What to remember:
- Recent studies have begun to establish an intriguing link between the gut microbiota and psychiatric disorders such as depression.
- Significant alterations in the composition and diversity of the gut microbiota have been demonstrated in people suffering from various diseases, including psychiatric disorders such as depression.
- There is a complex network of communication between the gut and the brain, called the "gut-brain axis," which is carried out through communication pathways.
- Diet, medications and chronic stress are examples of factors that can alter the intestinal microbiota.
- An imbalance of the intestinal microbiota is often found in people with depression.
- Probiotics help restore a healthy intestinal microbiota and thus combat symptoms associated with depression.
In recent years, a burgeoning field of research suggests that mental health may be influenced by unsuspected actors—the trillions of microorganisms that inhabit our gut, called the gut microbiota . Once thought of as simply a participant in digestion and immunity, it is now proving to be a potentially powerful player in mental health as well. Indeed, recent studies have begun to establish an intriguing link between the gut microbiota and psychiatric disorders such as depression. While this relationship may seem unusual at first glance, it opens up new perspectives in understanding and treating depression by connecting the gut and the mind.
The intestinal microbiota, a player in depression
What is the intestinal microbiota?
The intestinal microbiota, also known as intestinal flora, refers to the set of microorganisms , mainly bacteria, that reside in our digestive system. They play a crucial role, particularly in the digestion of food, the regulation of immunity and the synthesis of certain essential vitamins. It is a dynamic ecosystem in constant evolution, influenced by various factors such as diet, lifestyle and medical treatments.
What is depression?
Depression is a complex mental illness that can significantly impair an individual's quality of life and have a significant impact on their social interactions and daily life. The prevalence of this illness is alarming worldwide. In fact, according to the World Health Organization (WHO), more than 322 million people of all ages suffer from depression , making it the leading cause of disability worldwide. Did you know that in France, one in five people has suffered or will suffer from depression in their lifetime? But then, how is depression characterized? Depressive disorder is described as "a succession of characterized depressive episodes, resulting in numerous symptoms - including pathological sadness, loss of pleasure and cognitive symptoms - with a major impact on the life of the patient and those around them" (INSERM). We speak of depression if the symptoms found last at least two weeks .
A change in the intestinal microbiota observed in certain diseases
When we are healthy, our gut is home to a vibrant living ecosystem rich in microorganisms, primarily gut bacteria. However, not all of these bacteria have the same influence on our bodies. There are good bacteria that have beneficial effects on our health, but also bacteria that are referred to as “bad bacteria.” Recent studies have highlighted significant alterations in the composition and diversity of the gut microbiota in people suffering from various illnesses, including psychiatric disorders such as depression. Researchers have observed that some types of beneficial bacteria are often reduced in number, while other potentially harmful bacteria can proliferate. This disruption in the balance of the microbiota, called dysbiosis , has been linked to health problems, including mental disorders.
How does our gut communicate with our brain?
The idea that our gut can communicate with our brain may seem strange at first, but there's actually a complex network of communication between these two organs. This gut-brain connection, also known as the " gut-brain axis ," operates through four communication pathways: neuronal, blood, immune, and endocrine. Let's take a look at these pathways.
The neural pathway 🧠
This pathway involves chemical molecules synthesized by gut bacteria called "neurotransmitters," which include dopamine. The vagus nerve plays a central role in this pathway, allowing electrical and chemical signals to travel both ways, intimately connecting our gut to our brain. This two-way link between the two organs facilitates the transmission of essential information for regulating our mood, emotions, and even bodily functions.
The blood route 🩸
The gut-brain axis also functions through the bloodstream. Chemicals, nutrients, and signals produced by our gut are transported through our bloodstream and reach the brain, causing changes in its functioning. Different substances travel through this pathway, establishing a silent dialogue between the two organs.
The immune pathway ⚡️
Bacteria in our gut are capable of synthesizing molecules that can activate intestinal immune cells. Once activated, these cells produce molecules that act as chemical messengers. They can penetrate the brain and influence neurological processes, sometimes altering our mood and cognition.
The endocrine (hormonal) pathway 💪🏼
Hormones are key chemical messengers that orchestrate a variety of physiological processes in our bodies. Some of the molecules produced by our gut can interact with the hormonal system, influencing the regulation of appetite, stress , and emotions, among other things. Hormones such as cortisol, known as the “stress hormone,” can be modulated by our gut microbiota, affecting our emotional state.
These four communication pathways do not operate in isolation, but intertwine to form a complex network that connects our gut to our brain.
What are the factors responsible for an alteration of our intestinal microbiota?
As you may have guessed, the many intestinal bacteria that make up our microbiota play an essential role in our health. However, this delicate ecosystem can be disrupted by a variety of factors, which can have consequences for our health and well-being. Let's take a look at the main culprits responsible for an alteration of our intestinal microbiota.
1 - An unbalanced and unvaried diet
We often repeat it, but what we eat has a significant impact on the composition of our intestinal flora. It has been shown that a diet rich in processed (industrial) foods, refined sugars, and saturated fats can promote the growth of certain harmful bacteria, to the detriment of beneficial bacteria. Monitor the presence and number of additives found in the foods you consume. On the contrary, a diversified diet rich in fruits, fresh vegetables, oilseeds, legumes, and fermented foods provides fiber and essential nutrients to support the diversity and balance of our microbiota.
2 - Antibiotic treatments
Antibiotics, while essential for fighting bacterial infections, can also be veritable “nuclear bombs” and disrupt our microbiota by killing not only harmful bacteria, but also beneficial ones. This can lead to an imbalance in the bacterial composition, leading to various health problems.
3 - Chronic stress
Our emotional state can also play a role in the health of our microbiota. Chronic stress can disrupt the balance of our gut bacteria, as it can influence the production of certain chemicals that affect bacterial growth. In turn, a disrupted microbiota can also affect our stress response, creating a vicious cycle.
4 - Infections and diseases
Certain infections and illnesses, such as gastroenteritis, can temporarily unbalance our microbiota by disrupting the population of bacteria in our intestines. Chronic diseases such as irritable bowel syndrome (IBS) can also influence the composition of the microbiota.
5 - A sedentary lifestyle
Regular exercise appears to play a role in maintaining the diversity and stability of the gut microbiota. It's important to avoid a sedentary lifestyle to prevent the proportion of beneficial bacteria from decreasing and thus promote good overall health.
6 - The environment
Chemicals present in our environment, such as chemicals used in household products and pesticides, can potentially affect the composition of our microbiota.
An imbalance in the intestinal microbiota at the heart of depression
An imbalance in the gut microbiota can disrupt gut-brain communication mechanisms, which could have potential implications for the development of depression. Indeed, as we saw earlier in this article, when the gut microbiota is balanced, communication between the two organs occurs through different pathways. However, when the microbiota is disrupted, these pathways can be altered.
First, when beneficial and harmful bacteria are no longer in harmony in our gut, it can impact the production of neurotransmitters , the chemical messengers responsible for our emotions and mood. For example, serotonin, often dubbed the "happy hormone," is primarily produced in our gut. A disrupted microbiota can reduce serotonin production, which can alter our mood and make us more vulnerable to depression.
Additionally, an altered gut microbiota can induce chronic inflammation in the gut, generating inflammatory molecules that can reach the brain through the bloodstream. This brain inflammation is found in patients with depression and impairs brain function, such as the neurological processes involved in mood regulation. This contributes to the onset of depressive symptoms.
Finally, an imbalanced microbiota can influence the signals sent via the vagus nerve , disrupting communications between the digestive system and the brain. Chronic stress, often linked to depression, can also exacerbate dysbiosis, creating a vicious cycle where the disrupted microbiota can amplify symptoms of depression and vice versa.
Sources:
World Health Organization. (January 30, 2020). Key Facts on Depression. who.int. https://www.who.int/en/news-room/fact-sheets/detail/depression
Author unknown. Statistics and figures on depression in France and around the world. (July 30, 2020). https://www.la-depression.org/. https://www.la-depression.org/comprendre-la-depression/la- depression-en-chiffre/
INSERM. (December 6, 2019). Depression. Inserm - Science for health. https://www.inserm.fr/information-en-sante/dossiers-information/depression
ORBLIN, Marie et al. (2021, March 1). Gut microbiota imbalance and depressive disorders. ScienceDirect. https://www.sciencedirect.com/science/article/pii/S0515370021000343RABOT, Sylvie. (2015). Gut-brain axis: how the intestinal microbiota influences the stress response. Bulletin of the French Veterinary Academy, 3, 267. https://doi.org/10.4267/2042/57938
