It's a Tuesday evening in January, and Markus is sitting at the kitchen table. A glass of kefir sits before him, which he eyes skeptically. "For the lungs," his sister had told him when she brought him the fermented drink. Markus has suffered from asthma for years, uses his inhaler daily—and now a sour milk drink is supposed to help? What he doesn't know is that his sister is closer to the truth than he thinks. A connection exists between the gut and the lungs that medicine has only truly understood in recent years. An axis that determines how well we breathe—and how strongly our immune system protects us.
The story doesn't begin in the lungs. It begins deep inside our bodies, where trillions of microscopic inhabitants form a complex ecosystem: the gut. For a long time, the gut was considered merely a digestive organ, a kind of biological wastewater treatment plant. Today, researchers know that the gut is far more than that. It's a command center, a power plant, and a communication hub all in one. And it's in constant communication with an organ that, at first glance, seems to have nothing to do with it: the lungs.
When two worlds collide – The discovery of a silent partnership
This realization didn't come overnight. For decades, doctors observed a curious phenomenon: patients with chronic inflammatory bowel diseases suffered from respiratory problems more frequently than average. People with asthma showed changes in their gut flora. And after the administration of antibiotics – which massively disrupted the gut microbiome – lung diseases suddenly worsened.
What initially appeared to be a series of coincidences turned out to be a fundamental principle of our bodies. Scientists at the University of Marburg now describe it as the "gut-lung axis"—a bidirectional communication pathway through which both organs constantly exchange information. Metabolites, the metabolic products of gut bacteria, are transported to the lungs via the bloodstream. Simultaneously, signaling molecules from the lungs return to the gut. This ongoing dialogue can determine health or illness.
The gut microbiome – the community of bacteria, viruses, and fungi that colonizes our digestive tract – is far more than a passive companion. It is an active training partner for our immune system. From birth, these microorganisms train our immune cells to distinguish between friend and foe. They teach them tolerance to harmless substances – and simultaneously sharpen their response to real threats.
The gut microbiome is crucial for the development of the immune system. Changes in the composition of this microbial ecosystem not only affect the immune response in the gut itself, but also – via the gut-lung axis – in lung tissue. Researchers refer to this as an "immunological imprinting" that occurs in the first years of life and helps determine our later susceptibility to allergies, asthma, or other respiratory diseases .
The architects of the immune system – How gut bacteria shape the immune system
Deep in our abdomen, directly beneath the cells of the intestinal wall, lies one of the largest concentrations of immune cells in the entire body. It is estimated that around 80 percent of all immune cells are found here – more than in the bone marrow, lymph nodes, and spleen combined. The gut is not just a digestive organ. It is the headquarters of our immune system.
And here, in this microscopic arena, a daily training takes place that lasts throughout our entire lives. The gut bacteria produce substances that interact directly with immune cells. They send out signals, activating certain defense mechanisms and suppressing others. Particularly important in this process are the so-called short-chain fatty acids – butyrate, propionate, and acetate.
These molecules are produced when gut bacteria ferment dietary fiber. They are tiny, inconspicuous – yet of enormous importance. Butyrate, for example, serves as the most important energy source for the cells of the intestinal wall. It keeps the intestinal barrier intact and prevents harmful substances from entering the bloodstream. But its effects extend far beyond this. Butyrate regulates inflammatory processes and promotes the formation of regulatory T cells – those immune cells that curb excessive reactions and prevent autoimmune processes.
Acetate, in turn, supports the function of memory T cells, which recognize and quickly fight off familiar pathogens. During infections, the concentration of acetate in tissues rises sharply. And propionate? It protects nerve cells, lowers cholesterol levels, and modulates the activity of immune cells in the lungs. Yes, you read that right: in the lungs.
These short-chain fatty acids leave the intestines and enter the bloodstream, reaching all parts of the body – including the respiratory tract. There, they influence local immune cells, regulate inflammatory processes, and help determine how strongly the lungs react to allergens, viruses, or pollutants.
Studies show that people with a diverse gut microbiota that produces plenty of short-chain fatty acids are less likely to develop allergic asthma. Their airways react less severely to irritants. Conversely, asthmatics often have an imbalanced gut microbiota – a condition known as dysbiosis. The diversity of bacteria is reduced, and certain "good" strains are missing. Consequently, the signaling molecules that keep the lungs' immune system in balance are also lacking.
When the balance tips – dysbiosis and its consequences for the respiratory system
A characteristic of healthy lung flora is a wide variety of different bacteria. In chronic lung diseases such as asthma, this diversity is significantly reduced. However, the cause often lies not in the lungs themselves – but in the gut.
The question of whether changes in the microbiome trigger disease or vice versa has not yet been definitively answered. However, what is clear is that both organs influence each other. Dysbiosis in the gut can alter the immune response in the lungs. And respiratory infections demonstrably lead to changes in the bacterial composition of the gut.
This connection becomes particularly clear during times of acute stress. During the COVID-19 pandemic, researchers worldwide investigated the link between the gut microbiome and disease progression. The result was unequivocal: patients with a healthy, diverse microbiome – a so-called eubiosis – exhibited a controlled, appropriate immune response in the lungs. The production of interferons and the activation of T cells – all of these processes were coordinated.
In contrast, patients with dysbiosis more frequently experienced excessive reactions, the dreaded "cytokine storm," in which the immune system uncontrollably releases inflammatory messengers, thereby damaging the body's own tissue. The disrupted gut flora could no longer fulfill its regulatory function. The balance was thrown off.
Antibiotics also play a critical role here. While they save lives by fighting bacterial infections, they don't distinguish between "good" and "bad" bacteria. Antibiotic use can drastically reduce the diversity of the gut microbiome, indirectly weakening the lungs' defenses. Studies show that repeated antibiotic treatments in childhood significantly increase the risk of asthma. Immunological development in the first years of life is disrupted, with consequences that become apparent years later.
The Power of Fermentation – How Nutrition Strengthens the Axis
But there is also good news. The gut-lung axis can be influenced. And the key to this lies on our plates.
In recent years, an age-old process has experienced a renaissance: fermentation. Sauerkraut, kimchi, kefir, kombucha – foods produced by lactic acid bacteria and part of our diet for centuries. What our ancestors did for practical reasons – to preserve food – is now proving to be a highly effective method for strengthening the microbiome.
Fermented foods provide live microorganisms, known as probiotics. These colonize the gut, multiply, and displace potentially harmful germs. A study from Stanford University showed that people who regularly consumed fermented foods exhibited significantly increased microbial diversity after six weeks. Inflammatory markers in the blood measurably decreased. Participants reported improved well-being—and many also experienced relief from chronic respiratory symptoms.
But it's not just the live bacteria themselves that are valuable. The fermentation process also produces those short-chain fatty acids that are so crucial for the immune system. Acetic acid from vinegar and fermented vegetables, lactic acid from sauerkraut and yogurt – they all contribute to keeping the gut, and therefore the lungs, in balance.
Equally important are prebiotic fibers. These are food components that humans cannot digest themselves, but which serve as food for the "good" gut bacteria. Inulin from chicory and artichokes, pectin from apples, beta-glucan from oats, resistant starch from cooled potatoes – all these substances promote the growth of bacteria that produce butyrate and other short-chain fatty acids.
The German Nutrition Society recommends at least 30 grams of fiber per day. However, about 70 percent of adults in Germany do not reach this goal. No wonder dysbiosis is so widespread. And no wonder respiratory illnesses, allergies, and autoimmune diseases are on the rise.
The silent power of indoor air – When external factors disrupt inner balance
But even the best microbiome cannot function fully if the airways are constantly stressed. And this is where an often overlooked factor comes into play: the quality of the indoor air .
Today, we spend up to 90 percent of our time indoors. The air we breathe there is often drier, dustier, and more polluted than we realize. In winter, the humidity in heated rooms frequently drops below 30 percent – a desert-like climate that dries out our mucous membranes. Yet these mucous membranes are the first line of defense against inhaled pathogens.
Dry mucous membranes can no longer perform their cleansing function. The cilia, which normally transport mucus and trapped particles outwards, become paralyzed. Viruses and bacteria have an easier time. At the same time, dry air puts a strain on the airways themselves – it irritates, inflames, and weakens the local immune response.
This brings us full circle to the gut-lung axis. A stressed respiratory tract sends out inflammatory signals – including to the gut. Conversely, a healthy gut microbiome can support the regeneration of the respiratory mucosa by producing anti-inflammatory signaling molecules.
Salt plays a special role in this process. Salt-rich air – as traditionally used in graduation towers and salt caves – naturally moisturizes the airways. The osmotic effect of the fine salt particles binds moisture, liquefies stubborn mucus, and supports mucociliary clearance. At the same time, salt has a mild antibacterial effect.
Between tradition and science – The holistic approach
Markus finally understood. After weeks of drinking kefir daily, eating high-fiber meals, and humidifying his apartment with a mini saline solution, he noticed a change. The nighttime coughing fits became less frequent. The tightness in his chest subsided. He needed his asthma inhaler less often.
Was it just the kefir? The fiber? The salty air? Or was it the sum of all these measures that brought his immune system back into balance? Science would say: probably the latter. Because health is rarely a matter of adjusting a few things. It is the result of many small but significant decisions.
The gut-lung axis shows us just how interconnected our body truly is. How one organ, which we nourish with food, influences another that we need to breathe. How bacteria living deep within us play a role in determining whether or not we can defend ourselves against inhaled viruses.
An outlook – Where research and everyday life meet
Science is still in its early stages. Much remains a mystery. Which specific strains of bacteria protect against asthma? How much fermented food is needed to achieve a measurable effect? What role does an individual's genetic makeup play? These are all questions researchers worldwide are working on.
But while research progresses, we can already take action today. A high-fiber diet, the regular consumption of fermented foods, the avoidance of unnecessary antibiotics, and maintaining a healthy indoor climate – all these measures have been proven to strengthen the microbiome and thus indirectly support the respiratory system.
Some people also use small saltworks at home – a modern interpretation of the centuries-old graduation towers. These devices enrich the room air with fine salt particles and can serve as a supplementary measure for humidifying the respiratory tract. They do not replace medical therapy, but can – gently and incidentally – improve the indoor climate.
The gut-lung axis teaches us one thing above all: health is holistic. It doesn't begin with symptoms, but deep within us, with the invisible helpers that accompany us every day. With the trillions of bacteria that train us, protect us, and communicate with us. When we learn to pay attention to this silent partnership, we gain more than just a strong immune system. We gain a deeper understanding of what it means to be healthy.
(Images: Envato)
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Medical note:
This article is for general information purposes only and does not replace medical advice, diagnosis, or treatment. If you have health problems or existing medical conditions, please consult a doctor or qualified healthcare provider.
