Turkeys

Turkey contains an essential amino acid called tryptophan.

Tryptophan (C11H12N2O2) is formed during digestion by the action of proteolytic enzymes. One of the functions of tryptophan is to act as a chemical precursor to the synthesis in the body of serotonin. Serotonin affects our mood and helps us have happy thoughts, but it also can in turn be converted to melatonin. Melatonin is a hormone that functions in several ways, one of which is to regulate our sleep-wake cycles. A result of increased melatonin is sleepiness.

But, having said that, the turkey and its low amount of tryptophan is probably not what is making you sleepy: Tryptophan doesn't act on the brain unless it is taken on an empty stomach with no protein present (it is destroyed by acid, and therefore not measured when proteins are hydrolysed by acid in your stomach).

If you consider that a turkey is mostly all protein - and most people don't really have an "empty stomach" during Thanksgiving dinner - then the turkey appears to be innocent, or as put in his words: "Gobble gobble."

So, the amount of tryptophan eaten during a holiday meal is generally too small to have an appreciable effect. That lazy, lethargic feeling so many are overcome by at the conclusion of a festive meal is most likely due to overeating, having too many foods with simple carbohydrates* in them that metabolize very quickly to gush into the blood stream and increase the blood glucose levels, drinking alcohol, time of day or energy expended from dealing with in-laws.

People often think that eating turkey makes them sleepy. Traditionally, the cause has been blamed on the amino acid, tryptophan, which is found in turkey. Even though tryptophan is used in the synthesis of serotonin, which converts to melatonin that influences our sleepiness, the tryptophan is not specifically high in turkey. You can find similar levels of tryptophan in beef, pork,and lamb. Eggs and cheese have higher levels of tryptophan than turkey or these other meats, and these and other foods like them are not associated with making us sleepy even though turkey is. (One of the meats with the highest content of tryptophan is reindeer meat.)

Experts now believe that the sleepiness may come from a high carbohydrate intake that often comes along in the "fixin's" when eating turkey dinners - especially during Thanksgiving.

Here's how it works: A high simple carbohydrate diet increases insulin. Insulin increases the uptake of branched-chain amino acids into the muscles (but not tryptophan). This increases the ratio of tryptophan to branched-chain amino acids in the blood.

When blood levels of branched-chain amino acids are low and tryptophan is high, tryptophan competes with them and sort of wins the competition of getting into the central nervous system. There it is synthesized into a neurotransmitter called serotonin which can be converted to melatonin and we get sleepy. Alcohol is another directly involved factor in the sleepiness, too.

The Role of Simple Carbohydrates*

Simple carbohydrates are in many of the things eaten in large portions in Thanksgiving holiday meals, like white bread rolls; sugar, marshmallows, or corn syrup added to side dishes like sweet potatoes; white flour used in gravy; white potatoes; stuffing or dressings; sugar in tea or soda pops; and desserts. These are metabolized very quickly in our bodies into sugars that can make our blood sugar levels temporarily spike very high. When that happens, our body recognizes that the blood sugar level is moving up too fast and starts sending out insulin to reduce the blood sugars. With a quick and high spike in blood glucose, the body often misjudges how much insulin will be needed and sends too much. This can shift us into a rapid decline of blood glucose to a state of low blood sugar and when that happens, we "crash".