A Turkey Tall Tale

Navigating the blood-brain barrier is tricky business.

With Thanksgiving just around the corner, it’s only fitting we examine the age-old belief that eating turkey makes you drowsy.
Turkey contains tryptophan, an amino acid that the human body doesn’t produce naturally. Tryptophan is essential to good health and must be acquired through diet. L-tryptophan is metabolized by the body to create serotonin and melatonin, neurotransmitters that act as calming agents in the brain and regulate sleep. So its easy to see why the turkey gets blamed when you’re snoozing on the couch after dinner, but it’s not so simple.

Metabolism of L-tryptophan into serotonin
and melatonin. Transformed functional
groups after each reaction shown in red.

  First off, turkey contains about the same amount of tryptophan as most other meats, 0.24 grams per 100 grams of food. Our bodies only need about 0.2 grams per day, and we get more than five times that amount in an average meal. So eating turkey is not going to make a huge difference in and of itself. If one were to consume pure tryptophan on an empty stomach, then yes, it would make you drowsy. But you would have to take it as a supplement. In turkey, however, it’s only one of several amino acids and must compete to cross the blood-brain barrier. And because it has a large molecular weight, it’s not easily absorbed.
  So what is making you drowsy after your Thanksgiving dinner? Carbohydrates, mostly. Carbohydrates cause the pancreas to produce insulin. When this occurs, some of the competing amino acids leave the bloodstream and enter muscle tissue. This causes a relative increase in the concentration of tryptophan in the bloodstream. The tryptophan is carried to the blood-brain barrier through glucose transporters (GLUTs). Some of it then crosses the barrier and is metabolized first to serotonin and then melatonin which makes you sleepy.
  There are other factors involved in the conspiracy to make you miss that last quarter of football. Eating two days worth of food in one meal will have an effect. Your body will have to direct more blood flow to aid in digestion at the expense of your other organs, including your nervous system. If you drink alcohol with your meal it will act as a depressant and increase your drowsiness. Fatty foods will also slow down digestion and sap your energy. Maybe the best course of action is to just sit back, relax, and let Mother Nature take its course.