Found in 3 comments on Hacker News
Also, this book, The Second Brain, is a bit older, but relevant:

dillonmckay · 2020-08-03 · Original thread
The gut is a second brain, kinda...I have always been fascinated by this book:

The Second Brain

Author is Michael D. Gershon M.D., chairman of the Department of Anatomy and Cell Biology at Columbia University's College of Physicians and Surgeons at the Columbia-Presbyterian Medical Center in New York City.

TheBiv · 2016-05-19 · Original thread
There is a really good book written in 1999 that expounds on your research doctors point (and is written with a very approachable tone)

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