Thanksgiving feasters take heart. Contrary to popular belief, turkey's tryptophan dose doesn't cause drowsiness. In fact, the substance could possibly aid in the treatment of depression and multiple sclerosis.
Purified tryptophan is a mild sleep-inducing agent. That probably spawned the idea that turkey and other foods heavy in tryptophan cause drowsiness.
But tryptophan can't get to the human brain in large amounts when ingested as part of a massive Thanksgiving feast—it needs an empty stomach.
"Tryptophan is taken to the brain by an active transport system shared by a number of other amino acids [the chief components of proteins], and there's competition among them—like a crowd of people trying to get through a revolving door," said Simon Young, a neurochemist at McGill University in Montreal, Canada.
Consuming tryptophan-rich foods may cause blood levels of the amino acid to rise. But not enough tryptophan will reach the brain to have a sedative affect.
"Brain levels of tryptophan could even go down after a big meal because of the [amino acid] competition," Young said.
Turkey isn't even unusually high in tryptophan. Many foods, such as beef or soybeans, boast higher concentrations.
"Think about how much turkey you have in a turkey sandwich without getting tired," said Sherrie Rosenblatt, spokesperson for the National Turkey Federation. The Washington, D.C. nonprofit represents the turkey industry.
So why the traditional Thanksgiving nap?
The slumber may be caused by the stressful hustle and bustle of the holidays, alcohol consumption, and the massive caloric intake of the year's biggest feast.
"There have been many studies citing a post-lunch dip in performance, from factory output to single-car accidents," McGill's Young explained.
"These things tend to peak in the early afternoon. A thousand-calorie lunch causes a sedative effect that a smaller meal doesn't have."
Multiple Sclerosis Fighter?
Tryptophan won't put you to sleep this Thursday, but it can produce several helpful substances, including serotonin, melatonin, and kynurenines.
Serotonin affects mood, melatonin helps regulate sleep, and kynurenines may be useful in regulating the immune system.
A drug called tranilast, available in Japan as an allergy medication, is chemically similar to kynurenines and shows promise for the treatment of certain autoimmune diseases, including multiple sclerosis.
Multiple sclerosis and other autoimmune diseases result from overactive immune systems that attack important cells.
"If what we're seeing in mice is translatable in humans—and that's a very, very, very big if—it could have some quite beneficial effects," said Lawrence Steinman, a professor of neurology and neurological sciences at the Stanford School of Medicine in Palo Alto, California.
Steinman tested the drug on mice with a multiple sclerosis-like condition. He found that it relieved paralysis and other symptoms as effectively as any existing medicines for the condition.
His research was reported in the November 4 issue of the journal Science.
"These compounds have some remarkable qualities," Steinman said of the kyurenines.
"Through a very interesting mechanism, they reduce the strength of the immune signal. In general that is something to worry about, because we need a strong immune system to fight viruses and bacteria," he said.
But kynurenines seem to shut down only "bad" immune responses—responses that degrade the body's ability to defend itself.
But don't expect turkey treatment for such serious ailments. Eating even tryptophan-rich foods would have no effect, as the substance would be broken down in the body.
"I'm a strong believer that your diet is very important, but making manipulations in the diet to specifically improve the immune system is rather hard to do," Steinman said.
"If one wants to elevate the kynurenines in the body, it would be better to develop a drug that happens to look like kynurenines," rather than like tryptophan.
Tryptophan as Mood Booster
Purified tryptophan is available in some countries (though not in the United States) as a prescription drug for the treatment of depression. Another of its products, serotonin, has been strongly linked to mood.
"Tryptophan is reasonably effective in treating mild depression but probably not major depression," said McGill's Young.
Medicinal doses are three to six times as strong as the amount of tryptophan a person might eat in a day.
Studies on humans and other primates have linked low serotonin levels with low mood, increased aggression, and even suicide.
Young's recent studies of humans suggest that tryptophan may be effective in altering behavior.
"We've done a few studies where tryptophan decreased quarrelsome behavior, relative to a placebo, and [subjects'] behavior was changed without their knowing it," Young said.
"More recently we've seen an increase in agreeable behaviors. So tryptophan may not only have effects on mood but some effects on social interaction as well."
Though some holdouts may still cotton to the turkey-as-tranquilizer story, the National Turkey Federation's Rosenblatt has a rosy view.
"I think that there are some people who believe in myths," Rosenblatt said. "But it sure hasn't stopped 98 percent of [U.S.] consumers from having turkey at the center of their table for Thanksgiving."