I am depressed, but it is my intestinal bacteria that are harming me?

Review expert: Wu Xinsheng, deputy chief physician of gastroenterology department

Depression is an affective disorder. Patients mainly experience persistent low emotions. In severe cases, they also have slow thinking, slow behavior, and various somatic symptoms. With the increasing pressure of life, more and more people are suffering from depression.

The World Health Organization (WHO) predicts that by 2030, depression will become the disease with the highest global disease burden.

However, the exact cause and pathogenesis of depression have always been a "mystery".

Source: nature.com

Just this year, a study led by Finnish microbial bioinformatician Guillaume Méric analyzed the genetic composition and intestinal microbial status of more than 6,000 subjects and concluded that certain microbial characteristics in the intestines are related to depression, and the transmission mechanism behind it is related to genes.

Today, let’s talk briefly about how the bacteria in our intestines “influence” our emotions.

Humans are actually a "walking bacterial colony"

As we all know, humans do not live in a sterile environment. There are a large number of microorganisms living on our skin, respiratory tract, digestive tract, etc. Scientists have estimated the number of microorganisms in the human body. The number of bacteria on a "standard person" (20-30 years old, 170cm tall, 70kg in weight) is about 3.9×10^13, which is about 1.3 times the total number of cells in the body.

No wonder some microbiologists believe that humans are not just individuals, but also walking bacteria.

Distribution of normal human flora

Source: scq.ubc.ca

Our digestive tract is connected to the outside world. From the moment the fetus passes through the birth canal, along with the baby's breathing and eating, bacteria begin to enter our digestive tract, "settle" inside, and accompany people throughout their lives.

The relationship between intestinal flora and human health has always received widespread attention in the medical community. Researchers have found that many diseases such as ulcerative colitis and Crohn's disease are related to changes in intestinal flora. Therefore, oral probiotics to regulate intestinal flora and treat intestinal diseases have been adopted by many clinical departments as an effective treatment method.

The gut really is connected to the brain

"A rectum leads to the brain", I believe many of my friends have seen this Internet slang, it is often used to impolitely describe people as simple-minded (good children should not learn this).

However, what many people don’t know is that there is indeed a two-way signal communication network between our central nervous system and digestive tract, which is called the “gut-brain axis.” Of course, the principle of the gut-brain axis is much more complicated than “a rectum to the brain.” It requires information exchange between the digestive tract and the central nervous system through hormones and neural circuits, and ultimately regulates a variety of physiological phenomena.

Diagram of the gut-brain axis

Source: researchgate.net

Studies have found that the bacteria in the intestines can not only synthesize a variety of neuroactive substances such as serotonin, dopamine, γ-aminobutyric acid, butyrate, but also affect the metabolism of these substances in the human body, thereby affecting human emotions.

Not only that, intestinal microorganisms also affect the excitability of the vagus nerve and the function of the immune system, which may affect people's emotions and behaviors.

Source: Nik Spencer/Nature

The results of animal experiments show that intestinal flora may play an important role in the pathogenesis of Parkinson's disease, autism spectrum disorder and motor neuron disease. Currently, some researchers have tried to prevent and treat these neurological diseases by adjusting the microbiome.

Why gut bacteria can cause depression

So what is the relationship between gut flora and depression?

In the article mentioned at the beginning of this article, which was published in Nature Genetics in February last year, the author team found through a study of thousands of Finns that among the many combinations of intestinal bacteria, Morganella has the most clear correlation with depression and increases the risk of major depression. In addition, the study also found that Klebsiella may also increase the risk of depression.

Schematic diagram of Morganella

Source: assocarenews.it

The pathogenic mechanism of these specific intestinal bacteria is likely to be that after activating peripheral immunity and inflammatory response, the inflammatory factors produced enter the central nervous system, thereby leading to the occurrence of depression.

The current view is that genes are the internal factors that have the greatest impact on intestinal flora , among which genes related to blood type and genes related to lactose tolerance have the greatest impact on intestinal microorganisms. At the same time, people's eating habits will also affect the types and number of intestinal microorganisms in the human body.

Although this study revealed a possible correlation between intestinal flora and depression, the medical community's research on the "gut-brain axis" is still in its infancy. In addition, humans are a complex entity, and the manifestations and pathogenesis of depression are also diverse. Whether reducing or killing one or more specific bacteria can have a therapeutic effect on depression still needs further research.

I hope that in the near future, scientists will be able to transform these research results into clinical applications, so that more patients can get rid of depression as soon as possible!