Nov
21

Don’t blame the bird! The truth about tryptophan.

Author // Cathy
Posted in // Cathy's Blog

Does eating turkey make you sleepy? Is that post-Thanksgiving-stupor really the turkey’s fault? Research indicates that turkey alone doesn’t make you drowsy, but when combined with an excess of carbohydrates, it does.

When I was a child, my grandma set before us a cornucopia of good old Yankee fare: mashed potatoes, pumpkin bread, squash with maple syrup, cranberry sauce, a fancy jell-o mold with pineapple, homemade applesauce and sweet pickles. There was also the turkey in all its golden majesty, filled with stuffing, smothered in gravy. The food was endless. No one stopped at one serving – how could you? Then there was desert. She baked three kinds of pies from scratch: apple, custard and mincemeat. It would be rude not to sample all three. Would you like ice cream or freshly whipped cream with that my dear? We’d stagger through hand washing the dishes while my grandmother started the turkey soup, then retire to the warmth of the living room and the comfort of overstuffed chairs to slowly, one by one, nod off, book upturned in lap.

What is responsible for that after Thanksgiving meal drowsiness, or, in medical parlance, postprandial somnolence? I always blamed the bird – all that tryptophan in turkey meat.  Tryptophan is an amino acid that functions as a precursor in the manufacture of serotonin (a calming neurotransmitter that helps promote and regulate sleep) and melatonin (a sleep-inducing hormone) in the brain. I always blamed the bird, but I was wrong.

Eating turkey by itself will not make you sleepy. Proteins, like turkey, consist of long chains of amino acids. There are twenty types of amino acids, and tryptophan is one of the rarer ones. Once a protein has been digested (broken down) into its amino acids, those amino acids travel via the blood to various tissues including the blood-brain barrier. Once there, they must be shuttled across the barrier by specialized transport molecules before they can affect the brain. The different types of amino acids present compete for rides on these transport molecules, so poor tryptophan usually has a hard time crossing over the barrier in any significant amount.  After eating turkey, as with other food sources of tryptophan, the presence of these other amino acids in the blood naturally keeps tryptophan in check.

So if it isn’t eating turkey, per se, that causes the sleepiness, what does? Funnily enough, it is all the carbohydrates we also ingest. On Thanksgiving the average person eats more carbohydrates in one meal than are typically eaten in an entire day. Ingestion of a meal that is rich in carbohydrates triggers the release of insulin that, in turn, stimulates the uptake of glucose and many amino acids into cells, leaving behind a higher concentration of the turkey’s tryptophan in the bloodstream. According to Richard Wurtman of MIT (as reported in the November 2007 issue of Scientific American), insulin has little effect on tryptophan, so “by sopping up other amino acids from the blood, insulin reduces tryptophan’s competition; the transport system [into the brain] is no longer tied up and more tryptophan can reach the brain, so serotonin synthesis is stepped up.”

In other words, the drowsiness has more to do with what you eat alongside the turkey and, in particular, with eating too many carbohydrates, than with turkey itself. In fact, the amount of tryptophan in turkey is similar to that found in chicken, beef, pork and cheese, and lower than that found in soybeans and many seeds. So we must pardon the poor turkey after all!

So what is a person to do? You could choose this year to eat more moderate portions, plan on leftovers and take a brisk walk.  Or you could opt for swearing off such self-control and eat as you please – just save me a spot on the couch!  Wishing you peace and hoping you have much to be grateful for this Thanksgiving Day.