Serotonin is a chemical nerve cells produce. It sends signals between nerve cells. Serotonin is mostly produced in the digestive system. An important building block of serotonin is the essential amino acid tryptophan. This amino acid can’t be built by the body and therefore must enter through the diet and is commonly found in foods such as nuts, cheese, and red meat. A tryptophan deficiency can lower the serotonin levels. This can result in mood disorders, such as anxiety or depression.