The belief in question: Turkey, which contains tryptophan, causes drowsiness.
Why is the belief that turkey causes drowsiness so widespread? Tryptophan is an essential amino acid and the precursor to serotonin, which is often referred to as "the feel-good chemical." Serotonin can be converted to melatonin in the body, which causes drowsiness - and is colloquially known as the "sleep hormone."
If we look at the science simplistically, it’s understandable why we might make the connection between turkey and sleepiness. Tryptophan, which occurs naturally in turkey meat, is converted to serotonin and melatonin in the body, both of which are culturally associated with rest and relaxation. After a big meal, our parasympathetic nervous system becomes activated in what's colloquially known as the "rest and digest" stage.
Is turkey really responsible for making us sleepy after a meal?
While turkey does contain tryptophan, eating turkey on its own doesn’t result in much tryptophan entering the brain. Why? Because other amino acids that break down from the proteins in turkey heavily outcompete tryptophan for the transporter LAT1, which shuttles it across the blood-brain barrier. So, eating a regular portion of turkey by itself won’t cause sleepiness.
Most likely, the real culprit behind our post-meal coma is carbohydrates. If we eat our turkey with all the holiday favorites like stuffing, mashed potatoes, and bread, the carbohydrates stimulate beta cells in the pancreas to secrete insulin. When insulin enters the bloodstream, it causes the uptake of glucose and amino acids from the blood into the body’s tissues. However, insulin doesn’t cause tryptophan to be re-absorbed. As a result, fewer amino acids are competing with tryptophan for the LAT1 transporter, meaning more tryptophan crosses the blood-brain barrier.
You’d have to eat an inordinate amount of pure, isolated tryptophan to cause any immediate sleepiness. Still not convinced? Consider this: chicken contains about the same amount of tryptophan as turkey - yet we don’t associate a bucket of wings with drowsiness.
The truth: a lot of factors can lead to drowsiness, such as alcohol, a large meal, carbohydrates, and the body simply needing to rest and digest. If you want to stay awake this holiday season post-dinner, consider drinking a coffee (or skipping dessert). Alternatively, you can also just enjoy the extra sleep. 😴