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Every year, on the fourth Thursday in November, the United States celebrates a holiday called Thanksgiving. Part of the tradition, along with watching football (the American version), is eating mountains of food, including a roasted turkey. And this is where Uncle George regales the guests with the turkey tryptophan myth – that is, eating a mountain of turkey, which he claims is high in tryptophan, makes you sleepy.

Because I know the average reader of this blog is pro-science and snarky, I post this article for you to embarrass Uncle George. And you just know that Uncle George denies climate change and thinks vaccines are dangerous. But let’s get back to that to Thanksgiving and the turkey tryptophan myth.

Only a few countries celebrate Thanksgiving, and just a handful of countries eat turkey in any amount, other than the USA and Canada. Surprisingly, 87% of English holiday dinners will include turkey, a bird that is native to North America. So maybe your British Uncle George will tell the same turkey tryptophan myth during dinner. Or supper, I suppose.

Anyway, in case you want to impress friends and family, the other places that celebrate Thanksgiving, similar to the USA and Canada, are Liberia (which is populated by descendants of freed slaves who returned to Africa from the US), Grenada (a small English-speaking island in the Caribbean), Puerto Rico (a Spanish-speaking territory of the USA), and Norfolk Island, an Australian territory of like 1500 people.

The only thing I thought that was on Norfolk Island was the Norfolk Island pine. Apparently, American whaling ships would stop there and celebrate the holiday.

For Americans, the holiday celebrates white English settlers arriving in North America. The tales usually include some peaceful sharing of food between the white settlers and Native Americans (a nice myth without much actual historical support) prior to the first winter.

Canada’s backstory on Thanksgiving is much more complicated, including ships getting stuck in ice and other legends – it is very Canadian.

In both Canada and the USA, the celebration includes several tonnes of food (per person) which a roast turkey. Other foods may include mashed potatoes, yams (sweet potatoes), other meats, pies, corn, stuffing, and more food. It is a high-calorie meal of epic portions!

Just because this is my blog, let me state one simple fact – pumpkin pie is garbage. I hate that thing.

Generally, everyone, after finishing this dinner, would want to take a long nap. Thus, we find the origin stories of the turkey tryptophan math. However, the science of eating, sleeping, turkey, and tryptophan doesn’t support this myth. Not even close.

Well enough cultural history. This is a science blog, let’s talk about the science that debunks the turkey tryptophan myth. And because I think Uncle George is a blowhard.

Tryptophan molecule.

What is tryptophan?

This turkey tryptophan myth kind of falls apart before we get very far.

Tryptophan is one of the nine essential amino acids, meaning ones we cannot produce in our body and must be consumed. These amino acids, along with the other 12 amino acids that we can produce, the building blocks of every protein.

Yes, every single protein in the human body, no matter what its function – cell structure, enzymes, blood components, neurons, muscle fiber, and more things than I should write in a blog – is made up of some combination of these 21 amino acids.

Proteins don’t require all 21 different amino acids, just some subset of them, but all 21 amino acids are necessary for the cells to manufacture all the proteins necessary for one to live. There is nearly an infinite number of ways these 21 amino acids can create proteins. There are estimated to be from 250,000 to over 1 million different proteins in every human.

Like the myths about MSG, which is another amino acid found in nearly every single protein, a lot of physiological properties have been imparted to the tryptophan molecule without much evidence. To be completely clear, tryptophan is ingested from the breakdown of proteins (your body does not absorb the whole protein, just the constituent amino acids), then it is reused by new and different proteins necessary for a human to survive.

My explanation of proteins and amino acids may induce more sleep than the actual tryptophan in your turkey dinner.

Does tryptophan have other functions?

Yes, it has some other biological functions other than building proteins.

Tryptophan is important in manufacturing niacin, an important nutrient, and auxins, a plant hormone not necessarily important to humans. One of the reasons the myth of tryptophan and sleeping has exploded is that the amino acid is a precursor to the manufacturing of serotonin in the central nervous system of humans.

Serotonin is a neurotransmitter, a small molecule that transmits signals across the gap between two neurons. It is an essential part of most animal’s nervous system communication.

Serotonin also has a “feel good” component, in that excess serotonin stimulates neurons and gives a person a happy feeling. Alternatively, the lack of serotonin makes one sad and depressed. Many legitimate antidepressants have been developed to either increase production of serotonin, or to inhibit the breakdown of it by enzymes.

Serotonin itself has little effect on this sleepy feeling – however, serotonin can be converted by brain cells into melatonin, which does have some effect on causing sleepiness.

However, there is little evidence that has shown a direct effect between melatonin and sleep. Thus, tryptophan has an indirect sleep component– however, and this is important, there are two biochemical steps from tryptophan to serotonin, which is biologically regulated by specific enzymes.

Eating more and more tryptophan does not suddenly increase your concentration of serotonin, because cells have critical self-regulating mechanisms that either turn on or off serotonin production. Human physiology keeps things like serotonin and melatonin under tight control, producing enough to keep us running smoothly. Eating an excessive amount of tryptophan isn’t going to force the body to produce serotonin and melatonin in excessive amounts

Turkey and tryptophan – does it cause sleep?

Maybe, but there are a couple of issues.

First, turkey is not a huge source of tryptophan. Beef, chicken, turkey, lamb, and pork all have approximately the same amount of tryptophan per serving of meat–around 0.21-o.25 g per 100g of food.

In other words, if sleep were induced by eating turkey because of tryptophan, then we would observe the same effect with steak, burgers, or fried chicken. And this tryptophan myth has never been attributed to these other meats.

Second, even if turkey (or any meat) had excess tryptophan, the problem is that the amino acid is not selectively taken up by the brain. All amino acids are required by the nervous system to synthesize proteins, so the blood-brain barrier, a layer of cells between the brain and the blood, which selectively permits certain molecules to pass through to the brain, regulates how many amino acids pass into the brain.

So, if you eat 250 grams of turkey, it’s broken down into 22 different amino acids, all competing at the blood-brain barrier to cross into the brain. Tryptophan has zero advantage in being transported over all the other 21 important amino acids because the brain probably has no more need for tryptophan than most other amino acids.

Third, as I mentioned above, the serotonin synthesis system works independently of the concentration of tryptophan itself.

Finally, even if you consumed tryptophan on an empty stomach, which would negate most of the competition between tryptophan and other amino acids at the blood-brain barrier, there’s simply no evidence that supplementary tryptophan has any effect on mood disorders, which implies that tryptophan, whether from a turkey or not, really has no effect on sleep after consumption.

Pumpkin pie. Yuck. Photo by Element5 Digital on Unsplash

So what makes me so sleepy after eating on Thanksgiving?

Well, it could be Uncle George’s annoying stories or this 1500 word post about the turkey tryptophan myth. But seriously folks, it’s other things you eat for that big holiday meal.

Blame the carbohydrates. Because the turkey probably has no effect on your sleepiness, it could be the mashed potatoes, yams, pie, stuffing, gravy, bread, cranberry sauce, and whatever else that was served. Those carbohydrates cause the release of a lot of insulin, which signals muscles to start taking up all amino acids – except tryptophan (science isn’t sure why).

So, at the blood-brain barrier, there’s little competition between tryptophan and the other 21 amino acids for transport across the barrier – insulin has caused the other 21 amino acids to be absorbed by muscle cells, so they’re in lower concentration in the blood.

The blood-brain barrier is agnostic about amino acids – if the concentration of tryptophan is much higher, then it transports it. (And no, you could not increase the concentration of tryptophan high enough through supplementation to force it across the blood-brain barrier.)

At this point, the tryptophan levels in the brain go way up, overwhelming the enzymatic feedback regulation of serotonin production. As the body attempts to reduce serotonin by converting it to melatonin, one gets sleepier.

Let me TL;DR this

I know all of that science sounded complicated, so let me try to summarize it:

Tryptophan does not directly cause sleep, but it is converted into other chemicals, specifically serotonin and melatonin, which may induce sleep. Turkey really doesn’t have any excess tryptophan compared to any other meats. Even if it did, the brain isn’t going to get that extra tryptophan. However, the large amount of carbohydrates consumed during dinner causes the concentration of amino acids, other than tryptophan itself, to drop. As a result, there is a higher concentration of tryptophan in the blood, meaning more of it crosses the blood-brain barrier.

So, if drunk Uncle George starts pushing the turkey tryptophan myth as a reason for everyone want to go to sleep, this information will help put them in their place.

On the other hand, you could serve cannabis-infused gravy – that should mellow everyone!

Happy Thanksgiving USA, Grenada, Puerto Rico, Liberia, and Norfolk Island. And a belated Happy Thanksgiving to Canada, because I’m sure you have your own Uncle George lecturing everyone about the turkey tryptophan myth.

Notes

This article was first published in November 2017. Of course, it’s been revised (especially finding out why Norfolk Island celebrates Thanksgiving) and republished for Thanksgiving 2019.

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The Original Skeptical Raptor Lifetime lover of science, especially biomedical research. Spent years in academics, business development, research, and traveling the world shilling for Big Pharma. I love sports, mostly college basketball and football, hockey, and baseball. I enjoy great food and intelligent conversation. And a delicious morning coffee!

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