women with a higher proportion of Bacteroides, another
common type of gut bacteria, were much less likely to
experience negative emotions when shown the same
images. Structurally, their memory centers were larger, and
they also had more volume in their prefrontal cortexes,
which is the hub of executive function. It seemed as if the
women with less Prevotella and more Bacteroides were
emotionally stronger and more resilient.
Were the bacteria affecting the women’s brains, or were
the women’s brains somehow altering the mixture of
bacteria in the gut? No one knows. However, as they have
done with metabolism and weight, scientists have been able
to alter mouse behavior and what could be interpreted as
mouse mental health just by tinkering with their
microbiomes, suggesting that the types of bacteria present in
the gut do play a role in brain function.^39
As I’ve mentioned, the optimal gut makeup is a puzzle
that is a long way from being solved, and it is likely to be
different for you than it is for me. It is interesting to note,
however, that people who consume carbohydrate-rich,
grain-based diets tend to have higher proportions of
Prevotella bacteria residing in their guts.^40
Many scientists in the field seem to agree that the best
way to ensure that the beneficial bacteria maintain a
competitive edge in the tough and constantly changing
colonic environment is to consume a diet that is rich in fiber
and plant nutrients such as polyphenols, and to avoid sugar
and refined carbohydrates. This pattern will directly benefit
the helpful microbiota and will starve out the pathogens,
making it difficult for more malevolent species to gain a