Is there really any such thing as a turkey coma? Based on my anecdotal personal experience the answer is certainly “yes.” This became most apparent to me just this past Thanksgiving at about 4 in the afternoon while struggling to get back home from James Island, where I’d returned one of our dinner guests. Fighting to stay awake and not wreck the car, I was clearly under the influence of something, but what?

Was there alcohol involved? Well of course there was: A celebratory glass of champagne before sitting down, but that was a couple of hours previously. And I’ve had countless numbers of large midday meals over the years, and they don’t do it — or at least nothing like to the same extent. So that must leave the turkey. What else is there?

After finally getting home I collapsed on the sofa with the football game on and just went out, with visions of dressing, rice and gravy, collards, casseroles, pumpkin crisp, Bavarian (or “rock”) cream and pecan pie dancing in my head.

I asked my friends if they believed in the turkey coma and almost all of them said yes, absolutely, that it has something to with L-tryptophan, an essential amino acid which, in sufficient quantities and according to urban legend, is about as sedating as a hefty dose of Benadryl.

Fascinating, I thought: A plausible explanation for a well-recognized phenomenon. But like so many other things, there’s more to the story.

According to a web search and a posted article by Anne Marie Helmenstine, Ph.D., on the site “Thanksgiving Chemistry,” L-tryptophan can be absorbed and converted into serotonin and melatonin, two calming neurotransmitters, but needs to be taken on an empty stomach to have a direct effect. And it should be noted that other common foods such as chicken, pork and cheese actually contain as much as or more tryptophan than turkey.

However, large carbohydrate consumption can have an indirect effect on tryptophan metabolism (and note that our table was laden with cranberry sauce, rice and three desserts) by stimulating insulin secretion from the pancreas. This causes some other amino acids that “compete” with tryptophan to leave the blood stream, thus increasing the relative concentration of tryptophan and enhancing serotonin synthesis.

Fats are another issue. The Thanksgiving meal is high in everything, including fats, which take time to digest and use up energy in the process. More blood is directed to the digestive system to facilitate the process, which means that less blood goes elsewhere, including the brain, with the resulting effect being a sense of sleepiness.

Alcohol, a central nervous system depressant, will, in any quantity — and particularly under the above circumstances — make the couch appear that much more inviting.

Regardless of the actual food content, overeating itself will have soporific effects. It puts added demand on the digestive system and redirects blood flow in the same manner described above under fats.

And finally, relaxation is a key ingredient. Whereas the holidays can be stressful, there’s something very relaxing about sitting around the Thanksgiving table and sharing the moment with family and friends — assuming you’ve done the proper homework as far as the guest list is concerned. And relaxing is usually made easier by all the work that goes into the various preparations.

So whereas tryptophan may play a minor and indirect role concerning turkey coma induction, it’s really a combination of things. And to think that many of us will experience it at least once more over the next 15 days.

So bon appetit everybody and sweet dreams!

Edward M. Gilbreth is a Charleston physician. Reach him at edwardgilbreth@comcast.net.