Serotonin is an important neurotransmitter in the central nervous systems on mammals. In man, research has uncovered many functions for serotonin. Originally discovered in both Italy and American, the name serotonin derives from the word ‘sero’ for blood and ‘tonin’ for muscle tone, this being on account of the discovery that the substance was able to affect muscle contraction. Chemically serotonin is known as 5-hydroxytryptamine, and although it plays an important role in the brain and central nervous system, serotonin has effects on most physiological systems in the body, particularly the digestive system. Serotonin is synthesised primarily in the raphe nuclei in a pathways that required the essential amino acid L-tryptophan and therefore serotonin systems are reliant of dietary sources of protein to function. The amino acid L-tryptophan is converted to 5-hydroxytryptophan, and then to serotonin with tryptophan hydroxylase, an iron requiring enzyme, controlling the rate limiting step.
A number of symptoms of serotonin deficiency have been identified including low self-esteem, anxiety, eating disorders, cravings for sweet foods, impatience and impulsiveness, fatigue and a loss of libido. Although protein foods are the source of the precursor amino acid for serotonin synthesis, it is carbohydrate foods that might be most useful in increasing brain levels of serotonin. This is because carbohydrate foods stimulate insulin release and this drives branched chain amino acids into muscle tissue. Removing branched chain amino acids from circulation removes competition for the large neutral amino acid transporter to the brain, which L-tryptophan uses. As a result more L-tryptophan makes it into the brain and more serotonin is produced. A number of foods including bananas and pineapples are good sources of serotonin, but it is unclear how well they raise brain levels of serotonin. Taking dietary supplements of 5-hydroxytryptophan from Griffonia simplicifolia also increase brain levels of serotonin.