Your gut is home to an ecological community that harbors over 100 trillion microorganisms. Both “good” and potentially harmful microbes begin residing within human intestines shortly after birth to create the gut microbiome, which consists of different strains of intestinal microbiota. In the past month, two new human studies on the "gut-brain axis" are helping us better understand the link between specific microbiome colonies, brain structure, and emotional-cognitive function in both infants and adults.
In recent years, countless animal studies have provided compelling evidence that microorganisms in the gut play a role in neurodevelopment. More specifically, altering intestinal microbiota in rodents has been found to impact their cognitive, communicative, and exploratory behaviors. However, until recently, very few human studies have been conducted on the relationship between different types of gut microbiome colonization and brain development.
In recent weeks, two separate human studies on gut-brain-microbiota interactions from research teams at UNC School of Medicine and the University of California, Los Angeles (UCLA) were published online ahead of print in peer-reviewed journals.
The first human gut microbiome study, by researchers at the University of North Carolina School of Medicine, found that one-year-old infants with higher levels of specific gut bacterial colonies had better cognitive scores compared to infants with other gut microbiome colonies. Their paper, “Infant Gut Microbiome Associated with Cognitive Development,” is currently available online in the journal Biological Psychiatry.
The UNC research team, led by associate professor of psychiatry, Rebecca Knickmeyer, stated: “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.”
The main takeaway from this study is that infants who had more of the microbiota genus Bacteroides as one-year-olds displayed better gross motor skills, perceptual abilities, and language development by age 2. Although these findings may turn out to be a game changer, it's important to proceed with caution before drawing conclusions or assuming causal links. As we know: Correlation does not mean causation. And, as the researchers are quick to point out, this is just a first step.
The second pioneering human study on the gut-brain axis from UCLA identifies brain and behavioral characteristics clustered by someone's gut microbiota profile in middle-aged adults. I reported on these findings in a June 2017 Psychology Today blog post, “Gut Microbiota May Influence Mood and Behavior."
For this human study, Kirsten Tillisch, associate professor of medicine in the Division of Digestive Diseases and colleagues at UCLA used structural and diffusion tensor imaging (DTI) to obtain gray matter metrics (volume, cortical thickness, mean curvature, and surface area) as well as white matter fiber density connecting various brain regions in patients with different gut microbiota profiles.
The researchers also conducted fMRI brain scans to assess microbiota-based emotional responses to disturbing images between study participants with more of the aforementioned bacterium called Bacteroides or a genus called Prevotella.
Notably, the Prevotella group in the UCLA study displayed lower volumes in various brain regions including the hippocampus and higher levels of negative feelings such as anxiety and distress in the fMRI portion of the study. Regarding regional brain volumes, the Bacteroides cohort showed more gray matter in the cerebellum, frontal lobes, and hippocampus.
When I learned about the gut-brain findings of the UNC infant study this morning, the first thing that jumped out at me was their discovery of Bacteroides and cognitive development during the earliest stages of human life as it dovetails with the UCLA findings on Bacteroides and the gut-brain axis in adults.
Of course, much more research would be needed before drawing any type of conclusions about Bacteroides. But, speculatively, one could hypothesize that this specific gut microbiome colony may play a role in optimizing brain structure and function in both infants and adults based on these two studies. Therefore, I will keep my antennae up for more clinical research and empirical evidence about Bacteroides and will report on any new findings when they are published.
In a statement to UNC, first author, Alexander Carlson, described their latest research paper:
"This is the first study to show that cognitive development is associated with the microbiome, and so it's the very first step. We're not really at the point where we can say, 'Let's give everyone a certain probiotic.' But we did have a few big takeaways from what we found. One 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."
Identifying optimal communities of gut microbiome will undoubtedly be addressed in future research. According to Knickmeyer, another million-dollar question for upcoming studies should be: “Are the bacteria actually communicating with the developing brain?" She adds, “That's something that we are working on now, so we're looking at some signaling pathways that might be involved.”
As is often the case, the latest findings on brain-gut-microbiota interactions answer some questions...but reveal the need for further investigation—especially on how and why infant gut microbiome profiles play a role in overall child development.
Additionally, because the vagus nerve is the superhighway that keeps the bidirectional communication lines open along the gut-brain axis, it will be interesting to find out what psychophysiological role (if any) the vagus plays in the association between someone’s gut microbiota colonies, brain structure, and cognitive development across the human lifespan.
Future studies will help us better understand the influence that specific microbiome colonies have on the human mind, body, and brain. Hopefully, the scientific community will also identify actionable ways to shift microbiome profiles at various stages of child development to optimize the gut-brain-microbiota axis. Stay tuned for updates on this topic.
Carlson, Alexander L., Kai Xia, M. Andrea Azcarate-Peril, Barbara D. Goldman, Mihye Ahn, Martin A. Styner, Amanda L. Thompson, Xiujuan Geng, John H. Gilmore, and Rebecca C. Knickmeyer. "Infant Gut Microbiome Associated with Cognitive Development." Biological Psychiatry (2017). DOI: http://dx.doi.org/10.1016/j.biopsych.2017.06.021
Tillisch, Kirsten, Emeran Mayer, Arpana Gupta, Zafar Gill, Rémi Brazeilles, Boris Le Nevé, Johan ET van Hylckama Vlieg, Denis Guyonnet, Muriel Derrien, and Jennifer S. Labus. "Brain structure and response to emotional stimuli as related to gut microbial profiles in healthy women." Psychosomatic Medicine (2017). DOI: 10.1097/PSY.0000000000000493
Forsythe, Paul, John Bienenstock, and Wolfgang A. Kunze. "Vagal pathways for microbiome-brain-gut axis communication." In Microbial Endocrinology: The Microbiota-Gut-Brain Axis in Health and Disease, pp. 115-133. Springer New York, 2014.