Every three to five days, the gut replaces the cells lining its inner surface. At the same time, trillions of microorganisms live in the large intestine, feeding on everything we cannot digest ourselves. What this system needs is less mysterious than the flood of gut-health advice suggests: research keeps arriving at the same building blocks – dietary fibre, plant variety, fermented foods, physical activity and a solid supply of micronutrients.
The gut lining renews itself constantly – and depends on its residents to do so
The inner lining of the gut consists of a single layer of cells. It has two jobs at once: absorbing nutrients and forming a barrier against everything in the gut contents that should not enter the body. To manage this, it works at a remarkable pace. Stem cells at the base of small pockets called crypts divide without pause; the new cells migrate upwards and are shed into the gut after three to five days (narrative review, Gehart & Clevers, 2019). Hardly any other tissue in the body renews itself this quickly.
In the large intestine, a second layer of protection is added: a mucus layer that keeps bacteria at a distance from the cell wall. This is where the partnership with microorganisms begins. They break down indigestible plant components and produce short-chain fatty acids such as butyrate – the preferred energy source of the cells lining the colon. What we eat therefore helps determine what these cells are supplied with.
Dietary fibre: without food, gut bacteria turn on their own protective layer
How close this relationship is was shown by a widely cited animal study from the University of Michigan. Mice whose gut had been colonised with a defined community of human gut bacteria received either fibre-rich or fibre-free feed. When fibre was missing, the bacteria switched to a different food source: the gut's own mucus layer. It became thinner, and pathogens reached the gut wall more easily (animal study, Desai et al., 2016). How directly this mechanism applies to humans is still being investigated – but the image of a "starving" microbiome has shaped the field ever since.
In humans, the benefit of a fibre-rich diet is well documented. A series of meta-analyses commissioned by the World Health Organization pooled around 185 observational studies and 58 clinical trials: people with the highest fibre intake had 15 to 30 percent lower all-cause and cardiovascular mortality than those with the lowest, with the clearest benefits seen from 25 to 29 grams per day (systematic reviews with meta-analyses, Reynolds et al., 2019).
The European Food Safety Authority (EFSA) considers 25 grams a day adequate for normal bowel function in adults; the Swiss Society for Nutrition recommends at least 30 grams. Reality looks different: according to the Swiss national nutrition survey menuCH, 87 percent of adults in Switzerland fall short of 30 grams (cross-sectional study, Schönenberger et al., 2024). A follow-up analysis shows where the intake comes from – around 4 grams at breakfast, 6 at lunch and 6 at dinner, mostly from grain products (cross-sectional study, von Blumenthal et al., 2025). Choosing wholegrain bread instead of a croissant, or a muesli with oats and linseed, already shifts this balance noticeably.
Not all fibre is the same:
| Type of fibre | Typical sources | What happens in the gut |
|---|---|---|
| Soluble, readily fermentable fibres (e.g. pectin, beta-glucan, inulin) | Oats, barley, apples, chicory, onions, leeks | Fermented by gut bacteria, producing short-chain fatty acids |
| Insoluble fibres (e.g. cellulose, lignin) | Wholegrain cereals, bran, nuts, vegetables with skin | Bind water and increase stool volume |
| Resistant starch | Cooled potatoes and rice, pulses | Reaches the large intestine undigested and is fermented there |
Why the fridge of all things turns potatoes into a source of fibre is explained in our article on resistant starch. If your diet has so far been low in fibre, it is best to increase the amount over several weeks and drink enough alongside: bloating is common at first, because gut bacteria produce gas as they break fibre down. With an existing bowel condition, any change of diet should be accompanied by a doctor.
Thirty plants a week: variety matters more than quantity
Each bacterial species has its own preferences. One prefers to break down pectin from apples, another fibre from pulses, a third starch from cereals. A one-sided diet therefore feeds only part of the community. This idea is supported by one of the largest citizen-science studies on the microbiome: in the American Gut Project, with more than 10,000 participants, people who ate more than 30 different plants per week had a markedly more diverse microbiome than those who ate fewer than ten (observational study, McDonald et al., 2018).
The number 30 is a guide rather than a biological threshold – but a surprisingly practical one. Every plant species counts: vegetables, fruit, pulses, whole grains, nuts, seeds, herbs and spices. A handful of mixed seeds in your muesli, fresh parsley over a risotto, lentils instead of pasta in a stew: even small additions push the weekly tally up. How fibre even keeps the immune system busy by way of gut bacteria is described in our article Dietary fibre – how nutrition trains the immune system.
In a Stanford study, fermented foods increased the diversity of gut bacteria
One result from Stanford surprised even the researchers involved. For ten weeks, 36 healthy adults either ate a particularly fibre-rich diet or considerably more fermented foods such as yoghurt, kefir, sauerkraut, kimchi and kombucha. In the fermented-food group, the diversity of gut bacteria rose steadily, and 19 inflammatory markers in the blood decreased. In the fibre group, diversity remained stable on average over this short period (randomised intervention study, Wastyk et al., 2021).
This is not an argument against fibre but for a division of labour: fibre feeds the bacteria already present, while fermented foods continuously bring in new microorganisms. In everyday terms, that might mean plain yoghurt or kefir at breakfast and a portion of raw sauerkraut as a side dish. The difference from canned goods matters: pasteurised products or those pickled in vinegar contain hardly any live cultures. Anyone who tolerates histamine poorly or needs to watch their salt intake should go easy on sauerkraut and kimchi.
Exercise changes the microbiome – as long as you keep at it
That exercise does the gut good is old wisdom. What is new is that the effect can be traced right down to the composition of the bacteria. At the University of Illinois, 32 previously inactive adults did endurance training three times a week for six weeks, with increasing duration and intensity. Their microbiome changed, and in participants of normal weight the amount of short-chain fatty acids in the stool increased. After training stopped, the changes largely reversed (intervention study without a control group, Allen et al., 2018).
A six-month randomised trial from Copenhagen with 88 people with overweight or obesity confirmed the direction: those who trained vigorously in their leisure time had slightly higher bacterial diversity after three months than the control group (RCT, Kern et al., 2020). Both studies carry the same message: what matters is not the single workout but regularity. Cycling briskly to work, taking the stairs instead of the lift, three endurance sessions a week – whatever can be built permanently into daily life is what counts.
Micronutrients: the gut lining needs a steady supply of building material
A tissue that renews itself every few days constantly uses up building material. Alongside protein and energy, this requires various vitamins and minerals. Vitamin A, the B vitamins riboflavin (B2), niacin (B3) and biotin, and the trace element zinc are involved in maintaining the mucous membranes – which include the linings of the mouth, the airways and the digestive tract. Zinc also plays a central role in cell division.
| Nutrient | Good food sources |
|---|---|
| Vitamin A (or its precursor beta-carotene) | Carrots, pumpkin, sweet potatoes, spinach, eggs, dairy products |
| Riboflavin, niacin, biotin | Wholegrain cereals, pulses, eggs, dairy products, nuts, mushrooms |
| Zinc | Pumpkin seeds, oat flakes, lentils, cheese, meat |
A varied, plant-focused diet generally covers these nutrients well. When supplementing, more is not better. High-dose vitamin A in the form of retinol is not appropriate during pregnancy, and zinc should not be taken above the recommended amount over the long term, as it can impair the absorption of copper.
Supplements: where bacterial cultures and prebiotics fit into the picture
The scientific community has agreed on a clear definition: probiotics are live microorganisms that, when taken in adequate amounts, confer a health benefit on the host (ISAPP consensus statement, Hill et al., 2014). A central point of this paper is that effects are tied to individual strains and cannot automatically be transferred from one strain to another. A systematic review of randomised trials found no consistent change in the stool microbiota of healthy adults from bacterial preparations – which is why research is increasingly looking at what the bacteria do metabolically rather than simply how many there are (systematic review, Kristensen et al., 2016).
The same expert body defines prebiotics as substances that are selectively used by microorganisms – classically fibres such as inulin (ISAPP consensus statement, Gibson et al., 2017). When both components are combined, the result is called a synbiotic: the bacterial cultures bring their own food with them. Such a preparation makes sense as a complement to a fibre-rich diet, not a replacement for it. How inactivated microorganisms differ from this is covered in our article on postbiotics.
Bacterial cultures from food and supplements are considered well tolerated by healthy adults; temporary bloating may occur in the first few days. Anyone with a severely weakened immune system, a serious illness, or who is pregnant or breastfeeding should check with a doctor before taking them.
Kultur 13
If you would like to add bacterial cultures to your diet in a targeted way, Kultur 13 offers a synbiotic formula: 13 different lactic acid and bifidobacteria strains with 6 billion CFU (colony-forming units) per daily dose, combined with the plant fibres inulin and dextrin as well as calcium. Laboratory-tested batches from GMP-certified manufacturing, vegan capsule shell, in brown glass.
Calcium contributes to the normal function of digestive enzymes.
Conclusion: above all, the gut needs habits
What the gut really needs can be summed up surprisingly briefly – and the evidence behind each element is at a different stage:
| Building block | In everyday life | State of research |
|---|---|---|
| Dietary fibre | At least 25 to 30 g per day, increased gradually | Well documented by meta-analyses of observational and clinical studies |
| Plant variety | Around 30 different plants per week | Large observational study; an association, not yet proof of cause |
| Fermented foods | Yoghurt, kefir or raw sauerkraut daily | Promising randomised study with a small number of participants |
| Exercise | Regularly, several times a week | Intervention studies and an RCT; moderate effects that depend on regularity |
| Micronutrients | A varied diet, supplementation where needed | Well-described physiology of the mucous membranes |
No single food and no single preparation replaces this interplay. Starting with breakfast and side dishes, regularly including fermented foods and moving several times a week creates the conditions in which the gut lining and the microbiome can sustain one another. Supplements can complement this foundation in a targeted way.
This article provides general information on nutrition, physical activity and the gut microbiota and does not replace medical advice, diagnosis or treatment. If you have health concerns, please consult a doctor or pharmacist.
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