Tryptophan is often discussed in the context of holiday meals, particularly the traditional turkey dinner, due to a popular belief that this amino acid causes post-meal drowsiness. This association suggests that turkey is uniquely potent in delivering a sleep-inducing chemical dose. Understanding why turkey contains tryptophan, and whether it is truly the culprit for a “food coma,” requires a closer look at the body’s biochemistry. The presence of tryptophan in turkey is simply a matter of biology, but its effect on the brain is far more complex than the common myth suggests.
Tryptophan Defined
Tryptophan is one of the nine essential amino acids required for human health, meaning the body cannot manufacture it and must acquire it entirely through the diet. It is a regular component of any protein-rich food source. All amino acids, including tryptophan, function primarily as the fundamental building blocks used to synthesize various proteins throughout the body. This structural role is necessary for maintaining normal growth, tissue repair, and overall biological function.
Beyond its role in protein synthesis, tryptophan is metabolically diverted to produce several other substances. The necessity of obtaining it from food makes it a constant component in the protein content of all animal and plant sources.
Tryptophan in Context
Turkey contains tryptophan because it is a protein source, and all proteins are composed of various amino acids. The common perception is that turkey holds a disproportionately high amount of this compound compared to other foods. However, nutrition data reveals that turkey is not exceptional in its tryptophan content. Other common foods such as chicken, eggs, cheese, fish, and various seeds contain comparable, or sometimes greater, concentrations of the amino acid per serving.
A serving of cheddar cheese, pumpkin seeds, or ground pork can supply as much or more tryptophan than a similar portion of turkey. The presence of tryptophan in turkey is a standard feature of its protein composition, not a unique trait. The focus on turkey likely stems from its traditional role in large, heavy meals, which create conditions for post-meal fatigue.
The Tryptophan-Serotonin Connection
Tryptophan is associated with sleep and mood because it functions as a precursor molecule in the brain. Once consumed and absorbed, a small fraction of tryptophan is converted into the neurotransmitter serotonin. Serotonin plays a role in regulating appetite, mood, and social behavior.
The brain then uses this newly synthesized serotonin to produce the hormone melatonin, which controls the sleep-wake cycle. This conversion pathway—tryptophan to serotonin, and then to melatonin—is the theoretical basis for the link between consuming tryptophan and feeling drowsy. However, the mere presence of tryptophan in turkey does not guarantee its successful conversion into the sleep-inducing hormone.
The Myth Debunked: Crossing the Blood-Brain Barrier
The true story of tryptophan’s effect is governed by the selective mechanism of the Blood-Brain Barrier (BBB). Tryptophan must cross this barrier to enter the brain and be converted into serotonin and melatonin. However, it uses a shared transport system with five other Large Neutral Amino Acids (LNAAs), such as leucine, isoleucine, and valine, which are also abundant in protein-rich foods like turkey.
A meal consisting primarily of protein, such as a large turkey breast, floods the bloodstream with all these competing LNAAs. This intense competition means that only a small amount of tryptophan successfully crosses the BBB, limiting the production of serotonin and melatonin. In this high-protein scenario, tryptophan is largely blocked from entry.
The mechanism changes entirely with the consumption of carbohydrates, which are abundant in traditional holiday meals like stuffing, potatoes, and desserts. Carbohydrates trigger the release of insulin, which clears the competing LNAAs from the bloodstream by driving them into muscle cells. Tryptophan is less efficiently cleared by insulin than the other LNAAs.
This selective removal of competitors dramatically increases the ratio of tryptophan to the remaining LNAAs in the blood. With fewer competitors vying for the shared transport system, tryptophan is free to cross the BBB in higher quantities, boosting serotonin and subsequent melatonin production. Therefore, the sleepiness experienced after a large meal is less about the turkey and more about the combination of protein and high-carbohydrate side dishes. The feeling of lethargy is further compounded by the overall caloric load, increased blood flow to the digestive system, fat intake, and potential alcohol consumption.