Our gut microbiome is created shortly after birth, as both good and bad microbes slowly find their way to human intestines. They start creating the gut microbiome that consists of more than 100 trillion microorganisms. Up until recently, there hadn’t been any clear evidence that the gut microbiome influences cognitive development but, after several studies have been conducted, it has been proved that the gut microbiome is indeed linked to brain structure and emotional-cognitive function in infants, as well as adults, which means that it may play a huge role in how the human brain develops in the first years of someone’s life.
Evidence from Animal Studies
In the past several years, a lot of animal studies have been conducted to help understand the link between the gut microbiome and cognitive development. They have provided clear evidence that the two are really connected. Studies on rodents have proved that altering their intestinal microbiota clearly impacts their communication, cognitive function, and exploratory behavior.
Human Studies on the Gut-Brain Axis
There have been two human studies recently conducted by two research teams who wanted to delve deeper into the matter and find out whether or not the connection between brain development and gut microbiome is actually real. Both human studies have found that microorganisms in the gut do have an impact on cognitive development in infants and adults. Also, the research papers of both studies were published online in peer-reviewed journals before being printed.
The UNC School of Medicine Study
The first human study was conducted by the research team at the UNC (University of North Carolina) School of Medicine. The research team that conducted the study was led by Rebecca Knickmeyer, associate professor of psychiatry, who said: “This is the first study to demonstrate associations between the gut microbiota and cognition in human infants. As such, it represents an essential first step in translating animal data into the clinic.”
What the study found was that one-year-old infants who had greater amounts of specific bacteria in the gut showed much better results regarding cognitive development than infants who had other bacteria in the gut. What does this mean? It means that one-year-olds who had higher amounts of the so-called Bacteroides, the microbiota genus, showed much better gross motor skills and perceptual abilities, as well as much better and faster language development by age 2.
However, as the leading associate professor said, this is only the first step in proving the correlation between the gut microbiota and cognitive development in humans, so further studies will certainly take place, because correlation definitely does not indicate causation. Rebecca Knickmeyer stated: “Are the bacteria actually communicating with the developing brain? That’s something that we're working on now, so we’re looking at some signalling pathways that might be involved."
Alexander Carlson, a member of the UNC research team, agreed with the professor and stated: “This is the first study to show that cognitive development is associated with the microbiome, and so it’s the very first step." He added that one of the key takeaways from the study was that “when measuring the microbiome at age one, we already see the emergence of adult-like gut microbiome communities – which means that the ideal time for intervention would be before age 1."
The paper regarding this human study is available in the online journal of Biological Psychiatry, and it is titled “Infant Gut Microbiome Associated with Cognitive Development."
The UCLA Study
The second human study was conducted by the research team at the University of California, Los Angeles (UCLA). The study was focused on finding out whether or not gut microbiota influences brain and behavioral characteristics in middle-aged adults.
The UCLA and the associate professor of medicine in the Division of Digestive Diseases, Kirsten Tillisch, used DTI (diffusion tensor imaging) and structural imaging to see how brain matter behaves when different types of microbiota are in the gut. They analyzed the volume, surface area, mean curvature, and cortical thickness of grey matter, as well as fiber density of white matter that connects different regions in the brain.
Additionally, they used fMRI brain scans to analyze the connection between microbiota and emotional responses. Some of the study patients had higher levels of the bacterium Bacteroides, while others had more of a genus called Prevotella.
Regarding the brain volume metrics, the Bacteroides group of patients displayed more grey matter in the hippocampus, frontal lobes, and the cerebellum. The Prevotella group of patients showed lower volumes in different regions of the brain, including the hippocampus, while showing higher levels of distress and anxiety during the fMRI brain scans.
The Need for Further Studies
According to these two human studies, the bacterium Bacteroides definitely plays a great role in cognitive development and the optimization of brain function and structure in both infants and adults. However, although these studies have brought compelling evidence about the gut-brain axis, more studies and clinical research definitely need to be conducted in order to come to even more insightful conclusions.
This is because the main questions that still need to be answered are why and how exactly does the gut microbiome influence the overall cognitive development in infants, children and adults, and how the gut microbiome can be altered in order to optimize brain function and brain structure when it comes to both infants and adults.
Furthermore, the researchers want to find out whether or not the vagus nerve plays a psychophysiological role (and, if it does, what kind of role it actually plays) in correlation between brain structure, cognitive development, and the gut microbiota during someone's entire life.
This is precisely why further human studies need to be conducted, so that clear conclusions can be drawn regarding specific microbiome bacteria that influence human body, brain and mind. Most importantly, future studies will also try to influence various microbiome profiles, that is, find ways to shift them at different stages of development in children, so that the connection between brain and gut microbiota can be optimized.