Thanksgiving; the sacred and the tryptophan

Since it's Thanksgiving I was going to do a shortish post about the tryptophan hypothesis being the cause of sleepiness after a turkey dinner, but the real expert on sleep cycles, my ScienceBlogs comrade Coturnix, beat me to it by a day. Which is good because without his excellent post I would certainly have made a science fool of myself. Which of course I would blame on being tired after eating a big turkey dinner. I won't repeat most of his post. You should read it foryourself. It is extremely interesting. But I will give you a bit to whet your appetite (sorrry!) and take the opporutninty to raise another point that has bothered me for a long time. First Coturnix. After examining through the various steps that must be traversed for at least one putative mechanism for turkey tryptophan to make you sleepy, he summarizes thusly:

Now, as far as I know, nobody has done the complete study - all links in the chain - in any species, let alone humans. Still, as each link has been independently verified in one species or another, the whole hypothetical mechanism appears likely. Thus, until further research proves this wrong, you can tell your friends at dinner table this:

High levels of tryptophan in turkey meat and some other foods lead to increase in synthesis of melatonin in the intestine which results in more melatonin leaking from the GI tract into the bloodstream. Once in the blood, this extra melatonin phase-shifts the clock - your body thinks it is late at night and you feel sleepy as a consequence.

Furthermore, this shift of the clock will result in you being wide awake right after midnight - your body thinks it's waking-up time already - something that has been reported anecdotally. (Coturnix, Blog Round the Clock)

The issue of turkey, tryptophan and sleepiness seems to be a particularly controversial one. The folks over at Snopes don't believe it:

But tryptophan doesn't act on the brain unless it is taken on an empty stomach with no protein present, and the amount gobbled even during a holiday feast is generally too small to have an appreciable effect. That lazy, lethargic feeling so many are overcome by at the conclusion of a festive season meal is most likely due to the combination of drinking alcohol and overeating a carbohydrate-rich repast, as well as some other factors.

Two other factors that contribute to the desire to sleep at the dinner table are meal composition and increased blood flow to the gastrointestinal tract. Studies have shown that a solid-food meal resulted in faster fatigue onset than a liquid diet. The solid-food meal also causes a variety of substances to jump into action that ultimately leads to increased blood flow to the abdomen. This increase in blood flow and an increase in the metabolic rate for digestion can contribute to the "coma."

Those who still feel wary of turkey's purported sleep-inducing properties should find solace in the knowledge that many items we eat contain tryptophan. Milk, beef, and beans are among the foodstuffs which house this amino acid, and experts say the average serving of chicken or ground beef contains as much tryptophan as a serving of turkey does. If tryptophan were truly the sandman's henchman, we'd be falling asleep at the wheel on our way home from KFC or McDonald's. (Snopes Urban Legends)

Some of the same points were made by commenters over at Coturnix's site. There were other interesting issues raised, too, such as the rate limiting for melatonin synthesis from tryptophan and some others. Coturnix has helpfully posted these comments on the main page. The original post was last Thanksgiving, but it is clearly still a topic of interest.

Now to the point that has bothered me about all of this. It concerns melatonin, which you can buy over the counter and many people take for jet lag and self diagnosed sleep disorders. Here's a real nugget from Coturnix's post:

Not everyone knows that the complete enzymatic machinery for synthesis of melatonin is not active only in the pineal organ. It is also fully functional in the retina of the eye, in the Harderian gland (located in the ocular orbit just behind the eyeball), and in the intestine. After all, the GI tract possesses a large and complex semi-independent nervous system in which many of the same neurotransmitters and hormones are found as in the brain. Actually, more melatonin is produced in the intestine than in all the other sites combined.

I won't speak for how many others didn't know this, but I sure didn't. I do know that melatonin is a hormone and is involved to some extent in phase shifting in circadian rhythms and probably has other effect as well, many unknown. Coturnix mentions effects on intestinal motility which I didn't know about. One of the things that has interested me personally and professionally is the history of public health and pure food and drug regulations. It is quite striking to see magazine advertisments for over the counter nostrums containing radium and other radioactive materials, touted as a panacea for whatever ails you (some background here). When you see this, you ask yourself, "What were they thinking?"

Which makes me wonder this. Will some future historians of public health look back on this era, where we were selling a powerful hormone, melatonin, whose effects we aren't sure of, over the counter to people who take uncontrolled amounts of it and produce blood levels ten or more times the physiological levels and ask, "What were they thinking?"

I hope not.