Do gut microbes play a role in learning?

StockSnap, Pixabay.com, CC0 1.0
StockSnap, Pixabay.com, CC0 1.0

We all know that learning happens in the brain and is influenced by a child’s environment. But recent research shows that we may have been neglecting one important agent: our gut.

Over the past several decades, research into the human microbiome has exploded. The microbiome is the collective genome of billions of micro-organisms – bacteria, viruses, eukaryotes, and archaea – that inhabit the human body. Some estimates suggest that microbial cells outnumber host cells in the human body 10 to 1.

Unsurprisingly, the composition of the microbial community, particularly those microbes that inhabit the gastrointestinal tract, has been linked to many health outcomes, most notably metabolic and immune disorders. More recently this nascent field has shifted its focus to mental health and behavior.

Most of what we know about how the microbiome affects behavior comes from studies of animals. One very useful model has focused on the gnotobiotic mouse. Since these mice are born and housed without exposure to microbes, they are germ-free. Relative to other mice, they exhibit many behavioral abnormalities, such as altered social behavior, anxiety, and memory irregularities. These effects may be rooted in the development of the microbiome very early in life, perhaps even prenatally.

A recent report tested this hypothesis by colonizing neonatal mice with bacteria found in human infants. When compared in adulthood with their germ-free counterparts, the colonized animals showed improvements in all behavioral domains, but particularly in recognition memory. Another recent study has shown that diet-induced changes in the microbiome are associated with cognitive flexibility. These findings suggest that the microbiome may have long-lasting effects on learning and cognition.

“Recent work indicates that microbes may also play a role in learning and cognition in human infants.”

Recent work indicates that microbes may also play a role in learning and cognition in human infants. Alexander Carlson and colleagues have shown that infants whose enterotypes – that is, their classification based on the composition of their intestinal microbiomes – differ at the age of 6 months show differences in cognitive performance at 24 months old.

Specifically, infants with microbiomes defined by higher levels of Bacteroides perform at the highest cognitive level at 24 months. This research is the first to show that microbes are associated with cognitive function in human infants. Further work is needed to identify the mechanisms that underlie these relationships.

The microbiome may affect learning in several ways. One likely developmental pathway is through brain development. In germ-free mice, this may manifest itself in alterations in brain structure, particularly in brain regions related to memory and cognitive function, such as the frontal regions and the hippocampus. Germ-free mice also show alterations in neurochemistry. Effects in humans have been less compelling, but ongoing work is focusing on the interplay between the gut microbiota and brain development in human infants.

“One day, we may be able to use food not only to keep our children’s bodies healthy, but to keep their minds healthy as well.”

This line of work is exciting because of its implications for helping children, beginning potentially in utero, reach their cognitive potential. The microbiome is relatively easy to manipulate, although changes can be tricky to maintain. As Kathy Magnusson and colleagues have pointed out, diet-related changes in the microbiome are related to improvements in executive function. One day, we may be able to use food not only to keep our children’s bodies healthy, but to keep their minds healthy as well.

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