The new era of the 'brain-gut axis' has finally arrived!

There is an old saying: "To live long, one must first nurture the brain; maintaining brain health can lead to a hundred years of life." Maintaining good brain health throughout the entire life cycle is the primary goal of humanity's pursuit of health and longevity.

01 The Importance of Brain Health

The human brain, as the command center of the nervous system, completes complex functions such as thinking, memory, movement, and emotional responses. As a complex organ, the brain has at least three levels of functions that affect our daily lives: interpreting sensations and controlling movements; maintaining cognition, mental state, and emotions; and sustaining normal behavior and social cognition. Therefore, brain health can be defined as maintaining optimal brain integrity and mental and cognitive functions at a specific age without the presence of significant brain diseases that affect normal brain function.^{From birth, the brain has been in a state of development and change, and brain health can easily be influenced by various internal and external factors. With the aging population and the increasing burden of neurological diseases, the challenges of maintaining brain health are also growing. Major countries around the world have large-scale brain research programs, and research related to cognitive functions and brain functions is particularly important within these programs.}Cognitive function is a very complex physiological process involving a series of psychological and social behaviors such as learning, memory, language, thinking, mental state, and emotions. Cognitive function in the body experiences age-related decline; with the intensification of population aging, the incidence of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD) is on the rise. Additionally, modern society is increasingly facing emotional disorders such as anxiety and depression, as well as autism, all of which severely threaten human brain health.

Research Progress on the Association Between Brain Health and Gut Microbiota^{Previous research on brain health mainly focused on the brain itself. Given the very complex mechanisms of various cognitive function-related diseases, as research continues to deepen, people's attention has shifted from the brain to the whole body.}Gut microbiota is a very popular research area in recent years. They are the microbial communities living in our intestines, and numerous studies have confirmed that these special microorganisms are very important for the body's nutrient metabolism, development, immunity, and the occurrence of various diseases. People are also consciously regulating their gut microbiota by consuming probiotic products or adjusting their immunity to improve their health.

02 In recent years, many researchers have found that gut microbiota may influence the structure of the brain and the occurrence of many brain-related diseases through a special pathway, such as Alzheimer's disease, autism, depression, and Parkinson's disease. Various studies have shown that the microbial composition in the intestines of patients with these diseases differs from that of healthy individuals. Such findings have made the connection between the brain and the gut more diverse, and the "gut-brain axis" theory has gradually been revealed. So, can changing the gut microbiota also change the state of brain health? Let's explore how the two interact with each other.

Development and Mechanism of the Gut-Brain Axis Theory

The gut-brain axis is a bidirectional regulatory axis that communicates between the brain and the gut, mainly composed of the enteric nervous system, neuroimmune system, central nervous system, and hypothalamic-pituitary-adrenal axis. The gut microbiota affects the central nervous system through the mechanisms of the gut-brain axis, thereby influencing cognitive functions in the brain.

Figure: Brain-Gut-Microbiota Axis

03 The mechanisms of the gut-brain axis involve the vagus nerve pathway, immune pathways, and neuroendocrine pathways, which are very complex. In brief, gastrointestinal signals are projected to the central nervous system through the gut-brain axis, responding to various gastrointestinal stimuli; conversely, the central nervous system regulates the body's visceral activity through the gut-brain axis.

With the introduction of the concept of the gut-brain axis, research on

the impact of gut microbiota on brain health^{has gradually become a research hotspot in the fields of biology and medicine. Numerous animal and clinical experiments have demonstrated that probiotics show positive effects on improving cognitive functions of the brain.}

Key Neurotransmitter Related to Bifidobacterium in the Gut-Brain Axis - 5-HT (5-Hydroxytryptamine)^{5-HT is one of the most widely studied neurotransmitters and is a messenger that produces feelings of pleasure. Antidepressants such as fluoxetine work by increasing the levels of 5-HT in the brain. Individuals with lower levels of 5-HT are more prone to depression, impulsivity, alcoholism, suicide, aggression, and violent behavior. Scientists have found that the gut-brain axis can transmit substances such as 5-HT and dopamine to the brain, with about 95% of 5-HT stored in the gut. Recent studies have shown that bifidobacteria can increase the expression of Tph1, which breaks down tryptophan, and secrete 5-HT through intestinal epithelial enterochromaffin cells, which then enters the bloodstream and acts on the central nervous system (CNS), ultimately improving depressive symptoms. At the same time, the metabolic products of microorganisms, short-chain fatty acids (SCFAs), can also increase the production of 5-HT and improve depressive symptoms.}Gut microbiota is a very popular research area in recent years. They are the microbial communities living in our intestines, and numerous studies have confirmed that these special microorganisms are very important for the body's nutrient metabolism, development, immunity, and the occurrence of various diseases. People are also consciously regulating their gut microbiota by consuming probiotic products or adjusting their immunity to improve their health.

Figure: Mechanism of Probiotics and Depression in the Gut-Brain AxisReferences1. Wang Yongjun, Pan Yuesong, Li Hao. What is brain health? Why is brain health so important? [J]. Chinese Journal of British Medicine, 2021, 24(1): 1-4. DOI:10.3760/cma.j.issn.1007-9742.2021.01.102.

04 2. Jiang Yugang. Research Progress and Thoughts on Gut Microbiota and Cognitive Function [C]// Proceedings of the 23rd Academic Conference of Danone Nutrition Center - Nutrition and Cognition. 2020.

3. Kelly J R, Kennedy P J, Cryan J F, et al. Breaking down the barriers: the gut microbiome, intestinal permeability and stress-related psychiatric disorders [J]. Frontiers in Cellular Neuroscience, 2015: 392.^[6]Gut microbiota is a very popular research area in recent years. They are the microbial communities living in our intestines, and numerous studies have confirmed that these special microorganisms are very important for the body's nutrient metabolism, development, immunity, and the occurrence of various diseases. People are also consciously regulating their gut microbiota by consuming probiotic products or adjusting their immunity to improve their health.

图：脑肠轴中益生菌与抑郁症的作用原理图

参考文献
1. 王拥军, 潘岳松, 李昊. 什么是脑健康？为什么脑健康如此重要？[J] . 英国医学杂志中文版, 2021, 24(1): 1-4. DOI:10.3760/cma.j.issn.1007-9742.2021.01.102.

2. 蒋与刚. 肠道菌群与认知功能的研究进展和思考[C]//达能营养中心第二十三届学术会议——营养与认知论文集. 2020.

3. Kelly J R, Kennedy P J, Cryan J F, etal. Breaking down the barriers: the gut microbiome, intestinal permeability andstress-related psychiatric disorders[J]. Frontiers in cellular neuroscience,2015: 392.

4. Montiel-Castro A J, González-Cervantes RM, Bravo-Ruiseco G, et al. The microbiota-gut-brain axis: neurobehavioral correlates, health and sociality[J]. Frontiers in integrative neuroscience, 2013, 7: 70)

5. Li Ruiying, Yan Minghui, You Chunping. Research progress on the gut-brain axis and intestinal microbiota in mental diseases[J]. Food Industry Technology, 2021, 42(18): 419−426.

6. Tian P, Wang G, Zhao J, et al. Bifidobacterium with the role of 5-hydroxytryptophan synthesis regulation alleviates the symptom of depression and related microbiota dysbiosis[J]. The Journal of nutritional biochemistry, 2019, 66: 43-51.

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