The mechanism by which carbohydrate, including sugars, may affect mood is uncertain, but may involve the synthesis and release of serotonin in the brain. (fitnessandpower.com)
Rather, sour tastants evoke release of serotonin (5 - hydroxytryptamine; 5 - HT) directly from type III cells (Huang et al., 2005, 2009). (jneurosci.org)
Sugar gives you a temporary lift because it triggers an immediate release of serotonin. (mindbodygreen.com)