Gut bacteria can affect mental health by influencing how our brains, hormones, and the way our brains work immune system. It can also affect the brain by regulating neurotransmitters such as serotonin, dopamine, and GABA (gamma-aminobutyric acid). Serotonin has been linked to depression in humans and mice. Dopamine is associated with motivation and reward-seeking behavior. Low levels of dopamine have been linked to depression in both humans and animals. GABA has also been shown to reduce anxiety in some people when taken as a supplement or through diet changes that increase GABA production (such as reducing intake of refined carbohydrates).