Your gut produces 90% of your body's serotonin (the happiness hormone) and 50% of its dopamine. When gut bacteria are imbalanced, neurotransmitter production is disrupted, directly affecting your mood. The vagus nerve connects gut to brain, and an inflamed gut sends distress signals that manifest as anxiety, depression, or unpredictable mood swings. Many people find their mental health improves dramatically when they fix their gut.