Tryptophan, Turkey, and Sleep

By popular request, an explanation for why tryptophan in turkey does not actually make you sleepy. Neuroscientist Approved(tm)!

The wikipedia article on this is pretty good, and has nice references, but it’s not exactly in layman’s language. Let me try to explain it in my own words:

What is tryptophan?

Tryptophan is an amino acid, a building block of protein. Your brain uses it to produce serotonin, a neurotransmitter that plays an important role in sleep. So if you eat a food with lots of tryptophan, you should produce more serotonin, and thus get sleepy, right? Well…

Tryptophan in food ≠ tryptophan in brain 

Most things cannot move easily between your blood and your brain. It’s a delicate environment in there! Tryptophan (like all amino acids) needs another molecule to drag it bodily across that blood-brain barrier. However, that transport molecule is already working at full speed, hauling a full load of various amino acids. Unless you’re starving, your blood already has plenty of tryptophan, so a little extra from the turkey will make no difference. When the ferry is moving at full speed, and there’s already a backlog, adding more cars to the waiting list doesn’t get any more cars across the water.

So why do we get sleepy after Thanksgiving dinner?

Part of it’s just meal size — after a big meal your body diverts resources to digestion. But let’s focus on Thanksgiving dinner. It does contain something special, but not the turkey. When you eat lots of carbohydrates, your body produces insulin to pull sugar out of the blood, into storage. Insulin also makes your body pull some amino acids into storage… but not tryptophan. So after a lot of carbs, more of the amino acids in your blood are tryptophan. When a bunch of non-tryptophan cars bail out of the ferry line, you do end up with more tryptophan cars across the water.

In other words, tryptophan in your brain can make you sleepier — but you get more tryptophan in your brain from eating the potatoes and the stuffing, not the turkey.

On top of all that, turkey doesn’t even contain more tryptophan than chicken, pork, or cheese. Anyone know where the heck this urban legend comes from?

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