STEVE INSKEEP, host:

It's Morning Edition from NPR News. Good morning, I'm Steve Inskeep. In "Your Health" today, the dilemma of being nearly vegetarian on Thanksgiving. First we'll examine an old myth. It's the myth that the amino acid in turkey, tryptophan, makes you sleepy. There's also the notion that this same compound may make you more trusting and cooperative. We wanted to know if there was any truth to that, so we brought in NPR's Allison Aubrey.

ALLISON AUBREY: Sometimes the stories we tell ourselves have a way of becoming true. Take the turkey makes you sleepy tale.

Ms. CATHERINE NEWBERRY: I think personal experience indicates that, yes, it's true.

AUBREY: Holiday travelers, Catherine Newberry and Robin MacAdams(ph), who I caught up with in Washington's Union Station, say they're pretty certain.

Mr. ROBIN MACADAMS: Eating a lot of turkey does seem to make a lot of people tired.

AUBREY: But wait a second. Couldn't it just be, as traveler Mary Ann Burke suggests, the Thanksgiving feast gorge factor?

Ms. MARY ANN BURKE: I think it's true that anytime you fill you plate four times and lay down on the couch, you're going to fall asleep.

AUBREY: Burke's explanation is right on, according to scientists. Overeating makes you sluggish. Add a little wine and the mood gets very mellow very quickly. So why if the explanation is so simple does the story about tryptophan persist? Martha Stipanuk is a professor of molecular nutrition at Cornell University. When I called to get her thoughts on the matter...

Professor MARTHA STIPANUK (Molecular Nutrition, Cornell University): I said, oh, that same old story again.

(Soundbite of laughter)

Professor STIPANUK: But it keeps popping up. I think that it is based on a little bit of correct information, but misunderstood and misapplied.

AUBREY: The basic skinny is this. All foods that are protein rich - everything from turkey to tofu, dairy products, nuts, and virtually all meats - contain a bunch of amino acids, roughly 20 of them. And one of these amino acids is tryptophan.

Professor STIPANUK: There's nothing unique about turkey and its tryptophan content.

AUBREY: It contains about as much as other meats. So if there were a direct sleepy effect, we'd feel it with all protein. So here's the next interesting wrinkle. Once tryptophan enters the body, it becomes the building block or the precursor molecule of serotonin and melatonin, two brain compounds involved in regulating mood and sleep.

In lab studies when scientists jack up people's tryptophan levels by giving them supplements of purified tryptophan, they sometimes see changes in behavior. A surge of tryptophan can alter sleep and perhaps levels of cooperation too. Take one study by British scientist Robert Rogers of Oxford. He documented what happens when players engage in games of prisoner's dilemma. In this game you can either compete with your opponent to try to win everything, or you can cooperate and divide the reward. Roger says tryptophan-deprived players were significantly less likely to cooperate.

Dr. ROBERT ROGERS (Reader in Cognitive Neuroscience, Department of Psychiatry, University of Oxford): I was surprised by the magnitude of the effect.

AUBREY: So this takes us back to the Thanksgiving dinner table. Surely, if loads of purified tryptophan delivered in lab studies can produce such effects, can't a turkey feast do the same? Not exactly, explains Stipanuk.

Professor STIPANUK: If you feed people a high protein containing diet, you don't get changes in brain tryptophan or serotonin.

AUBREY: So, here's the rub. It basically boils down to a little traffic jam at the blood-brain barrier. Remember how we said that protein-rich foods like turkey contain about 20 amino acids? Well, after a meal, all of these amino acids are competing to get from the bloodstream to the brain. Researcher Simon Young of McGill University says, think of it as a revolving door.

Dr. SIMON YOUNG (Neurochemist, Department of Psychiatry, McGill University): And if you imagine you're a tryptophan molecule, you want to get through that door. And if there are more other people around - those are the other amino acids - it takes longer for you to get in.

AUBREY: So it turns out there is no surge of tryptophan to the brain after eating turkey or any other protein. Having heard the evidence, Robin MacAdams has a new theory about the post-Thanksgiving feast letdown.

Mr. MACADAMS: Maybe it's because I'm around my family, so I just want to sleep.

(Soundbite of laughter)

AUBREY: Not a bad theory. Food anthropologist Christine DuBois says the social interactions of the holidays may trump specific chemical changes brought on by food.

Professor CHRISTINE DUBOIS (Anthropology, Johns Hopkins University): Here we are thinking that some substance is going to alter our destinies, instead of thinking about how the way we treat people and interact with people is so paramount.

AUBREY: Americans, she says, just seem to love creating stories about the power of food. Allison Aubrey, NPR News, Washington. Transcript provided by NPR, Copyright NPR.