There are well established connections. About 90% of the neurotransmitter serotonin in your body is produced by enterochromaffin cells in the stomach. Though

There are well established connections. About 90% of the neurotransmitter serotonin in your body is produced by enterochromaffin cells in the stomach. Though this does not directly affect the brain (the brain has it's own serotonine synthesis) it modulates a multitude of responses like blood pressure, body temperature, which, in turn, affect the mood. Quote from Wikipedia: Serotonin is a neurotransmitter and is found in all bilateral animals, where it mediates gut movements and the animal's perceptions of resource availability. [citation needed] In less complex animals, such as some invertebrates, resources simply mean food availability. In more complex animals, such as arthropods and vertebrates, resources also can mean social dominance. In response to the perceived abundance or scarcity of resources, an animal's growth, reproduction or mood may be elevated or lowered. This may somewhat depend on how much serotonin the organism has at its disposal.