You may be tempted to blame turkey’s high levels of sleep-inducing tryptophan for sleepiness after a big Thanksgiving or Christmas dinner. But, most experts agree the link between drumsticks and drowsiness is not as clear-cut as commonly thought.
The amino acid tryptophan, present in high amounts in turkey meat, is converted within the brain into the sleep-inducing substances serotonin and melatonin.
“Tryptophan and sleep actually have a long history dating back to the disease pellagra in the U.S.,” said Dr. Craig Hudson, who completed his M.D. and specialist training at U of T. Considered an expert on tryptophan, Hudson is also the founder of Biosentials Inc., a “nutraceuticals” company that offers “enhanced” natural food products touting health benefits for problems such as sleep disorders, sexual dysfunction and chronic pain.
“Among other symptoms, pellagra showed us that [tryptophan] deficiency leads to anxiety and loss of sleep.”
Unfortunately, for proponents of the turkey-sleepiness link, increased levels of dietary tryptophan in the blood do not necessarily translate into increased levels in the brain, or the subsequent sedating effect.
Hudson has speculated on dietary tryptophan’s sedating potential. “There’s something there but clearly it’s a matter of, ‘Can you get enough in you at one time?’ There are a number of pharmacokinetic hurdles to overcome.”
Pharmacokinetics, among other things, is a discipline that describes the way substances move throughout the body. And there are many “pharmacokinetic hurdles” for tryptophan. Tryptophan competes with other amino acids in the blood for entry into the brain thereby keeping brain levels low despite high consumption.
Tryptophan can only circumvent this system if the body has high levels of insulin, which eliminates the compounds tryptophan competes with, before tryptophan is ingested. Unfortunately for the turkey-sedative theory, this is unlikely to occur naturally for two reasons.
First, the necessary insulin boost would mean loading up on carbohydrates (for example, by eating a high-sugar dessert) before eating the turkey, and avoiding foods that contain any of the “competitor” amino acids.
Second, studies suggest that the amount of turkey required to affect wakefulness would be difficult to obtain in a single holiday meal. In fact, studies linking dietary tryptophan to modest improvements in sleep use enhanced preparations of tryptophan in order to elicit a sleepy response.
“You have to take nature and tweak it a bit.” Hudson explained.
So, why do many of us nap after large holiday meals? Aside from the “too-much-wine” explanation, the most likely answer relates to a phenomenon called “gastric steal.” Gastric steal is a bodily response to large volumes of food, in which the body reroutes blood from the brain to the digestive system to facilitate digestion. By commandeering blood that the brain would otherwise use to facilitate consciousness, “gastric steal” results in sleepiness.
Since light meals and no turkey over the holiday season aren’t likely options for anyone, post-meal sluggishness will probably stay a part of many festive dinners.
