There has been research suggesting that the vagus nerve might carry serotonin from the gut to the brain: https://www.psychologytoday.com/us/blog/the-athletes-w

There has been research suggesting that the vagus nerve might carry serotonin from the gut to the brain: https://www.psychologytoday.com/us/blog/the-athletes-way/201910/the-vagus-nerve-may-carry-serotonin-along-the-gut-brain-axis https://www.psychologytoday.com/us/blog/the-athletes-way/201... I'd note that this is a five year old article and I haven't heard of any followups. And the article itself may be misinterpreting the underlying actual research, which suggests that it's more about SSRIs: https://www.nature.com/articles/s41598-019-50807-8 https://www.nature.com/articles/s41598-019-50807-8 Still, I thought it was worth mentioning.