The gut microbiome includes bacteria, viruses, fungi, bacteriophages, protozoa, and archaea. This community of organisms is critical to the maintenance of human health, and heavily influences the progression of various diseases. The community in your gut is unique, much like your fingerprint.
Your microbiome began to colonize you the moment you were born, and can change throughout your lifetime. Good bacteria in your gut help you absorb nutrients from your food. They also take up space and hog vital nutrients so harmful microorganisms are not able to colonize, and they even teach immune cells how to identify harmful invaders. However, did you know that your gut microbiome also affects your mental health? These tiny beings help you break down food traveling through your intestines, hence producing metabolites which influence all of your cells, including those of your nervous system.
Additionally, immune responses to harmful pathogens produce molecules that can affect brain physiology. But that's just scratching the surface. A healthy and diverse microbiome is essential for normal cognitive and emotional processing. Your microbiome communicates with the central nervous system, the brain and spinal cord, through nervous, endocrine, and immune signaling mechanisms.
We don't yet have a good understanding of how the gut microbiome and central nervous system influence one another, but it has been shown that changes in gut flora composition can result in increased intestinal permeability, which allows neuroactive compounds to enter the blood. Other microbiota can produce compounds that affect gene expression in the nervous system. Research has shown that changes in microbiota can cause depression, influence one's reaction to social situations, and protect one's immune system from stress-induced changes. Amazingly, research using fecal transplants has shown that the microbiome can cause physiological changes that are even transferable between species. The microbiome and nervous system don't just have a one-way relationship, however.
The microbiome can affect how we think, But also, our nervous system and lifestyle have a major effect on the composition of our microbiome. Although the human gut microbiome is generally fairly stable and resists change in community composition, the brain can actually modulate the composition of our gut community by changing intestinal permeability and secretions, as well as by releasing hormones that affect microbial gene expression. Our gut flora composition can also be perturbed by changes in hormones, diet, antibiotics, emotional state, and stress.
For instance, antibiotics reduce one's normal gut biota population, providing an opportunity for pathogenic biota to colonize the gut epithelium. It has been known for some time that the gastrointestinal system communicates with the brain. The enteric nervous system is a mesh of 500 million neurons governing the gastrointestinal tract. That's five times as many neurons as there are in your spinal cord.
No wonder the enteric nervous system is sometimes called the second brain. Hypothetically, it could even operate autonomously. In reality, the enteric nervous system communicates with the central nervous system via the vagus nerve and prevertebral ganglia. This biochemical signaling between the gastrointestinal tract and central nervous system is called the gut-brain axis. However, it is only now being realized just how much of an effect the microbiome has on the brain, and so the bidirectional interaction between the microbiome and the central nervous system is now being termed the microbiome-gut-brain axis.
Absence of a normal gut microbiome in early life significantly impacts one's response to stress in adulthood. One of the first studies that turned the microbiome-gut-brain axis into a hot research topic was a study in 2004 that showed differences in behavior between germ-free and non-germ-free laboratory mice. The mice lacking a microbiome showed an exaggerated stress response. This was reversed when their gut was colonized by a Bifidobacterium species. Elimination of the gut microbiome in mice resulted in problems with spatial and working memory.
In other studies, dietary modifications also altered the performance of mice on memory tasks. One potential mechanism for these changes is the nerve growth factor BDNF, which is short for brain-derived neurotrophic factor. This substance influences neuronal development, protects against stress-induced damage, and is important in determining stress tolerance, mood, and cognitive function.
Mice with healthy microbiomes have higher expression of BDNF in their brains, which might be why they have better memories. Another study showed that, even in the absence of obesity, the microbiome associated with obesity can cause neurophysiological changes. Researchers used donor mice for the development of two different types of microbiomes. One group was fed a diet with 13% fat calories, while the other was fed a diet with 60% fat calories.
Ten weeks after the diets commenced, the researchers harvested their microbiomes. At this time, mice from the leaner group weighed an average of 24.5 grams, while mice from the high-fat diet group weighed an average of 37 grams. Next, another group of mice, the microbiota recipients, were given antibiotics daily for two weeks to eliminate their original microbiomes.
Three days after the end of the two-week course of antibiotics, the mice were recolonized by donor microbiota from either the group that had been fed the high-fat diet, or from the group that had been fed the control diet. Behavioral testing was conducted and results indicated a decrease in exploratory behavior and an increase in anxiety-based behavior for the high-fat microbiota receiving recipients. However, the locomotor activity and total distance traveled was the same for both groups of recipient mice, indicating no effect of the different microbiomes on motor function. The mice which received the high-fat diet microbiota also had increased intestinal permeability. The authors also examined markers for brain injury and inflammation, and found that the high-fat diet microbiota receiving mice had higher numbers for these markers.
It is, of course, much more difficult to conduct studies regarding the relationship of the brain and the microbiome in humans. One interesting link that is still being investigated is the correlation between autism and high levels of Clostridium bacteria in children's stools. Around 70% of people with autism suffer from gastrointestinal problems. These gastrointestinal problems may be associated with an altered gut microbiome.
one that causes increased intestinal permeability. Although much more research needs to be done before any conclusions can be made, it is possible that certain developments in gut flora may trigger autism, or that the two develop concurrently. One question this raises is whether C-sections increase risk of children developing autism. During their exit through the birth canal, babies come into contact with some of their mother's microbiome, the first bacteria they are exposed to, which then play an important role in the development of their own microbiome. However, Babies born via c-section do not get exposed to these microbes.
In fact, one study found a 21% higher risk of developing autism in children born by c-section. Other human studies have found that probiotics can reduce anxiety and OCD-like behavior, and can even help normalize emotion-related behavioral development after early life trauma. Consumption of probiotics can alter activity in areas of the brain involved in cognitive functions. Change in diet can also have profound and rapid effects on the structure of the gut microbiome in both human beings and mice, and these changes have been shown to influence memory and learning.
More research on the microbiome-gut-brain axis will help us get further insights into disorders of both the gut and the central nervous system. This is exciting news because it may be that one day, neuropsychiatric disorders will be treated through gut microbiota. So what can you do to maintain the health of your gut flora?
Eat a healthy diet! Additionally, This gives us a reason not to overuse antibiotics. Part of our long-term health is protecting our gut biome. If you liked this video, like and subscribe.
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