The microbiota is a set of microorganisms (bacteria, viruses, fungi, etc.) present in the mouth, intestine, vagina and skin. In this post, I'm going to talk to you about the gut microbiota and how it influences our response to stress.
The microbiota: A modular organ, a genetic fingerprint and numerous functions
Just like the DNA that characterizes us genetically, each of us has our own unique microbiota. Each microbiota differs in quality and quantity. The quality of the microbiota refers to the diversity of the strains and species that compose it (firmicutes, lactobacilli, bifidobacteria, streptococci, lactococci, etc.). Quantity refers to the numbers of microorganisms for each strain and species.
Today we consider this microbiome (= area occupied by the microbiota) as a real organ located between nutrients and others coming from the food bolus and the cells of the intestinal mucosa.
The microbiota is transmitted by the mother when the fetus passes through the vagina. This is why during a cesarean section, we encourage skin-to-skin contact with the newborn so that his/her mother transmits her microbiota to him/her through the skin. This primary microbiota evolves and becomes mature at the age of 3 years. From there, it will evolve according to environmental factors (diet, lifestyle, pollutant, etc.). When it is in balance, it is called a eubiotic microbiota. It is often subject to imbalances, but which are only transitory. When the disturbance becomes chronic, there is a breakdown in its balance and we then speak of dysbiosis.
It is given many functions such as:
1. Digestive functions: fermentation of certain foods (this is particularly the case when we do not chew enough), digestion of fibers, resistant starch and polysaccharides.
2. Metabolic functions: metabolism of the intestinal epithelium, of certain peptides and proteins, energy metabolism, synthesis of vitamins, synthesis of neurotransmitters, development of intestinal vascularization.
3. Detoxification functions: contributes to the process of detoxification of toxins via the transformation of bile acids.
4. Immune functions: maturation and functioning of immune cells.
So many functions that give the microbiota an important role in the physiology and balance of the functioning of the human body. But the function I would like to talk to you about in this post is the link between the gut and the brain that scientists call the “gut-brain axis”. He is very intriguing and is only at the beginning of his investigation. But studies agree that there are several pathways of communication between the intestine and the brain and that it would be bilateral. So how is this relationship able to modify our response to stress?
The gut-brain axis and modulation of the response to stress
Bilateral communication
It seems that this communication between the brain and the intestine is bilateral. That is to say, the brain sends information to the microbiota which will react accordingly and it also receives information from the microbiota via the vagus nerve and the blood circulation. Thus, the areas of our brain which control our behaviours and our emotions are under the influence of our intestinal bacteria. QED: Who has never had a stomach ache when stressed or upset?
This communicative loop is done through different channels:
• Metabolic pathway: the cells of the intestine absorb molecules from our food, transmit them to the blood circulation, the route by which the molecules reach the brain.
• Nervous pathway: the vagus nerve (enteric nervous system) sends information to the brain to keep it informed of the nutrients it is receiving and the metabolites that come from them thanks to the microbiota. An interesting study carried out on rats where the vagus nerve was cut and supplemented with probiotics (intestinal bacteria that we provide as supplements). This study shows that the beneficial effects of probiotic supplementation are cancelled out, which clearly demonstrates the importance of the vagus nerve in gut-brain communication.
• Immune pathway: a healthy microbiota is capable of producing anti-inflammatory immune cells (cytokines) which will be transported to the brain via the blood circulation, thus protecting it from neuroinflammation.
• Endocrine pathway: the intestinal membrane contains specific cells called endocrine cells capable of synthesizing and releasing metabolites (such as hormones, neurotransmitters, enzymes or precursors) in reaction to chemical stimuli (coming from the brain or our food for example) or mechanism (such as intestinal peristalsis). The metabolites produced can reach the brain through the blood circulation or activate the vagus nerve and thus send information to the brain.
How stress impacts the microbiota
One of the mechanisms by which stress negatively influences the microbiota is ultimately quite simple. In my last post, I mentioned the disruption of the leptin/ghrelin balance (two hormones that control hunger and satiety) and the increase in cortisol levels in the event of stress. It is because of the disruption of these imbalances that hunger increases. We will then be attracted to “comfort food”, that is to say foods that comfort us. Unfortunately, it is rarely the carrot stick that we turn to but rather fatty and sweet foods (Cf. the tub of ice cream in front of the TV after a difficult day 🙄). And of course, even if we think we are “recomforting ourselves”, in reality we are mainly comforting ourselves in our pain. Because in fact, these foods disrupt the balance of the microbiota, and you will have understood, an unbalanced microbiota is guaranteed stress!
How the microbiota influences the stress response
Many avenues are being studied, some are more valid than others. The most obvious and studied is the impact of the microbiota on the Hypothalamic-Pituito-Adrenal axis (HPS axis; See September post). Studies carried out in rats show that rats deprived of microbiota have a higher level of corticosterone (stress hormone) and show anxious behaviours compared to rats, therefore the microbiota is healthy. The absence of microbiota increases the reactivity of the HPS axis which no longer self-regulates correctly, explaining the increase in corticosterone levels. Some studies show the same effects when taking antibiotics: antibiotics kill intestinal bacteria, both good and bad (this is why you should always take probiotics in parallel) and therefore disrupt the balance of the microbiota. The microbiota would therefore have a regulatory effect on the HPS axis and thus modulate our response to stress. A study caught my attention and I take this opportunity to recall the importance of the microbiota in young children: “the quality of the microbiota at young ages is essential for the maturation of the brain areas constituting and regulating the corticotropic axis”. An avenue to investigate to explain anxious type personalities…
The second widely documented pathway is that of serotonin. Serotonin is the target neurotransmitter of antidepressants which are actually serotonin reuptake inhibitors. However, 80% of serotonin synthesis is done by the cells of the enteric nervous system (in other words, in the intestine) and only 20% by the central nervous system (CNS). In addition, 95% of 5-HTP (precursor to serotonin) is stored in certain intestinal cells and neurons of the enteric nervous systems compared to 5% stored in the CNS… However, it was found that the levels of 5-HTP were 2 times lower in mice without microbiota compared to mice with a healthy microbiota. Short-chain fatty acids and secondary bile acids are largely involved in the stimulation of intestinal endocrine cells. However, these two metabolites are the result of a healthy microbiota. Finally, the intestinal microbiota also contributes to the bioavailability of Tryptophan, an essential amino acid (= which must be provided through food) precursor of 5-http. A great avenue to consider in the management of depressive disorders.
Another phenomenon documented in studies is the effect of the microbiota on the inflammatory state and more precisely on the inflammation of neurons in the brain, which is called neuroinflammation. An unbalanced microbiota alters intestinal permeability and thus allows the increased passage of molecules called LPS (lipopolysaccharide). These bind to a specific receptor (TLR4) and cause the formation of pro-inflammatory molecules (cytokines). Inflammation which affects the brain and negatively modulates our response to stress.
Also, the microbiota strongly influences the production of neurotransmitters (dopamine, GABA, serotonin, acetylcholine, etc.), molecules which, as you will have understood, play an essential role in the stress response as explained previously.
stimulation of the vagus nerve which sends messages to neurons in the brain activating or inhibiting the synthesis of neurotransmitters.
Finally, one last hypothesis that I find interesting, particularly for the management of stress and anxiety. Certain strains (like Lactobacilli) enhance the production of endocannabinoids (eCB). eCBs are molecules produced naturally by the body and modulate the action of all neurotransmitters. FYI, cannabis and CBD have the same effects as eCB on our brain...better to boost your natural production 😉.
Psychobiotics
The influence of the microbiota on the response to stress, anxiety and depressive disorders is becoming so obvious that some laboratories specializing in prebiotics offer supplements they call “psychobiotics”. Scientific research has in fact been able to identify certain strains such as Lactobacillus farciminis, Lactobacillus helveticus R0052 and Bifidobacterium longum R0175, having a significant impact on the modulation of the stress response.
The modulation of the stress response by the microbiota via neurological, metabolic and endocrine pathways is therefore undeniable. And it seems that we are only at the beginning of our discoveries... To be continued 🧐!
For a personalized investigation of your microbiota in collaboration with the Intus laboratory, get in touch.
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