The Gut-Brain Connection: How the Microbiome Affects Our Health
The human gut is home to more than 100 million nerve cells, which are responsible for producing 95% of serotonin – a neurotransmitter associated with bodily health.
Recent studies have emphasized the importance of gut microbes for both physical and mental health.
The gut microbiome consists of trillions of bacteria, viruses, fungi, and other microorganisms.
This means that the gut and the brain are connected, and they influence each other. Have you ever noticed the feeling of nausea before an important meeting, or the emotional stress when suffering from constipation?
But how did this connection between the brain and gut emerge? Can we improve this link for better health and a happier life?
These two organs are connected in three different ways, according to researcher Salha Mahmoud Ahmed, a specialist in digestive diseases at the Gut Research Center in the UK:
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The Vagus Nerve: This is a crucial nerve that connects the brain to many organs, including the heart and gut.
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Hormonal Communication: The brain and gut communicate with each other using hormones. These hormones, such as ghrelin and glucagon-like peptide-1 (GLP-1), are secreted by glands and help transmit signals throughout the body.
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The Immune System: Many people believe that immune cells are only found in the blood or lymph nodes, but the reality is that a large proportion of immune cells reside in the gut and act as mediators between the brain and the rest of the body.
Pankaj J. Pasricha, a gastroenterology consultant at the Mayo Clinic in the United States, believes that this connection between the gut and brain exists because the brain requires a lot of energy to perform its tasks, and the gut is our energy generator.
Pasricha points out that the brain represents only 2% of the body’s weight but consumes 20% of its energy. He explains that the function of the gut is to "break down food" into small molecules before absorbing and converting them into energy for the entire body.
But this is a two-way relationship; the brain affects the gut, and the gut affects the brain.
We can give many examples from daily life that illustrate this brain-gut relationship.
When we face a dangerous or frightening situation, or when we are preparing for an important event, one of the first physiological responses occurs in the gut: we might feel nauseous, experience stomach cramps, or even suffer from diarrhea.
On the other hand, when we experience love, we often feel "butterflies" in our stomachs, or a psychological sensation linked to the excitement of someone we deeply care about approaching us.
Conversely, if you're constipated and haven’t felt the urge to use the restroom for several days, it could leave you vulnerable to irritability and stress.
A Whole World Inside You
Our guts host between 10 to 100 trillion microbial cells, including bacteria, viruses, fungi, and protozoa, which outnumber the human body’s cells.
Specialists believe that these trillions of microbial cells coexist in a symbiotic relationship with humans. They extract nutrients from the food we consume and aid in digestion, while also helping break down components that we would not be able to digest without their assistance.
Over the last 20 years, research has revealed much about these microbial cells and their impact on human health.
Researcher Salha Ahmed points out that new tools and tests have been developed by scientists to calculate the number of microorganisms residing in the gut, as well as to understand how these microorganisms affect the development of certain diseases.
Pasricha explains that "imbalance in the gut microbiome," scientifically known as "dysbiosis," is linked to several common diseases.
In 2011, Pasricha led a pioneering study on laboratory rats exhibiting symptoms of stomach disorders in the early stages of life.
The study found that these symptoms could develop into "depression and anxiety-like behaviors that may last for a long time."
Another study showed that dysbiosis is linked to conditions such as obesity, cardiovascular diseases, and even cancer.
However, Pasricha notes that there is not enough evidence to definitively establish a cause-and-effect relationship, or to claim that gut microbiome imbalances are the root cause of many diseases.
Pasricha states: "There is some evidence, found in animal and human studies, that suggests that an imbalance in the gut microbiome could be associated with anxiety or depression. But whether these diseases are caused by the gut remains unclear."
A Prescription for Beneficial Bacteria
In light of recent discoveries about bacteria and the gut-brain connection, it is possible to achieve an optimal balance of the microorganisms residing in our intestines.
However, researcher Salha Ahmed sees it as challenging because each person has a unique composition of bacteria, viruses, and other microorganisms.
She explains: "Each person has their own unique composition of microorganisms living in their gut, and there is no single starting point that is the same for two humans."
Experts, however, suggest that there are some general interventions that could be beneficial for gut health, such as adopting a balanced and diverse diet.
Foods that are recommended in this context include "probiotics" – foods that contain certain beneficial bacteria for the digestive system, such as natural yogurt, kefir, and kombucha; and "prebiotics" – foods rich in fiber that nourish gut microorganisms, such as fruits and vegetables.
Salha Ahmed emphasizes: "I can say that a varied diet is crucial, especially with regard to the number of plant-based foods people consume."
She advises ensuring that one’s diet includes a substantial amount of fruits, vegetables, whole grains, legumes, nuts, seeds, and spices.
Salha Ahmed adds: "I am not vegetarian, but I believe there is a need to increase the share of vegetables in our diet."
She refers to studies that showed a healthy microbiome balance in people who consume about 30 different types of plants each week.
But does changing the diet have a significant impact on emotions, and can such a change help treat conditions like depression?
To answer this question, a study was conducted at the University of Oxford, UK.
Researchers gathered a sample of 71 people suffering from depression and divided them into two groups. The first group consumed "probiotics" – foods containing specific types of beneficial bacteria for the digestive system for four weeks, while the second group took a placebo.
This experiment was "randomized and double-blind," meaning neither the researchers nor the participants knew who took what.
During the experiment, researchers conducted several tests to measure factors such as mood, anxiety, sleep, and salivary cortisol (a substance related to stress).
Clinical psychologist Rita Baiao, who led the study, notes that people with depression tend to focus more on negative emotions and facial expressions compared to neutral or positive emotions.
She explains: "We want to know if consuming probiotics affects the brain's processing of emotional information."
Baiao adds: "With the group that took probiotics, we noticed a tendency to focus less on the reasons behind negative emotions."
She believes probiotics may help alleviate some symptoms of depression, but further research is needed. "We still need stronger data, though there are indications that probiotics can have a positive effect, resulting in a considerable level of resilience with fewer side effects," according to Baiao.
Pasricha points out that changing the gut microbiome's composition can take decades before results appear.
He adds: "We know that it is very difficult to maintain some behaviors in certain people, otherwise, we wouldn’t have the obesity epidemic."
"But we are trying to gather the necessary pieces to complete the picture," Pasricha concludes.
