Your Brain and Your Microbiota…Which Forms Which?

This is a great follow up to my post on Tuesday about the relationship of personality to the gut bacteria.   Today I’m covering a new study from UCLA which looks at how the gut bacteria affect the actual structure and function of the brain.[i] Our understanding of the gut-brain axis continues to grow; it really is fascinating watching the slow but steady progress!

40 healthy women provided stool samples, and were then divided into two groups, based upon the composition of their gut flora.  33 of them had higher levels of Bacteroides than Prevotella; the other 7 had the opposite profile, with Prevotella-family bacteria predominating.

It turns out that their bacterial profiles were predictors of both the amount of gray matter in various parts of the brain as well as their reactions to different stimuli.

In terms of structure, “…white and gray matter imaging discriminated the two clusters, with accuracy of 66.7% and 87.2% respectively.” That is, there was statistically significant correlation between the make-up of the gut bacteria and the structure of the brain. Women with higher levels of Bacteroides (versus Prevotella) had thicker gray matter in their cerebellum and frontal region which are “…involved in the complex processing of information” and more brain matter in the hippocampus, which is a big player in memory processing.  The Prevotella group had less brain matter in several regions including “functional and structural differences in the hippocampus” (which is the brain region involved in emotion regulation), as well as differences in areas involved with attentional and sensory processing.

This latter group of women, when viewing negative images, showed less activity in their hippocampus, and greater levels of anxiety and irritability than the Bacteroides group.

Of course, we still face a chicken-or-the-egg scenario in that it is currently unknown whether or not it is the brain dictating the gut flora or the gut flora dictating the structure/function of the brain. The authors point out that while “…some aspects of the microbiota’s influence on the CNS are likely to be established early as traits, other aspects may be malleable and are vulnerable to environmental factors…”  Timing too may matter:  what you eat long term, or during early years of life while the brain is still developing, may have lasting influence.  It does appear that diet is a major factor.  In fact, these researchers have already published work demonstrating that in healthy women, 4 weeks of ingestion of “…a fermented milk product with probiotics can shift functional brain responses to an emotional attention task…providing more direct evidence in humans that brain function can be affected by modulation of gut microbiota.”

In Western populations, Bacteroides and Prevotella species tend to dominate, with the former outnumbering the latter –as they did in this sample of women. A diet high in fat and animal protein (i.e. a standard Western diet) is more associated with higher Bacteroides species than Prevotella, which is associated with a plant-based diet.  With so few women fitting into the Prevotella group then, the authors advise caution in interpreting their results.  Much larger studies are needed to confirm whether this “…propensity towards developing a more negative affect during this paradigm is a marker of personality traits, a risk factor for developing clinically relevant negative moods states, or just a healthy variant.”

The interpretation that a plant-based diet (which leads to higher amounts of Prevotella than Bacteroides but also, a more negative emotional state under these laboratory conditions) is therefore less desirable is fallacious.  Someone’s emotional state may, in fact, dictate their diet.  I, for example, thought about a healthy diet a lot less when I was younger and hadn’t yet faced my son’s autism diagnosis.  Yes, I am under a great deal more stress but we also eat way healthier.  Prevotella levels are markedly higher in non-Westernized societies, which consume plant-based diets, where people are actually less prone to “mental” illnesses like depression or anxiety. And don’t forget, low levels of Prevotella  have also been associated with leaky gut and also, Parkinson’s disease.  So the likelihood is that higher Prevotella amounts are better for health.  (Wouldn’t it be interesting if they followed these women over the next 20 years to see how they fare, health-wise?!)  In fact, the structure of the brain in these women may not actually be caused by their gut bacteria, or these particular gut bacteria.  This is a correlation study, not a causation one.

As always, I will watch and wait to see further research on this.


[i] Tillisch K, Mayer E, Gupta A, et al.  T, Zeevi D, Zmora N, et al. Brain structure and response to emotional stimuli as related to gut microbial profiles in healthy womenPsychosomatic Medicine. 2017. DOI: 10.1097/PSY.0000000000000493

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