L-Tryptophan & Serotonin

Tryptophan is one of 20 amino acids that your body uses to build proteins and other biological molecules or to burn for fuel. Of these 20 amino acids, your cells can manufacture 14 from other molecules. The other eight -- nine in children, whose metabolic capabilities are underdeveloped -- are considered "essential" amino acids because they cannot be synthesized in your body and must be obtained from your diet. Tryptophan, a molecule that is needed to produce the neurotransmitter serotonin, is one of the essential amino acids.

Basic Chemistry

L-tryptophan, the form of this amino acid that is biologically active in your cells, is a stereoisomer. Stereoisomers are two molecules of the same compound that form mirror images when they are compared side by side. Chemists have developed a convention for naming stereoisomers as dextro- or levo- forms, depending on which direction polarized light is bent when it passes through a pure solution of a given molecule. Most of the amino acids found in nature are of the levo-, or L, form.

Sources of Tryptophan

According to Elson Haas, M.D. and author of "Staying Healthy with Nutrition," your dietary intake of L-tryptophan is probably lower than it is for most other amino acids because this nutrient is not particularly plentiful in any food. Not surprisingly, foods with higher protein contents typically contain higher levels of L-tryptophan. Foods with the highest levels include beef, pork, poultry, eggs and milk. Plant sources of L-tryptophan include flaxseed, cowpeas, lentils, nuts, soy, asparagus and legumes.

Serotonin from L-Tryptophan

Serotonin is a neurotransmitter that is present in high concentrations in the gastrointestinal tract, in the central nervous system and in platelets, which are fragments of cells that are involved in forming blood clots. Serotonin is synthesized in your body from L-tryptophan via a simple metabolic pathway that converts L-tryptophan to 5-hydroxy-L-tryptophan, or 5-HTP, and then converts 5-HTP to serotonin. Serotonin that is consumed in your diet or that is produced elsewhere in your body cannot pass from the bloodstream into the central nervous system, so your brain must manufacture the serotonin it needs from the precursors L-tryptophan and 5-HTP. Notably, these precursors do cross the blood-brain barrier relatively easily.

Functions of Serotonin

Serotonin serves a variety of functions in your body. Whenever food enters your intestine, serotonin stimulates intestinal contraction, thus moving the food through your gut. Irritants in food increase serotonin secretion, which pushes the food through the intestine more quickly and might even stimulate vomiting, which helps to rid your body of potentially toxic material. Platelets release serotonin at the sites of injury, which causes blood vessels to constrict and reduces blood loss. Serotonin's functions in the central nervous system are complex and include appetite and mood regulation, sleep cycles, pain perception, reproductive behavior, socialization and the ability to assess environmental status. Your brain is capable of altering both its production of and its responses to serotonin.

Practical Applications

Among the most commonly prescribed drugs in the world are those that influence levels of serotonin or the receptors that bind to it. Medications for depression, social phobia, anxiety disorders, migraines and post-chemotherapy vomiting have sprung from our expanding knowledge of serotonin's protean nature. 5-HTP and L-tryptophan are popular over-the-counter supplements among people who want to increase their levels of serotonin without resorting to prescription drugs, but too much serotonin can cause unpleasant and even dangerous side effects. According to a 2005 article in "The New England Journal of Medicine," signs and symptoms of excess serotonin can include nausea, vomiting, diarrhea, muscle twitching, sweating, agitation, rapid heart rate and increased blood pressure. If you are already taking a medication that changes the way your body responds to serotonin, do not take additional similar medications or serotonin-boosting supplements without consulting your physician.