Turkey does not make you drowsy after you eat it. Now to be clear, turkey does contain tryptophan. Tryptophan is an amino acid which the human body doesn't naturally produce, but is essential to proper body function and, thus, must be acquired from food.
The reason tryptophan is tied to drowsiness is that it is used by your body to create serotonin. Serotonin, among other things, acts as a calming agent in the brain and plays a key role in sleep. So with turkey containing tryptophan, one would logically say it would make you sleepy after you eat it… except, this isn't actually the case.
So, why not? Primarily because of how we eat turkey and more importantly how the body deals with tryptophan. If you tend to eat turkey on a completely empty stomach and don't eat anything else but a little turkey, then there's a very small chance that the tryptophan in the turkey will make you a little drowsy right after you eat it. Add it to a sandwich or have it with some mash potatoes or really pretty much any other food eaten at the same time or just eat it on a non-empty stomach and it won't be the turkey that is making you drowsy if you feel drowsy after.
It turns out, tryptophan needs to not only be taken on an empty stomach to have any instant effect, but also with little to no other amino acids or protein present in order to make you drowsy after you eat something that contains it. Given that there is a lot of protein in turkey and other amino acids, even if you are a bit malnourished, you are not likely to get drowsy from the tryptophan after you eat turkey, though it will increase your body's store of the tryptophan.
What's going on here is that when there are a lot of amino acids around, this causes competition among the amino acids as far as crossing the blood-brain barrier. Tryptophan is a particularly bulky amino acid so it ends up being at the back of the line, so to speak, and will generally stay there until those other amino acids are gone or, at the least, until quite a lot of them are gone so the ratio of tryptophan to those other amino acids is drastically increased.
So why do you get drowsy after, say, eating turkey on Christmas? Well, there are a lot of reasons, but primarily because, with your Christmas meal, you likely just crammed a couple days worth of food into your body in the span of an hour or so, possibly with some alcohol, which is a central nervous system depressant that has a mild sedative effect.
Also, with a full stomach, your body directs blood away from your central nervous system and other organ systems to help with your digestive system. More than anything, this is probably causing most of the drowsiness after a large turkey dinner, such as on Christmas.
The "drowsy" effect is even more noticeable when your food contains big amounts of various fats, which take a lot of energy to digest, and carbohydrates. Carbohydrates in particular release insulin. This results in various amino acids being more or less swept out of your blood, being absorbed into your muscle cells, and thus the tryptophan to other amino acid ratio increases quite a bit, giving the tryptophan a better shot of crossing the blood-brain barrier and raising your serotonin levels.
Now there is typically always a nice store of tryptophan in your body at any given time due to the fact that it is not only prevalent in turkey, but also can be found in chicken, beef, pork, cheese, chicken eggs (which contain nearly four times as much tryptophan per gram over turkey), sunflower seeds, and many types of fish; it also is found, in lesser or similar amounts as in turkey, in milk, beans, sesame seeds, lamb, wheat flour, chocolate, white rice, oatmeal, potatoes, and bananas, among a lot of other foods.
So your body is typically stocked full of tryptophan most of the time and eating some turkey isn't going to significantly change your body's store, though it will add some.
So, if you want to point to a specific food that can make you quite drowsy, it would be things high in carbohydrates such as pasta, mashed potatoes, breads, and the like; these not only cause your body to direct more blood towards your abdomen and away from other organ systems, which will make you drowsy, but also result in some of the tryptophan hanging out in your body to actually get a chance to make it passed the blood-brain barrier to eventually produce some serotonin.
Daven Hiskey writes for the mildly popular interesting fact website TodayIFoundOut.com. To subscribe to Today I Found Out's "Daily Knowledge" newsletter, click here or like them on Facebook here. You can also check 'em out on YouTube here.