Tryptophan is an essential amino acid in the human diet. It is needed for normal growth in infants and for nitrogen balance in adults. The body also uses tryptophan in order to make niacin and serotonin (5-hydroxytryptamine, 5-HT). Tryptophan is an important building block in protein biosynthesis. Good sources of tryptophan are eggs, sesame seeds, cheese, milk, nuts, all sorts of meat,…
Some of the foods rich in tryptophan are :
- Peanut butter
- Pumpkin seeds
- Sesame seeds
At this time appropriate recommended dose is not established as there is not enough scientific evidence. However, data suggest that there is an optimum range for plasma tryptophan concentration and that the therapeutic response will decline above or below this range. For treating depression, dose should be lower than 6 g daily. 5 grams was superior dose for human growth hormone secretion compared to 2 g and 8 g.
Can L-Tryptophan Increase Human Growth Hormone?
As mentioned in the introduction, tryptophan is a direct precursor to serotonin. The synthesis of serotonin can be accelerated by increasing the availability of tryptophan. Tryptophan loading has been used for stimulating the release of growth hormone in order to evaluate the role of serotonin in the regulation of growth hormone release . Müller and colleagues  reported that orally ingested L-tryptophan (70 mg/kg) caused a slight rise in plasma human growth hormone in 32 subjects.
Koulu  wanted to find optimum dose of L-tryptophan for human growth hormone secretion. 2, 5 and 8 g L-tryptophan was given orally in random order to 8 healthy male volunteers. When comparing L-tryptophan with placebo, significant stimulation of human growth hormone release was observed (p<0.001) and 5 grams seems to be most suitable dose.
L-Tryptophan and Exercise
It has been suggested that branched-chain amino acid supplementation exert its effect by counteraction of increased tryptophan levels . If levels of tryptophan are increased this leads to an increase in the rate of transport of tryptophan across the blood-brain barrier which increases the rate of synthesis of serotonin (5-HT) in the brain . Elevated concentration of serotonin in the brain may be responsible (at least to some extent), for the development of physical and mental fatigue during exercise . Therefore, it seems unreasonable that L-tryptophan supplementation can decrease fatigue as high levels of brain 5-HT leads to fatigue.
Segura and Ventura  found that total exercise time was 49.4% greater after receiving L-tryptophan than after receiving the placebo. They suggested that exercise time was increased due to an increased pain tolerance as a result of L-tryptophan supplementation. A randomized double-blind study by Javierre and assistants  noted increased average anaerobic power output (at 50% of VO2 max for 10 min followed by maximal intensity exercise for 30 s) in twenty well-trained young males with L-tryptophan supplementation. They explain that some types of physical exercise aren’t always terminated due to fatigue but because of inadequate neural drive in the serotonergic system .
When foods high in glycemic index are consumed, glucose is rapidly transported into the bloodstream. This causes insulin levels to rise to drive glucose in to body’s tissues and maintain normal range of blood glucose levels . Insulin increases the uptake of branched-chain amino acids but not tryptophan (aromatic amino acid), increasing the ratio of tryptophan to BCAA in the blood stream . This change in ratio reduces the competition for large neutral amino acid transporter (or LAT1, which transports branched-chain amino acids and aromatic amino acids) at the blood–brain barrier , which increases uptake of tryptophan by the brain (across the blood–brain barrier into the cerebrospinal fluid). Tryptophan is than converted into serotonin , which is then converted to melatonin (a hormone that affects the modulation of wake/sleep patterns) by pineal gland . Increased brain serotonin and melatonin levels result in sleepiness .
Ahmad and colleagues  were the first to show that meal rich in high glycemic carbohydrates indirectly increases the production of sleep-promoting melatonin in the brain in healthy subjects.
Studies are showing that tryptophan deficiency, either due to inadequate diet intake or malabsorption, causes a number of symptoms, such as decreased protein synthesis and low levels of niacin and serotonin (5HT). There are some case reports with symptoms that include emotional lability, depression, confusion and hallucinations .
Tryptophan supplementation is known to have side effects such as nausea and gastrointestinal distress. However, of greater concern is the possible association with Eosinophilia-Myalgia Syndrome (EMS). In 1989, nearly 1 500 tryptophan users were affected which also led to over 30 deaths . It is not clear whether this was a chance association only or it was due to excess tryptophan itself.
(Other common names: L-Trypt, L-2-amino-3-(indole-3-yl) propionic acid, L-tryptophane)
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Müller, E. E., et al. “Slight effect of L-tryptophane on growth hormone release in normal human subjects.” Journal of Clinical Endocrinology & Metabolism 39.1 (1974): 1-5.
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Segura, R., and J. L. Ventura. “Effect of L-trypt supplementation on exercise performance.” International journal of sports medicine 9.05 (2008): 301-305.
Javierre, C., et al. “L-trypt supplementation can decrease fatigue perception during an aerobic exercise with supramaximal intercalated anaerobic bouts in young healthy men.” International Journal of Neuroscience 120.5 (2010): 319-327.
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