has suggested that overweight people have more Firmicutes
than Bacteroidetes (or Capulets than Montagues, in our
Shakespeare analogy). At this point, it is unknown whether
this or any other signature is causally related to, or merely
reflective of, the health of its human host. However, animal
studies using fecal microbial transplants are paving a road
toward greater clarity. With this method, we’re able to
answer the question: can we change aspects of an animal’s
health and appearance by changing its microbiome?
In one such example, scientists wanted to see what
would happen if they transplanted the microbiomes of obese
mice with insulin resistance into the digestive tracts of lean
mice. As if by magic, the lean mice, when given the obese
mice’s microbes, began to gain weight, displaying the same
metabolic dysfunction as their obese counterparts.^37 While
humans are more complex than mice, this study does
suggest that microbes, in many ways, call the shots—at least
where our weight is concerned. But what about our mental
health and cognition?
For the first time, groundbreaking research illustrated a
link between brain structure and function and gut bacteria in
healthy humans. In this UCLA study, healthy women had
their microbiomes sequenced and brains scanned and were
given a test that assesses risk for depression. Those women
with a higher proportion of Prevotella, a type of bacteria, in
their guts had heightened connectivity between emotional
and sensory brain regions while having smaller and less
active memory centers.^38 When they were shown negative
imagery, these women seemed to experience stronger
emotions, as if they were distressed. On the other hand,