Don’t blame the turkey!

In honor of the upcoming Thanksgiving holiday, I wanted to write about a scientific topic that I’m sure will be brought up in many households across the country. Sometime after dinner, you may find yourself sitting on the couch, trying to stay awake. And you just might think:

The tryptophan in the turkey I just ate is making me sleepy.

A Newsweek article that I came across today definitely bolsters this idea: “Four Reasons Thanksgiving Makes Us Sleepy.” Reason #1? The turkey and the trimmings.

But is it true? Does eating a tryptophan-rich food like turkey really make you sleepy? If yes, why? If not, why does everyone think it does? For that matter, what is tryptophan, anyways?

Tryptophan is an amino acid, one of 20 standard amino acids used as building blocks to make proteins. (Actually, its proper name is really L-tryptophan, but hardly anybody uses the L.) Tryptophan is a pretty complex amino acid, considered hydrophobic (which means it doesn’t like water) and aromatic (which refers to its chemical structure). It is actually one of the essential amino acids, which means that our bodies cannot make it ourselves. We have to get it from our diet. (Nine of the twenty standard amino acids are essential; we can produce the other eleven.) Tryptophan is found from several food sources, including poultry (both chicken and turkey), pork, cheese, beef, fish, peanuts and soybeans. (Basically, foods rich in protein are also rich in tryptophan.) In addition to being used as a building block for proteins, tryptophan is also used by our bodies to make hormones. For example, tryptophan is used to make niacin (a B-vitamin). Niacin, in turn, is used to make serotonin. And serotonin is a remarkable hormone that exerts a calming effect on your brain, and plays a key role in making you sleepy. Tryptophan is also used in the production of melatonin, another hormone that regulates sleep.

With all that, it makes sense to think that eating a tryptophan-rich meal would result in increased levels of serotonin in your brain, which would then make you drowsy, right? In fact, in the 1980s, many people took L-tryptophan supplements to help combat insomnia. It’s not taken in the US anymore, as the FDA has banned it due to problems with production. But L-tryptophan supplements are still used to treat insomnia in Canada – though it’s only available by prescription.

Unfortunately, while the idea sounds good in theory, it doesn’t really work that way. In order for tryptophan to cause sleepiness, it must get to the brain. To do that, it must be eaten on an empty stomach, with no other protein source. If eaten with other proteins, all of the amino acids are trying to go to the brain at once. And since they all use the same transport system to get there, too many amino acids at once make it slower for all of them. (Just like driving during rush hour.)

So why do we get sleepy after Thanksgiving dinner? One of the biggest factors is that we simply eat a lot. To digest food, your body diverts blood away from your brain and down to your digestive system. The more food that has to be digested, the more blood has to be diverted to your stomach. This is especially true if you ingest a lot of fats in the meal. Fats (for example, in the meat, gravy, butter and desserts) take a longer time to digest than other components of your meal. So not only does your body need to use a lot of blood to digest the quantity of food, it needs extra time to digest the quality of food. And all that blood diverted from your brain to your stomach means sleepiness.

Another big food culprit is the carbohydrates (found in potatoes, bread, yams, stuffing and dessert). A carbohydrate-rich meal causes your pancreas to secrete insulin. When the insulin hits your blood stream, it causes your muscle cells to absorb glucose out of the blood. Since glucose is essentially energy, reducing your blood glucose level results in a feeling of low energy. Have you ever eaten a lot of sugar? You probably got a big boost of energy that quickly wore away, leaving you feeling very tired. That’s because sugar causes a rise in blood glucose, which gives you energy. But your body compensates by producing insulin, which pulls that glucose out of your bloodstream, leaving you tired.

I do want to address an issue raised in the Newsweek article I mentioned above. This article cites research published in the Canadian Journal of Physiology and Pharmacology in 2007. The research report, entitled “Protein-source tryptophan as an efficacious treatment for social anxiety disorder: a pilot study,” argues that tryptophan reaches the brain better when eaten in combination with carbohydrates than when eaten by itself. That’s because insulin causes specific absorption of some amino acids into your muscle cells, but not tryptophan. These other amino acids can no longer compete with tryptophan for delivery to the brain. The authors of the research say that serotonin increases in the brain, as measured by an anxiety test. (Remember, serotonin helps calm you down.) Two big problems with this study, though are (a) they didn’t test sleepiness, and (b) they tested pure components – deoiled gourd seed (a tryptophan source) and pure glucose (a carbohydrate). So it’s a stretch to say that this proves that turkey makes you sleepy on Thanksgiving. Relative to the other 2 factors mentioned above (blood diversion and insulin production), sleepiness induced by serotonin in your brain from ingested tryptophan is going to be small. You’re going to fall asleep because of the amount of food you ate much sooner than you would from the tryptophan in your turkey.

So while you may feel sleepy after dinner on Thursday, don’t blame the turkey. It gets a bad reputation for keeping us prone on the couch, when we should probably be helping with the dishes!