Tryptophan, Turkey and Post-Prandial Drowsiness

Since he was just a pup, about 30-45 minutes after wolfing down a Thanksgiving turkey dinner, WC would be asleep on the couch. In the late 1970’s, a physiologist who was WC’s Thanksgiving guest explained it as a reaction to all of the tryptophan in turkey meat. Just before dropping off to sleep.

As a service to all of his Thanksgiving Day readers who even now are struggling to stay awake, WC will examine the reality of the claim that it is “all that tryptophan.”

First, tryptophan is is an essential amino acid. This means that it cannot be synthesized by our bodies and therefore must be part of our diet. Amino acids, including tryptophan, act as building blocks in protein biosynthesis. In addition, tryptophan functions as a biochemical precursor for the production of serotonin, a neurotransmitter, synthesized via tryptophan hydroxylase. Serotonin, in turn, can be converted to melatonin (a neurohormone), via N-acetyltransferase and 5-hydroxyindole-O-methyltransferase activities. The melatonin signal, in turn, forms part of the system that regulates the sleep-wake cycle by chemically causing drowsiness and lowering the body temperature, but it is the central nervous system (specifically the suprachiasmatic nuclei, or SCN) that controls the daily cycle in most components of the paracrine and endocrine systems rather than the melatonin signal (as was once postulated).

Trytophan can be synthesized, although it is a fairly complex, multi-step synthesis and the manufacture is regulated. It’s an interesting process:

Simplified tryptophan biosynthesis

You still awake? Oh, good.

So tryptophan is linked, albeit remotely, with the sleep/wake cycle. The problem is that while turkey does contain high levels of tryptophan, the amount is comparable to that contained in most other meats.

Post-meal drowsiness on Thanksgiving may have more to do with what else is consumed along with the turkey and, in particular, carbohydrates. Studies on both animal models and humans show that ingestion of a meal rich in carbohydrates triggers release of insulin. Insulin in turn stimulates the uptake of large neutral branched-chain amino acids (BCAA), but not tryptophan, which is an aromatic amino acid, into muscle, increasing the ratio of tryptophan to BCAA in the blood stream. The resulting increased ratio of tryptophan to BCAA in the blood reduces competition at the large neutral amino acid transporter (which transports both BCAA and aromatic amino acids), resulting in the uptake of tryptophan across the blood-brain barrier into the cerebrospinal fluid (CSF).

Once in the CSF, tryptophan is converted into serotonin in the raphe nuclei by the normal enzymatic pathway. The resultant serotonin is further metabolised into melatonin by the pineal gland.

All of this suggests that “feast-induced drowsiness” — and, in particular, the common post-Thanksgiving turkey dinner drowsiness — may be the result of a heavy meal rich in carbohydrates, which, via an indirect mechanism, increases the production of sleep-promoting melatonin in the brain.

Wake up, there. This is very nearly important.

So the answer is that the turkey doesn’t have much to do with it. It’s the mashed potatoes and your metabolism. The turkey/tryptophan business is mostly untrue.

Any readers who are still awake may take their naps now. And Happy Thanksgiving.

[Credit for much of this article to Wikipedia, which WC has shamelessly pasted into this blog post. WC slept through most of organic chemistry, which he took shortly after Kekule discovered benzene rings. It couldn’t have been melatonin that made WC nap through organic; it hadn’t been discovered yet.]