It is well tolerated in short courses. Bone broth and slow-cooked stews supply glutamine in food form, which suits long-term use better than powders.

Butyrate is a short-chain fatty acid made by bacteria in your colon when they ferment fibre. It’s the preferred fuel for the cells lining the large intestine.

A 2025 review sets out its role in barrier function and immune regulation, and describes resistant starch and fibre as the main drivers of its production.

The bacteria responsible are mainly Faecalibacterium and Roseburia. They ferment fibre in your colon and produce butyrate as a byproduct.

When researchers tested butyrate directly on human colon tissue, it did not prevent induced leakiness, despite strong results in cell and animal work.

The dietary route may be more reliable. Vegetables, beans, lentils, oats and whole grains all provide the fibre your bacteria need. Resistant starch from potatoes, rice or pasta cooked, then cooled, adds to it. Fermented foods help to support those bacteria too.

If you’ve had antibiotics recently, the long-term effects on gut bacteria can affect how well this step works.

Vitamin D behaves more like a hormone than a vitamin. The cells lining the gut carry a vitamin D receptor, which is how it influences this tissue directly.

Reviews describe vitamin D signalling as supporting tight junction structure and reducing inflammation in the gut lining. Low levels are common in inflammatory bowel disease, though much of the barrier work is in laboratory and animal models rather than trials in people.

Low vitamin D is common through a UK winter, regardless of gut health. A vitamin D test helps you to choose the right dose for you. I’ve written more on vitamin D in a UK winter.

If you take thiazide diuretics, digoxin or long-term steroids, check with your GP before supplementing.

Herbalists have used two groups of plants in this phase for a long time.

Demulcents are rich in mucilage, a soluble fibre that becomes gel-like in water. They coat and soothe irritated surfaces, and they also feed gut bacteria. Marshmallow root and slippery elm are the classic examples. Psyllium seed behaves similarly. Take all of these with plenty of water so the fibre can swell properly.

Slippery elm’s wild populations have been depleted, so choose a supplier who can tell you the bark was sustainably harvested. Marshmallow root does much the same job and is easily cultivated.

Vulneraries are the tissue repair herbs. Calendula, chamomile and gotu kola all belong to this group, and all three have a long history in inflamed gut tissue. Chamomile is the easiest to use daily, as a good quality loose leaf tea.

Liquorice is also traditional here, and it soothes irritated linings. It needs more care than the others. Regular use can raise blood pressure and lower potassium, so keep it to short courses. Deglycyrrhizinated liquorice, sold as DGL, avoids that effect.

Astringent herbs have a place too, particularly where there is looseness and irritation together. Green tea, raspberry leaf and ribwort all tighten and tone surface tissue.

Most of the research on turmeric and gut health uses curcumin, an isolated extract, not the culinary spice. The two are not interchangeable.

The irritant is still there. Alcohol, painkillers, ongoing stress or an untreated infection will keep undoing the work.

Fibre went up too fast. A sudden jump causes bloating and wind, and most people stop at that point. It’s better to build up gradually.

The bacteria were never rebuilt. Repair depends on the microbes from step three doing their part.

Or, it simply hasn’t been long enough. Six weeks is not long for tissue that has been irritated for years.

Some things need your GP, not a home approach.

Blood in the stool, unexplained weight loss, a persistent change in bowel habit, ongoing pain, or fever all need assessment. So does anything that came on abruptly and hasn’t settled.

Ongoing gut trouble in your forties and fifties needs looking into. The gut ecosystem changes with age, and what worked at thirty may not be enough now.

If you take prescribed medicines, some herbs interact with them. Liquorice with blood pressure medication is one example. That’s something to check with a herbalist who knows your case.

In that order, each step makes the next one easier. If you skip ahead, repair has to do work that belongs to the earlier steps.

Your gut lining is designed to renew. Most of what you’re doing in this programme is creating the conditions for it.

If you’d like help working out where you are in this sequence, a consultation is a good place to start.