What is tryptophan? Does eating turkey really make you sleepy?

14 December 2008

Question

What is tryptophan? Does eating turkey really make you sleepy?

Answer

We put this to John Fry, consultant in food science, nutrition and dietetics.

There's been a question about sleepiness caused by tryptophan in turkey and this is a popular myth in the United States that a feeling of sleepiness arises after the Thanksgiving meal and it's caused by the Thanksgiving turkey having a high content of a substance called tryptophan. Tryptophan is an amino acid, one of the building blocks of protein which means that pretty much all proteins contain some Tryptophan but turkey's not unusual in its tryptophan content. It has about the same amount as chicken or beef. Tryptophan is involved in the desire to sleep after a heavy meal but only indirectly. The root cause of the drowsiness but only indirectly. The root cause is the large carbohydrate intake that usually accompanies a celebratory festive meal. All those roast potatoes, the stuffing, not to mention sugar-rich puddings. They all result in a burst of insulin in the blood stream as the body tries to cope with this influx of sugars. One of the side-effects of this secretion of insulin is that tryptophan gets into the brain more easily and once there part of the tryptophan is transformed into a substance that's called melatonin. Melatonin is a hormone involved in sleep regulation and it can encourage sleep. But really it's the carbohydrate in a heavy meal that triggers drowsiness and tryptophan is just a bit player in the biochemical consequences of over-indulgence in carbohydrates.

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