Over half of all neurotransmitters originates in the gut, but the barrier preventing the entry of neurotransmitters into the brain is very effective in healthy individuals.

Signals from the gut to the brain arrive via the vagus nerve and probably also the other cranial nerves.

Interesting! The way I've often heard it is that "over half of neurotransmitters originate in the gut" — making it sound like it's talking about the neurotransmitters already at the brain. Do you have any citations that could clarify this?

Neurotransmitters are used all over the body. A lot is centralized in the brain, but the periphery also uses neurons and neurotransmitters.

Neurotransmitters in general don't particularly imply they're being used in the brain. Some neurotransmitters travel through the body, but a larger fraction are produced locally by a cell and intended only for close neighbors.

I probably learned that fact from an episode of neuroscientist Andrew Huberman's podcast--more likely than not episode one:

https://youtu.be/H-XfCl-HpRM