Neurotransmitters are chemicals that transmit signals between nerve cells. Interestingly, many neurotransmitters, including serotonin, dopamine, and gamma-aminobutyric acid (GABA), are produced in the gut. In fact, about 90% of the body’s serotonin, a key regulator of mood, is synthesized in the digestive tract. An imbalance in gut bacteria can affect the production of these neurotransmitters, potentially leading to mood disorders such as depression and anxiety.