Tryptophan is an essential amino acid found in several kinds of food. It can't be created by the body and must be consumed, and so follows a path of digestion and absorption in the body to reach the brain, where it assists in hormonal production and regulation. Tryptophan is found in many protein-rich foods, such as meats and fish, and as those foods are broken down in the stomach, the tryptophan is released from its chemical bonds and allowed into the bloodstream. While there, tryptophan is rapidly metabolized by body cells to help build necessary proteins; additionally, tryptophan is blocked by the blood-brain barrier, which protects the brain against pathogens. Tryptophan shares a pathway with tyrosine, another amino acid, and so a large concentration of one can block the other. Once tryptophan reaches the brain, it is metabolized to create serotonin, niacin, and auxin, all necessary compounds for healthy body function.