1. Your gut is home to more than tens of trillions of microorganisms.
The intestines are not just part of the “tube” through which food passes and where digestion and absorption occur – they are home to a complex ecosystem composed of tens of trillions of microorganisms. These include bacteria, viruses, archaea, fungi, and even single-celled parasites. Scientists estimate that about 38 trillion microorganisms live in our bodies, which is nearly equal to the number of our own cells. The total mass of microorganisms in the gut can reach up to 2 kilograms, comparable to the mass of larger organs, such as the liver or brain.
2. The gut – our “hidden organ”
The microbiota is not just a passive resident of our intestines – it lives in symbiosis with humans and has a profound impact on nearly every aspect of our health. Its composition and activity influence:
- metabolism (it participates in the breakdown of complex carbohydrates; a disrupted microbiota can contribute to obesity, insulin resistance, type 2 diabetes…);
- functioning of the immune system;
- brain activity, mental and emotional state;
- cardiovascular health;
- digestive abilities (vitamin synthesis, fiber breakdown);
- response to medications;
- the health of other organs, such as the skin and liver.
Although the microbiota is not an organ in the anatomical sense, due to its key role in metabolism, the immune system, and maintaining the body’s internal balance, scientists are increasingly referring to it as a “hidden organ.”
3. Diet as a selective force
Diet is one of the most powerful environmental factors that can significantly change the composition, diversity, and function of the gut microbiota in a very short time – as quickly as within one to two days. What we eat nourishes not only us but also the trillions of bacteria that inhabit our gut.
That’s why food acts as a selective force – it encourages the growth of certain bacterial species while suppressing others. For example, a diet rich in fiber promotes the proliferation of bacteria that produce anti-inflammatory compounds, while a diet high in saturated fats can encourage the growth of those that contribute to inflammatory processes.
Our gut bacteria are, in fact, competing for space and nutrients. This is why dietary changes are one of the most effective ways to improve the health of the microbiota in the long term – and therefore our own health as well.
4. The gut is our second brain
The wall of the intestine is interwoven with a neural network of approximately 100 million neurons, more than are found in the entire spinal cord. This nervous system is called the enteric nervous system, and it allows the gut to react independently, contract, and manage digestion without needing constant “commands” from the brain. Because of this incredible ability to function completely autonomously, the gut is rightly referred to as the “second brain.”
5. Diversity and stability – the secret to a healthy microbiota
The two most important characteristics of a healthy gut microbiota are diversity and stability. Just as with plant and animal ecosystems, the same applies to the gut microbiota: the more diverse it is, the more resilient and healthier it becomes.Imagine a field where only one crop is grown – the soil becomes depleted and more vulnerable to diseases and pests. On the other hand, when we plant a variety of crops, the soil becomes richer, more resilient, and more fertile.
The same happens in our gut – if only a few types of bacteria dominate, the gut becomes less resilient, and the risk of inflammation, digestive issues, and disease increases. In contrast, a diverse diet (rich in fiber, fermented, and whole foods) supports microbiota diversity and strengthens internal health.
6. More than 70% of immune system cells are located in the gut, highlighting its exceptional importance for the body’s defense.
The most intense exchange between the external and internal world of the body takes place here, so immune cells are constantly “patrolling,” monitoring, and responding to potential threats.
The gut microbiota and its metabolic products also regulate the functioning of the immune system, determining when and how immunity will be activated to protect us from danger, but also to ensure it doesn’t overreact (as in allergies or autoimmune diseases).
The intestinal wall, together with the mucosa (mucous membrane), acts as a physical barrier – the first line of defense that prevents harmful substances, pathogenic bacteria, and toxins from entering the body. That’s why a healthy gut microbiota is the foundation of a strong and balanced immune system. When the microbiota is disrupted, the immune system weakens, increasing the risk of infections and disease. Sometimes it can even become overactive, as seen in cases like allergies or atopic dermatitis.
7. As much as 90 to 95% of total serotonin, the neurotransmitter also known as the “happiness hormone,” is produced in the gut.
Although we most often associate serotonin with the brain and emotions, the majority of it is actually produced in the gut, thanks to local cells and substances generated by gut bacteria.
Serotonin has numerous functions in the body. For example, it regulates sleep, appetite, digestion, mood, and even pain sensitivity. In the gut, serotonin influences peristalsis, that is, the movement of contents through the digestive system.
Although serotonin produced in the gut cannot cross the blood-brain barrier, research suggests that an imbalance in the microbiota can affect serotonin signaling in the brain, which may be linked to depression, anxiety, and other psychological conditions.
8. The microbiota begins to develop at birth and stabilizes around the age of three
The microbiota begins to develop intensively at birth, when the newborn is first exposed to the external environment, although some scientists believe that the first microbes appear already in the womb. The method of delivery (vaginal or cesarean) and type of feeding (breastfeeding, formula, introduction of solid foods) also play a significant role.
Around the age of three, the microbiota becomes functionally more stable and similar to that of adults, but it still remains sensitive to changes. This is why a diverse diet in early childhood is crucial not only for growth and development but also for shaping the gut microbiota, which has long-term effects on immunity, metabolism, and overall physical and mental health. For example, children who consume more fiber and fermented foods have a more diverse microbiota and a lower risk of allergic diseases.
That’s why early childhood nutrition needs to be diverse, rich in prebiotics (fiber) and probiotics (fermented foods), and minimally processed.
9. The gut-brain axis – communication that changes the perspective on health
The gut and the brain communicate with each other through the gut-brain axis, a complex communication system that operates bidirectionally. This connection occurs through multiple parallel pathways: the nervous system (particularly via the vagus nerve), hormonal and immune signals, and metabolites produced by the microbiota.
Increasing research is revealing how stress and diet strongly influence this axis, and in turn, affect our mood, digestion, and overall health.
You can read more about the connection between diet, stress, and digestion in this article.
10. The inhabitants of your gut play a role in regulating body weight
The influence of the gut microbiota on body weight occurs through several mechanisms – via hormonal signaling, metabolites, inflammatory markers, and even through its impact on behavior. For example, the microbiota, through its metabolic products, stimulates the release of hormones such as GLP-1 and PYY, which regulate appetite and feelings of fullness.
Certain bacteria even produce substances that affect the brain, potentially influencing your food choices and cravings, especially for sweet or fatty foods.
The microbiota can also influence how much energy the body absorbs from food, as well as how that energy is used or stored. Research has shown that individuals with less diverse microbiota have a higher risk of weight gain and difficulties in regulating it.
You can read more about how a disrupted gut microbiota may contribute to weight gain here.
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