The Gut, The Brain, And the Immune System

It’s not often that a true paradigm shift occurs in a well-established field such as Neurology, but two recent research trends seem to be building towards a very exciting one. The first relates to the microbiome, that ecosystem of friendly bacteria that live in our gut and help us to digest food. For a decade or more, this has been an active area of interest with regards to diet and obesity, but lately, there’s been some research on the effects of the microbiome on the brain. The most striking of these studies has to do with Autism. It seems that people with this disorder have a higher proportion of digestive ailments than the rest of us and consistently different microbiomes. In a recent study, when autistic mice were fed B. Fragilis, a species of friendly bacteria commonly found in the gut, their symptoms improved dramatically. In another study, 12 healthy women were given probiotic yogurt twice a day for 4 weeks and then had their brains scanned while they were shown images of different emotional facial expressions. This group was compared against 13 healthy women who did not eat the yogurt. The yogurt-consumers had significantly calmer brain reactions to the facial expressions. All of this new evidence suggests a much more direct connection between the gut and the brain than was previously supposed. The exact mechanism of this connection is still unknown, but a very recent study on Parkinson’s disease implicates the Vagus nerve as one potential conduit.

The second trend has to do with the elusive connection between the brain and the immune system. Intuitively, it has always made sense that the nervous system and the immune system somehow communicate with one another, but until last month, no one had any evidence that they did. In a stunning discovery, researchers at the University of Virginia School of Medicine uncovered an unknown system of lymphatic vessels in the brain. That we could have overlooked this system for so long is a marvel in and of itself. But this discovery is quite literally the missing link that will open a whole new world of understanding. This avenue may even lead in our lifetimes to new treatments for intractable neurological diseases. A few years ago, if you had told me that there was a direct connection between the gut and the brain, I would have probably smiled and filed the comment away under unsubstantiated alternative mumbo-jumbo. I would have had a similar reaction to talk of a connection between the immune system and the nervous system. But in 2015, we have solid scientific evidence to support both claims. And it’s a very exciting time to be a neurologist.