Gut health & immune system - probiotics, prebiotics and nutrition

Our gut is much more than just a digestive organ - it is also known as the seat of the immune system. In fact, around 70 % of immune cells are located in the intestinal area. The trillions of microorganisms in our digestive tract, known as the intestinal flora or intestinal microbiome, are in active exchange with the immune system. A balanced microbial ecosystem in the gut helps to keep pathogens at bay and keep the immune system "trained"【2】【1】. Conversely, a disturbed intestinal flora (dysbiosis) can promote inflammatory reactions and is associated with certain diseases【1】【8】. Recent research even speaks of the gut-brain axis: a healthy gut also influences mood and stress resistance【16】. But how can we promote our gut health? This is where probiotics (beneficial bacteria), prebiotics (food for these bacteria) and a gut-friendly diet come into play. This article sheds light on how you can strengthen your gut flora and what impact this has on your immune system. (Note: claims about health effects are carefully researched, but many connections are not yet EFSA-confirmed and are still being researched).

Why the gut is so important for the immune system

The gut is lined with a mucous membrane, behind which is a dense network of immune cells (the gut-associated immune system). Intestinal bacteria constantly interact with this mucosa and the immune cells. Beneficial bacteria help to keep the mucosal barrier intact and compete with potentially harmful germs for space and food. Some even produce anti-inflammatory substances. For example, research shows that certain gut bacteria produce short fatty acids (SCFAs) such as butyrate, which inhibit inflammation and strengthen the intestinal barrier【7】【8】. The links between the formation of such fatty acids and anti-inflammatory effects have not yet been confirmed by the EFSA; further studies are required. In addition, beneficial microbiota promote the formation of regulatory immune cells (Tregs), which keep the immune system in balance【2】. Put simply, you could say: **Good gut flora = immune system in balance**. Experiments with germ-free mice prove that this is not just theory: without gut microbes, these animals develop an underdeveloped immune system【2】. The gut-brain axis, in turn, shows that the gut and psyche are connected - stress can weaken the gut barrier and, conversely, gut bacteria can influence mood via nerve signals and messenger substances (disturbed gut flora has been linked to anxiety and depression, for example【16】).

If the intestinal flora is imbalanced(dysbiosis), this can have a variety of effects. On the one hand, unwanted germs can multiply excessively and trigger inflammatory reactions. Such inflammations can occur locally in the intestine (e.g. in chronic inflammatory bowel disease) or become systemic. Chronic dysbiosis is associated with diseases such as irritable bowel syndrome, allergies and even autoimmune diseases【1】【8】. For example, patients with rheumatoid arthritis have been found to have altered intestinal flora; certain bacteria there could help stimulate the autoimmune reaction【8】. Important to emphasize: Research in this field is still young and complex, and causal relationships are difficult to prove. However, it is undisputed that the gut and the immune system are closely intertwined.

Probiotics - friendly helpers for the gut

Probiotics are living microorganisms (usually bacterial strains such as Lactobacillus or Bifidobacterium) which, when consumed in sufficient quantities, can bring health benefits【6】. They are found naturally in fermented foods: Yogurt with active cultures, kefir, sauerkraut, kimchi, kombucha or miso, for example. Eating such foods regularly provides the gut with "good" bacteria. There are also probiotic capsules available as food supplements that contain defined strains in high concentrations.

But do probiotics really work? Some studies show **positive effects**. Meta-analyses suggest that probiotics can slightly reduce the duration and frequency of colds【5】【9】. In a review of 6 clinical studies with over 1,500 participants, the regular intake of probiotics reduced the likelihood of catching a cold(upper respiratory tract infection) by around 23%【5】. In addition, the duration of the illness was reduced by an average of almost 2 days. Although this is not a miracle cure, it shows a connection between the gut and the immune system - because probiotics work in the gut, while the cold affected the respiratory tract. The intestinal bacteria presumably indirectly stimulate defense mechanisms throughout the body【17】(however, this link has not yet been confirmed by the EFSA; further studies are required). According to studies, probiotics are actually quite effective against diarrhea, especially traveler's diarrhea and antibiotic-associated diarrhea. Certain strains such as *Saccharomyces boulardii* or *Lactobacillus rhamnosus* GG have a proven effect here【10】. (However, there are no approved EFSA health claims for probiotics - with the exception of special lactic acid fermented yogurts that improve lactose digestion).

Important: There are hundreds of strains of probiotics and their effects are very **specific**. Not every probiotic helps with every problem. It often requires a high dose (billions of colony-forming units) and regular intake over a longer period of time. In addition, many added bacteria do not settle permanently - they exert an effect during their passage through the intestine, but disappear again when they are discontinued. Nevertheless, they can have a positive effect on the existing microbiome during this time. In general, taking probiotics is considered safe; side effects are rare and usually mild (e.g. temporary flatulence). However, people with severely weakened immune systems should consult their doctor before taking live cultures.

Prebiotics - food for the good intestinal bacteria

Prebiotics are indigestible food components - usually certain dietary fibers or plant-based carbohydrates - that serve as "food" for our health-promoting intestinal bacteria. Examples are **inulin** and **oligofructose** (e.g. in chicory, Jerusalem artichokes, onions), resistant starch (e.g. in cooled potatoes or green bananas) or beta-glucans (in oats). By eating prebiotic fiber, you selectively promote the growth of beneficial bacteria such as bifidobacteria and lactobacilli in the large intestine. These then produce important metabolites from the fiber, such as the aforementioned short-chain fatty acids (e.g. butyric acid), which have an anti-inflammatory effect and strengthen the intestinal barrier. Prebiotics therefore indirectly help the immune system by strengthening the "good" intestinal inhabitants. However, the correlation between the intake of prebiotic fibers and a stronger immune defense has not yet been confirmed by the EFSA; further research is needed.

A variety of foods provide such prebiotics: In addition to the aforementioned onions, garlic, leeks, asparagus, bananas, oats, pulses and barley, flaxseed, artichokes and salsify are also good sources. **Tip:** Incorporate a few of these gut-friendly foods into your diet every day to "feed" your microbes. It is important to increase your fiber intake slowly - especially if you are currently eating little fiber. Otherwise, there is a risk of increased bloating and stomach rumbling because the intestinal flora will suddenly "step on the gas". This is not bad in itself, but it is often unpleasant. Excessive flatulence can be reduced by slowly increasing your intake and drinking plenty of fluids.

There are also prebiotic supplements (powder or capsules with inulin, FOS etc.). These can be useful if you are not getting enough fiber from your diet. However, "real" food is usually preferable, as it also provides vitamins, phytochemicals and a variety of different prebiotics.

Build up intestinal flora - practical tips

After a course of antibiotics or if you have digestive problems, you often want to build up your intestinal flora. Here are a few tips for a gut-friendly diet and lifestyle:

• Eat a varied plant-based diet: **Variety is key** - different plants provide different prebiotics. Goal: "Eat the rainbow" - i.e. different colored fruits and vegetables, whole grain products, nuts, seeds, legumes. They all promote a diverse intestinal flora and provide you with fiber.
• Integrate fermented foods: Consume a small portion of natural probiotics every day - e.g. a glass of kombucha, a few forks of raw sauerkraut or kimchi, a piece of fermented vegetable or a bowl of yogurt (plain, without sugar). The microorganisms they contain can help to colonize the intestines or at least have a positive effect through their metabolic products.
• Reduce sugar and white flour: Harmful germs (e.g. some yeasts) thrive particularly well with high sugar consumption. Highly processed foods can also have an unfavorable effect on the intestinal flora (partly due to emulsifiers or additives). Try to make "real", unprocessed foods the basis of your diet and only enjoy sweets and white flour products in moderation.
• Stress management: Chronic stress can make the intestinal barrier more permeable via stress hormones (keyword leaky gut) and also change the microbiome composition. Autogenic training, meditation, yoga or simply daily relaxation phases can therefore indirectly help your gut. (Interesting fact: meditation has been shown to improve sleep quality and stress resistance).
• Moderate lifestyle: Sufficient sleep, regular exercise in the fresh air and plenty to drink generally support good digestion and immune function. Make sure you drink at least 1.5-2 liters of liquid (water or herbal tea) per day - fiber needs liquid to swell well and develop its effect. Studies show that people with chronic sleep deprivation are more susceptible to infections【12】.

Scientific insight: Such measures are effective - this is also backed up by research. A recent study found, for example, that a diet with daily fermented foods (such as yogurt or kombucha) significantly increased the diversity of the intestinal flora within just a few weeks and at the same time reduced certain inflammation levels in the blood【4】. The positive effects of a high-fiber wholefood diet and probiotic food can therefore be measured.

Some people have an intestinal flora test (microbiome analysis) carried out to find out which bacteria are predominant in their own intestines. These tests can provide interesting insights, but are still difficult to interpret. A "bad" microbiome profile does not automatically provide specific recommendations for action - apart from the general tips that we list here anyway. Although research is attempting to derive personalized nutritional tips from this, science is still in its infancy in this respect. It is usually sufficient to implement the general principles for a healthy gut (see above).

Leaky gut - fact or trend?

The term "leaky gut" is often used in health forums. What is meant is that the intestinal wall loses its barrier function and unwanted substances (e.g. bacterial components such as LPS) increasingly enter the body, which can lead to chronic inflammation【7】【8】. In fact, the intestinal barrier is a key factor: it determines what stays in the intestine and what passes into the bloodstream. An impaired barrier is associated with various diseases - from autoimmune diseases to depression. However, "leaky gut syndrome" is not a recognized clinical diagnosis, but rather a concept. Intestinal permeability can be measured using certain tests (e.g. lactulose mannitol test). Some studies show: Permeability can actually be increased with stress, an unhealthy diet (lots of fat/sugar) or chronic illness【7】【8】. What can be done about it? A lot comes down to diet and lifestyle again. Dietary fiber and prebiotics can strengthen the tight junctions (the "closing strips" between the intestinal cells), probiotics can reduce inflammation and thus indirectly protect the barrier【3】. Omega-3 fatty acids and certain amino acids (e.g. L-glutamine) are also being discussed as intestinal wall-strengthening agents - but official health claims are still lacking. It is clear that a "leaky gut" is a real phenomenon in the context of illness, but the blanket claim that all possible non-specific complaints come from leaky gut is not scientifically proven. It is better to focus on specific measures - namely those that generally promote healthy bowel function (as described above).

Conclusion: A healthy gut - a strong immune system

Our gut and our immune system form a single unit. By maintaining a gut-friendly diet and lifestyle, you are also indirectly doing a lot for your immune system. Probiotics (in food or as a preparation) can help to restore balance to a disturbed intestinal flora - for example after antibiotics or in the case of digestive problems. Studies show moderate but noticeable benefits: for example, fewer infections and a milder course of colds【5】【9】. Even more important is a permanent prebiotic diet, i.e. sufficient fiber from various sources. This creates an environment in which the "good" bacteria feel at home. It is also worth including more traditional fermented foods in your diet. They provide millions of beneficial bacteria and often have direct health-promoting properties (sauerkraut juice, for example, contains organic acids and some vitamin C).

Lifestyle factors** should not be neglected: Reducing stress, getting enough sleep and exercise also have a positive effect on the structure of the gut and immune system. Interestingly, there is a lot of overlap between what is good for the gut and what is generally considered healthy - no coincidence. Ultimately, the immune system *and* overall health benefit from a holistically healthy lifestyle. Supplements such as special pro- and prebiotics or so-called intestinal detox cures should be used with caution. Terms such as "eliminating harmful substances" or "detox cure" are popular, but the body detoxifies mainly via the liver and kidneys. The intestines support this through regular excretion - for which fiber is important. Instead of questionable intestinal cleansing cures (which can sometimes lead to nutrient loss or electrolyte imbalances), a continuous high-fiber, wholesome diet is the better way to achieve a "detox effect".

To summarize: **Take care of your gut and your immune system will thank you.** Many connections - for example between microbiome changes and certain diseases - are still being intensively researched【18】, but it is already clear that a diversely colonized, well-nourished gut is a protective factor for health. With a varied diet, probiotics and prebiotics as well as a healthy lifestyle, you can do a lot yourself to maintain your gut health. The reward is not only better digestion, less bloating and a better gut feeling (more intestinal well-being), but often also a more robust immune system that is better able to cope with infections and stress. When the stomach and gut are in balance, the whole person usually feels more balanced - it's not for nothing that the gut is said to be our "second soul". With this in mind: **Good nutrition, good bacteria, good defense!

*Disclaimer:* Information on the health effects of foods and supplements in this article is for information purposes only. Many effects - especially in the area of microbiome and immunity - are scientifically plausible, but vary from person to person and are not confirmed by the EU Health Claims Regulation. This article does not replace medical advice.

1. [1] Salvadori, M. & Rosso, G. (2024). Update on the gut microbiome in health and diseases. *World Journal of Methodology, 14*(1), 89-196. DOI: 10.5662/wjm.v14.i1.89196
2. [2] Belkaid, Y. & Hand, T.W. (2014). Role of the microbiota in immunity and inflammation. *Cell, 157*(1), 121-141. DOI: 10.1016/j.cell.2014.03.011
3. [3] Ghorbani, Z., Shoaibinobarian, N., Noormohammadi, M., Lober, U., Mahdavi-Roshan, M., Bartolomaeus, T.U.P., et al. (2025). Reinforcing gut integrity: A systematic review and meta-analysis of clinical trials assessing probiotics, synbiotics, and prebiotics on intestinal permeability markers. *Pharmacological Research, 216*, 107780. DOI: 10.1016/j.phrs.2025.107780
4. [4] Wastyk, H.C., Fragiadakis, G.K., Perelman, D., Dahan, D., Merrill, B.D., Yu, F.B., et al. (2021). Gut-microbiota-targeted diets modulate human immune status. *Cell, 184*(16), 4137-4153.e14. DOI: 10.1016/j.cell.2021.06.019
5. [5] Li, L., Hong, K., Sun, Q., Xiao, H., Lai, L., Ming, M., & Li, C. (2020). Probiotics for preventing upper respiratory tract infections in adults: A systematic review and meta-analysis of randomized controlled trials. *Evidence-Based Complementary and Alternative Medicine, 2020*, 8734140. DOI: 10.1155/2020/8734140
6. [6] Hill, C., Guarner, F., Reid, G., Gibson, G.R., Merenstein, D.J., Pot, B., et al. (2014). The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. *Nature Reviews Gastroenterology & Hepatology, 11*(8), 506-514. DOI: 10.1038/nrgastro.2014.66
7. [7] Bischoff, S.C., Barbara, G., Buurman, W., Ockhuizen, T., et al. (2014). Intestinal permeability - a new target for disease prevention and therapy. *BMC Gastroenterology, 14*, 189. DOI: 10.1186/s12876-014-0189-7
8. [8] Dawson, S.L., Todd, E., & Ward, A.C. (2025). The interplay of nutrition, the gut microbiota and immunity and its contribution to human disease. *Biomedicines, 13*(2), 329. DOI: 10.3390/biomedicines13020329
9. [9] Quick, M. (2015). Cochrane Commentary: Probiotics for prevention of acute upper respiratory infection. *Explore (NY), 11*(5), 418-420. DOI: 10.1016/j.explore.2015.07.012
10. [10] Ivakhnenko, O. & Nyankovskyy, S. (2013). Study of the efficacy of a new synbiotic composition in infants with rotavirus infection. *Pediatrics International, 55*(5), 639-647. DOI: 10.1111/ped.12099
11. [11] Black, D.S., O'Reilly, G.A., Olmstead, R., Breen, E.C., & Irwin, M.R. (2015). Mindfulness meditation and improvement in sleep quality and daytime impairment among older adults with sleep disturbances: A randomized clinical trial. *JAMA Internal Medicine, 175*(4), 494-501. DOI: 10.1001/jamainternmed.2014.8081
12. [12] Prather, A.A., Janicki-Deverts, D., Hall, M.H., & Cohen, S. (2015). Behaviorally assessed sleep and susceptibility to the common cold. *Sleep, 38*(9), 1353-1359. DOI: 10.5665/sleep.4968
13. [13] Langade, D., Kanchi, S., Salve, J., Debnath, K., & Ambegaokar, D. (2019). Efficacy and safety of Ashwagandha (Withania somnifera) root extract in insomnia and anxiety: A double-blind, randomized, placebo-controlled study. *Cureus, 11*(9), e5797. DOI: 10.7759/cureus.5797
14. [14] Mah, J. & Pitre, T. (2021). Oral magnesium supplementation for insomnia in older adults: A systematic review & meta-analysis. *BMC Complementary Medicine and Therapies, 21*(1), 125. DOI: 10.1186/s12906-021-03297-z
15. [15] Choi, K., Lee, Y.J., Park, S., Je, N.K., & Suh, H.S. (2022). Efficacy of melatonin for chronic insomnia: Systematic reviews and meta-analyses. *Sleep Medicine Reviews, 66*, 101692. DOI: 10.1016/j.smrv.2022.101692
16. [16] Foster, J.A. & Neufeld, K.A.M. (2013). Gut-brain axis: how the microbiome influences anxiety and depression. *Trends in Neurosciences, 36*(5), 305-312. DOI: 10.1016/j.tins.2013.01.005
17. [17] Sanders, M.E., Benson, A., Lebeer, S., Merenstein, D.J., & Klaenhammer, T.R. (2021). Shared mechanisms among probiotic taxa: implications for general probiotic claims. *Current Opinion in Biotechnology, 68*, 163-169. DOI: 10.1016/j.copbio.2020.10.006
18. [18] Campbell, I.W., Almamun, A., Aggarwal, S., & Esteve-Saavedra, J.M. (2023). Microbiota, gut-brain axis and innate immunity: therapeutic implications. *Frontiers in Immunology, 14*, 1124234. DOI: 10.3389/fimmu.2023.1124234