L-TRYPTOPHAN
L-TRYPTOPHAN by Robert Pollack, PhD Copyright © 2007
All living forms, including plant life, contain amino acids. Only plant cells are able to synthesize amino acids. Animals obtain the needed amino acids by directly eating protein contained in plants or by eating animals that ate plants.
Of the 20 amino acids that occur naturally in proteins, L-Tryptophan2 is, perhaps, the one which has been the most publicized and received the most attention.
Of the approximately 57 compounds that comprise the nutritional requirements of cells, tryptophan is one of the essential amino acids. In protein, tryptophan occurs in the smallest concentration, 0.5-1.6%, of the amino acids, in contrast to 25% of a group of amino acids termed the large neutral amino acids, LNAAs: isoleucine, leucine, phenylalanine, tyrosine, and valine. The LNAAs are of particular interest because they are transported across the blood-brain barrier by the same carrier system as tryptophan, and play a specific role in the way they influence tryptophan’s passage into the brain.
Body cells, including the brain, obtain their nutritive requirements from the circulating blood and fluids that surround them. However, the brain is very sensitive to the surrounding concentration of materials in the blood supply. In the process of traveling from the blood capillaries (extending through the brain) into the brain itself, there is in operation a control mechanism by which the entrance of nutrients into the brain, such as amino acids, are rigidly controlled: the blood-brain barrier. This barrier protects the central nervous system from the changing metabolite concentrations of the blood and serves to provide a constant chemical environment for optimum brain function.
Tryptophan not only has to compete with the LNAAs for access to the same transport carrier mechanism but compete with them despite the fact that these interfering amino acids (especially phenylalanine) have a greater affinity for the transport system than does tryptophan itself, which tends to decrease the amount of tryptophan that actually crosses the blood-brain barrier into the brain.
The blood level concentrations of tryptophan and its relationship to the LNAAs are expressed as the ratio of the concentration of tryptophan to the concentration of the LNAAs:Tryptophan, Isoleucine, Leucine, Phenylalanine, Tyrosine, Valine.
Changes in the concentration of either tryptophan or the LNAAs in this ratio affects the degree to which the tryptophan can pass through the blood-brain barrier. The higher the level of tryptophan, the greater is its ability to compete with the LNAAs for passage through the blood-brain barrier.
Because the body’s sole source of tryptophan is obtained from the food supply, it would seem logical that an increased intake of protein would augment the blood and brain supply of tryptophan – with a resultant increase in the serotonin level.
Paradoxically, exactly the opposite was observed in animal studies: a protein-rich meal results in a decrease of brain tryptophan. Tryptophan studies showed that the ratio of the plasma tryptophan levels to the sum of the other LNAAs tended to decrease as the dietary protein intake increased; the greater the quantity of protein eaten, the less will be the amount of tryptophan getting across the blood-brain barrier into the brain.
The effect of protein-rich food intake on brain serotonin level becomes even more significant in view of the fact that the daily intake of total protein for the average American is generally greater than the Recommended Daily Dietary Allowances, Food and Nutrition Board, National Academy of Sciences – National Research Council, Recommended Daily Allowances, Revised 1980: “The recommendation for the daily protein intake for the adult female is 44 grams per day and for the adult male, 56 grams per day”. Since an ounce of meat, fish, or poultry contains an average of 7 grams of protein, two three-ounce hamburger patties will provide all of the protein necessary for the average adult woman, and three of these same patties will provide all of the daily protein needed for the average adult male.
SEROTONIN
Once tryptophan enters the brain, it is converted to serotonin, a major neurotransmitter in the brain.
Serotonin performs a number of functions in the body from acting as a powerful vasoconstrictor to its role as a neurotransmitter sending messages throughout the brain. Its best known function is its effect on nerve receptors in the brain involved with the neurotransmission of various types of nerve impulses.
In the central nervous system serotonin plays a role in the regulation of pain, depression, sleep, and appetite. Lower than normal levels of serotonin have been associated with aggressive and anger disorders, suicide, depression, obsessive compulsive disorder, and migraines.
