Adequate gut serotonin production is necessary to maintain brain serotonin levels, impacting mood.

“First of all, some of you, but perhaps not all have heard that more than 90% of the serotonin that we make is in our gut. And indeed we have a lot of serotonin in our gut. We have neurons in our gut that makes serotonin. We have neurons in our brain that make serotonin, but here's the deal. Most of the serotonin that impacts our mood and our mental state is not in our gut. Most of it is in the neurons of the brain in an area called the raphae nucleus of the brain, there are a few other locations too, and those are the neurons that control whether or not we feel satiated or not. Whether or not we feel happy and calm.”