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. Well, he’s probably a Trump supporter who wouldn’t know any science because it isn’t a pedophile in Alabama. Oh sorry, I did go there.
Back 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, I guess when gobby Uncle George (loyal Chelsea football fan) starts with the turkey tryptophan tosh, you can tell him to bugger off with this article.
Just 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. And now I wonder if they import turkeys for the dinner.
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 tonnes of food (per person) usually including 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!
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.
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, that are, 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 are 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 supporting 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 is reused by new and different proteins necessary for a human to survive.
My explanation may induce more sleep than the actual turkey tryptophan myth.
Does tryptophan have other functions?
Yes, it has some other biological functions other than building proteins. It’s 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. Most 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, although there are no clinical trials which have 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 are biological controlled through 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 the myth of sleep has never existed with those 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.
Third, like I mentioned above, the serotonin synthesis system works independently of the concentration of tryptophan itself.
Finally, even if you consume 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.
So what makes me so sleepy after eating on Thanksgiving?
Well, it could be Uncle George’s annoying stories. But seriously folks, it’s other things you eat for that big holiday meal.
Blame the carbohydrates. Because the turkey really has no effect, it’s the mashed potatoes, yams, pie, stuffing, gravy, bread, cranberry sauce, and repeat. Those carbs 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 force up 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.
There we go, the end of the turkey tryptophan myth, and the start of the mashed potatoes myth.
- Turkey doesn’t have extra tryptophan
- Even if it did, extra turkey and tryptophan isn’t going to make you sleepy.
- Extra turkey and tryptophan can make you sleepy if consume a boatload of carbohydrates
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. I’m going to use it!
And for those of you in the USA, Happy Thanksgiving. Enjoy the food. For Canadians, I’ll try to remember to wish you one next year.
- Ravindran AV, da Silva TL. Complementary and alternative therapies as add-on to pharmacotherapy for mood and anxiety disorders: a systematic review. J Affect Disord. 2013 Sep 25;150(3):707-19. doi: 10.1016/j.jad.2013.05.042. Epub 2013 Jun 12. Review. PubMed PMID: 23769610.