Lies My Turkey Told Me: Tryptophan

Learning By Doing

This week's activity is pretty easy. Grab a turkey leg (or spirulina or cashews if you're vegan), and take a big bite. Now wait while your body processes the food. Make a mental note of the time of day, any other food you may have eaten, and how alert you feel.

From Gut to Grey Matter

In this special holiday edition, I wanted to cover a topic related to Thanksgiving. The choice was obvious. I need to blog about Tryptophan! I'm sure you've probably heard the following explanation for why we get sleepy after a massive Thanksgiving feast. Turkey is full of essential proteins, fatty acids, and nutrients. In particular, turkey is said to contain an unusually large amount of Tryptophan, which is a necessary amino acid. Amino acids, as you may recall, combine with other compounds to create proteins that your body can use.

It turns out that Tryptophan is a biological or chemical precursor for other neurotransmitters. The body isn't able to produce its own Tryptophan, so it must be supplied by the food that we eat. Enter the drumstick. Tryptophan, then, is chemically altered by the body to form Serotonin, which is a neurotransmitter that serves many different functions. One such function is to regulate mood (it is associated with feelings of contentment and happiness) and sleep as well. Serotonin, in turn, can be converted into Melatonin, which is a hormone the brain sends the body to tell it that it's time to go to bed [1].

In a nutshell, here's the chemical chain of events: 

Tryptophan → Serotonin → Melatonin

Tryprophan is a precursor for Serotonin, which gets converted to Melatonin, which then tells the body to go to sleep. That seems logical. Turkey makes us sleepy because if fuels this bio-chemical waterfall.

Armchair Neuroscience: Getting off the couch

Ah...but there's a huge problem with this explanation. Actually, there are a couple of problems. 

First, feeling sleepy after a large Thanksgiving dinner is hopelessly confounded with the time in which people typically eat. The body has a natural rhythm (called a "circadian rhythm") in which people feel wide awake and alert, and other times of the day when we feel sleepy and tired. For most people, there's a natural dip in the afternoon (siesta, anyone?).

Second, eating a large meal containing a glut of protein is a taxing process that your body has to then deal with. Most of the blood in your extremities goes to your stomach to aid in the digestive process and to carry off the newly absorbed nutrients. What better way to pass the time than dozing off for an hour or so?

Finally, the chemical process outlined above, while true, tends to take some time. I would be shocked if this chemical conversion process happens between the time you push away from the table and pass out on the couch. 

The S.T.E.M. Connection

Wouldn't it be cool if we were teaching a chemistry class, and we could synthesize melatonin in the lab? A more likely connection might be to ask a science class to use the scientific method to confirm or disconfirm a hypothesis that everyone seems to take for granted (e.g., "turkey causes people to become sleepy because it contains tryptophan"). 

That could also lead to an interesting discussion about applying critical thinking to claims that sound "scientific." We could then discuss the following questions: 

• How do you know turkey causes sleepiness? 
• What other foods cause people to become tired? 
• What other foods that contain tryptophan don't generally induce sleep? 
• How do you counteract alternative explanations, such as time-of-day effects (e.g., circadian rhythms) and other confounding factors (e.g., the size of the meal or the amount of protein)? 

I admit...I thought that I was going to end up blogging about how turkey makes people sleepy. Then again, I believe all sorts of lies that my teachers, family members, tv shows, movies, and friends have taught me. I just need to remember to keep asking myself, "How do I know that it's true? What's the evidence? Is it any good?"

Share and Enjoy!

Dr. Bob

Going Beyond the Information Given

[1] It's useful to distinguish between hormones and neurotransmitters. A neurotransmitter is a chemical that cells in the brain use to communicate with other cells or regions of the brain; whereas, a hormone is how the brain communicates with the rest of the body.