[jinder]

While the serotonin used in your gut doesn’t directly impact the brain, it does impact the enteric nervous system which communicates via the vagus nerve to your brain. So I don’t think it’s correct to say this doesn’t (or couldn’t) impact your mood (and indeed murine experiments demonstrate it does).

[avisser]

Perhaps my favorite science experiment, next to dual-slit, https://radiolab.org/podcast/197242-gut-feelings

They cut the vagus nerves of rats and see behavior change.

Which makes sense to me - when "brains" first evolved, the connection to the stomach would've been the prime connection.

[jinder]

Not just that. In mice, SSRIs stop having any anti-depressant effects if the vagus nerve is severed: https://www.nature.com/articles/s41598-019-50807-8

So they also seem to be effective via the enteric nervous system!