The gut is the only organ system most people experience as a mood. It announces itself after a course of anti-inflammatories, a stretch of bad sleep, a hard training block, or a stressful month, and it is easy to treat that as background noise rather than as a tissue doing difficult work under load.
This category is small on purpose. There is one prescription treatment here, and it is here because its research is genuinely about the gut rather than borrowed from somewhere else.
