Tripping On Tryptophan
Tryptophan is an essential amino acid, meaning that the body cannot produce it and must get it from foods such as meats. Tryptophan stimulates the production of serotonin, which is a chemical that helps keep people happy by calming anxieties, relieving depression, and promoting sleep.
One way to get the effects of tryptophan is indeed through foods heavy in protein such as meat, but also dairy products, eggs, soy products, and nuts. However, the tryptophan in a lot of these foods competes with other amino acids to get into the brain and do its work. So you might actually feel more of tryptophan’s effect after eating a meal heavy in carbohydrates.
That’s because many carbs cause the body to secrete large amounts of insulin, which clears the bloodstream of most of these other amino acids that compete with tryptophan. Thus, there’s more room for tryptophan in the brain and it ends up having a stronger effect. In addition, though, eating a large meal can stimulate gut hormones that cause a sleepy feeling unrelated to tryptophan. This is the body’s way of ensuring that you’re still and quiet, so that it can better digest.
“Is There Something In Turkey That Makes You Sleepy?” (How Stuff Works)