As an amino acid, L-tryptophan -- or more simply, tryptophan -- comes from the protein-containing foods you eat. Your body uses it to build the new proteins it needs. Beyond this vital job, tryptophan has other roles. After it gains entry to your brain, it's converted into the neurotransmitter serotonin. It also protects you from a niacin deficiency, because your body can turn tryptophan into niacin.
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Tryptophan and Your Brain
The blood-brain barrier determines which substances in your blood can pass into the brain. At least nine amino acids, including tryptophan, compete with one another for access to the same carrier that transports them across the barrier. The amino acids present in the highest amount in your blood are more likely to win the competition. Tryptophan occurs in the smallest amount in most proteins, so it has a hard time gaining access, according to the "Encyclopedia of Neuroscience." You can increase tryptophan’s chances by consuming it with carbohydrates. Carbs trigger the release of insulin, which lowers the amount of other amino acids in your blood without affecting levels of tryptophan.
About 80 percent of the serotonin in your body is in your gut, where it regulates activity in your intestines, according to "Medical News Today." The rest is in your brain, which is where tryptophan becomes essential. After tryptophan gets in your brain, it's turned into serotonin. As a neurotransmitter, serotonin has a role in learning and memory. It also regulates appetite and mood. Low levels of serotonin can cause depression. Research exploring the potential for L-tryptophan to treat depression, however, has produced inconsistent results, according to the July 2011 issue of “Alternative Therapies in Health and Medicine."
Regulates Sleep Cycles
After tryptophan is converted into serotonin, your body uses serotonin to produce the hormone melatonin. In this way, tryptophan helps regulate your sleep-wake cycle, because melatonin promotes sleep. The amount of melatonin produced is determined by the light in your environment: Levels of the hormone in your blood are low during the day, and they rise in response to the dark. Melatonin supplements help improve some sleep problems, such as those caused by jet lag and a condition called delayed sleep phase syndrome, but evidence for their ability to treat insomnia isn't strong, according to the University of Rochester.
Your body converts tryptophan into the B vitamin niacin, which is essential for metabolizing food into energy and maintaining a healthy nervous system. Niacin deficiency only develops when your diet lacks tryptophan as well as niacin, according to the "Merck Manual." But your body needs an adequate supply of vitamin B-6, riboflavin and iron to successfully make niacin from tryptophan, reports MedlinePlus. Most of the top sources of tryptophan -- poultry, meat, fish, cheese, beans and nuts -- also contain the other nutrients needed to make the conversion.