A tiny handful of countries, most notably the US and Canada, celebrate a holiday called Thanksgiving. In the USA, the holiday is held on the fourth Thursday in November and more or less starts the so called holiday season which ends with New Year. In most of Canada (excluding the Atlantic provinces), the holiday is held on the second Monday in October. Invariably, there are stories about turkey and tryptophan, making you just want to take a nap hearing about it.
For trivia purposes only, the other places that celebrate a similar Thanksgiving 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 Australia. Australia? I have no clue if they all talk about turkey and tryptophan over dinner.
Generally, 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 tons 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!
There’s a legend that eating this meal, specifically the turkey, fills your body with tryptophan, and you fall asleep.
Nice story, but the science of eating, sleeping, turkey and tryptophan doesn’t support this myth. Not even close.
What is tryptophan?
This turkey and tryptophan myth kind of falls apart before we get very far.
Tryptophan is one of the 22 standard amino acids that form proteins. 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 22 amino acids. Proteins don’t require all 22 different amino acids, just some subset of them, but all 22 amino acids are necessary for the cells to manufacture all the proteins necessary for one to live. If you want even more trivia, there are estimated to be from 250,000 to over 1 million different proteins in every human, so those 22 amino acids have to combine in all different combinations to end up as a protein.
Tryptophan is unique because it is considered a human “essential amino acid,” which means it is an amino acid that is not manufactured in our own body but must be consumed. Different organisms manufacture some or all of the amino acids, which are incorporated into proteins, so most diets that include adequate protein happen to include all the necessary amino acids.
Like the myths about MSG, which is another amino acid found in nearly every single protein, a lot of physiological properties have been imparted onto 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 is reused by new and different proteins necessary for a human to survive.
My explanation may induce more sleep than the turkey and tryptophan.
Does tryptophan do anything else?
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. I told you I was going to science the shit out of this.
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.
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. And 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 independent of the concentration of tryptophan itself.
Furthermore, 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 means that we’re finding little obvious change in serotonin levels.
So what makes me so sleepy after eating on Thanksgiving?
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. Thus, the tryptophan levels in the brain go way up, overwhelming the enzymatic regulation of serotonin production. As the body attempts to reduce serotonin by converting it to melatonin, one gets sleepier.
Nevertheless, blame the carbs that make up the Thanksgiving meal, not necessarily the turkey.
- 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 the arrogant brother-in-law at your Thanksgiving dinner claims that the turkey and tryptophan is going to make him 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.