Serotonin is an essential neurotransmitter responsible for regulating many body and brain activities. Though people associate serotonin only with the brain due to its role as a neurotransmitter, the brain only synthesizes and houses about 5 percent of the body’s supply of serotonin. The small intestine produces the rest, and this serotonin never finds its way to the brain; rather, it plays other roles in the body.
Two areas product serotonin: the intestines and the brain. Neurons produce serotonin in the brain. The serotonin in these two areas are distinct. That is, the serotonin from the intestine cannot go to the brain because of the blood-brain barrier, which the body uses to protect the brain from toxins.
Serotonin in the brain regulates mood, appetite and sleep. Serotonin also creates a general “good” feeling. A serotonin deficiency can result in depression, anxiety, violent behavior and obesity.
Until recently, scientists thought that the serotonin made in the duodenum (the “gut”) regulated digestion. In the November 26, 2008 issue of "Cell," Dr. Gerard Karsenty of the Columbia University research group discovered that a gene in the gut, Lrp5, controlled serotonin synthesis. Manipulating this gene caused changes in bone formation. Thus, Dr. Karsenty concludes, “The findings demonstrate without a doubt that serotonin from the gut is acting as a hormone to regulate bone mass.”
The amino acid tryptophan is essential in serotonin synthesis, but the body does not make it; it must be ingested. Simon N. Young, the Editor-in-chief of the Journal of Psychiatry, states that, contrary to popular belief, eating turkey (which is associated with tryptophan and drowsiness) and other high-protein foods (which actually contain the same amount of tryptophan) will not drastically increase serotonin levels in the brain. The blood-brain barrier only allows regulated amounts of amino acids to enter the brain, and tryptophan must compete with more abundant amino acids in order to enter.
A common drug for serotonin deficiency is a serotonin-selective reuptake inhibitor (SSRI) like Prozac. SSRIs try to preserve the serotonin already in the brain instead of helping produce more. Many scientists believe that the best treatment for serotonin deficiency is ingestion of 5-HTP, an amino acid made before serotonin in the synthesis process, because it raises serotonin levels. It also crosses the blood-brain barrier easier than tryptophan.