Gut microbes are linked to a vast array of conditions, influencing digestion, immunity, metabolism and brain function, with imbalances associated with increased risks for metabolic issues, gastrointestinal diseases, neurological disorders, autoimmune conditions and even certain cancers.
Now, a new study from Northwestern University has linked gut microbes to something positive—brain size and function.
Humans have the largest relative brain size of any primate, but little is known about how mammals with larger brains evolved to meet the energy demands required to support brain growth and maintenance. This paper, published in PNAS, provides the first empirical data showing the direct role the gut microbiome plays in shaping differences in the way the brain functions across different primate species.
In a controlled lab experiment, the Northwestern researchers implanted gut microbes from humans and squirrel monkeys, who are both large-brain primate species, and macaque, who are small-brain primate species, into microbe-free mice.
Within eight weeks, the researchers noted changes between the mice implanted with large-brain primate microbes as opposed to small-brain. For example, they found increased expression of genes associated with energy production and synaptic plasticity—the physical process of learning—in the mice with microbes from humans and squirrel monkeys. In the mice microbes from macaque, the researchers say there was less expression of these processes.
Importantly, the researchers also saw a pattern of gene expression associated with ADHD, schizophrenia, bipolar and autism in the genes of the mice with the microbes from the smaller-brained macaque.
While there is existing evidence showing correlations between conditions like autism and the composition of the gut microbiome, there is a lack of data showing causation. It is an area of study that is hotly researched, currently.
“This study provides more evidence that microbes may causally contribute to these disorders—specifically, that the gut microbiome is shaping brain function during development,” said PI of the study Katie Amato, associate professor of biological anthropology at Northwestern. “Based on our findings, we can speculate that if the human brain is exposed to the actions of the ‘wrong’ microbes, its development will change, and we will see symptoms of these disorders. [In other words,] if you don't get exposed to the ‘right’ human microbes in early life, your brain will work differently, and this may lead to symptoms of these conditions.”