Past research has shown that the microbiome and the gut-brain axis contribute to health in a variety of ways, such as influencing the onset of allergies, obesity and inflammatory bowel diseases -- and now, scientific evidence indicates that the microbes inhabiting the intestines can play a substantial role in behavior, to boot.
The developing brain and the microbiome
During the first few years of a child's life, it's estimated that between 700 and 1,000 new connections between neurons -- or synapses -- form every single second. But after this period of tremendous growth, the number of synapses is reduced through a process known as "pruning." Special immune cells in the brain, known as microglia, break down some of these newly formed synapses. This pruning process allows the remaining synapses to become stronger and more efficient, and it's considered essential for normal postnatal development.
Ultimately, the child's environmental conditions, experiences and interactions have a substantial impact on what synapses are preserved. Those connections that are activated consistently will be maintained, while those that are not used are pruned away. (See more news about brain health and brain physiology at Brain.news)
Does the internal environment of the intestines play a role in these connections as well? And what would happen if microbes were removed from this process entirely? That's what German researchers wanted to know. By using germ-free mice, they were able to determine that the absence of an intestinal microbiome prompted abnormal microglia function. Mice raised without a microbiome experienced abnormal development of their central nervous systems, compared to conventional mice with microbes.
The researchers questioned if microbial metabolites could have played a part in this phenomenon. As Chris Kresser explains, "The gut microbiota are constantly processing fermentable fibers from the diet and producing a wide range of metabolic end products, including short-chain fatty acids (SCFAs). SCFAs are known to be absorbed into circulation and influence host physiology by binding to free fatty acid receptors (FFARs) on cells throughout the body."
This inspired the researchers to create mice that lacked FFAR2. They found that these specialized mice exhibited similar abnormalities of the microglia seen in the germ-free mice. Due to their findings, the research team concluded that microbial metabolites are essential for the microglia to mature and function properly.
Microbiome and temperament
Researchers from Ohio State University have also found that the microbiome plays a significant role in childhood development. In a study of 77 toddlers, scientists discovered that the abundance and diversity of microbial populations in the gut appeared to impact behavior -- and the effect was most noticeable in boys. This correlation persisted even after the researchers controlled for other factors like diet, method of childbirth and breastfeeding, which are all known to impact microbial populations.
Lisa Christian, Ph.D., a researcher with Ohio State's Institute for Behavioral Medicine Research, commented, "There is substantial evidence that intestinal bacteria interact with stress hormones- the same hormones that have been implicated in chronic illnesses like obesity and asthma."
Christian says that a toddler's temperament gives them a good idea of how they react to stress, and posits that when combined with an analysis of the microbiome, this information could help them identify opportunities to prevent chronic illnesses.
The team also discovered that children with the most diverse microbial populations exhibited behaviors indicative of positive mood, curiosity, sociability, and impulsivity more frequently. The researchers believe that there is definitely some kind of communication between the brain and the gut.
In boys, extroverted behavior was associated with an abundance of microbes from the Rikenellaceae and Ruminococcaceae families and Dialister and Parabacteroides genera. However, in girls, an abundance of Rikenellaceae family microbes was actually associated with more fearful behaviors compared to those with a more balanced microbial population. Overall, less diversity of gut bacteria seemed to promote more focused attention, "cuddliness" and "self-restraint" in girls.
While it is not yet known if this relationship is merely casual or more involved, the researchers posited that there is a connection between the microbiome and the hypothalamic–pituitary–adrenal (HPA) axis. The HPA axis has been an area of significant interest when it comes to temperament. Studies have indicated that changes in the function of the HPA axis are linked to changes in temperament. For example, research has shown that germ-free mice exhibit a much more exaggerated HPA response compared to mice with microbes.
If microbes impact the function of the HPA axis, in this way they could also play a role in temperament.
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