Above: image © istockphoto.com/bhofack2
Ever wonder why you feel incredibly tired and sleepy after a large holiday meal? Popular culture would have you believe it’s the turkey’s fault. However, your drowsiness is more likely to be a side-effect of your entire indulgent meal.
Turkey does contain a large amount of an amino acid called tryptophan. But a variety of other foods—including egg whites, bacon, and even Cheddar cheese—actually contain larger amounts of tryptophan per serving. So why don’t these foods cause the same effect?
Did you know? The human body uses 21 different amino acids as building blocks for proteins. Half of them are non-essential and can be produced by the body. The other half, including tryptophan, are essential and must be obtained from food.The answer can be found in the other foods on your plate. Chances are, your holiday meal includes lots of high-carbohydrate foods such as bread, potatoes, and stuffing. Carbohydrates are essentially long, polymeric chains made of many glucose monomer units. As these chains pass through your digestive system, they are broken apart into smaller chains. By the time the food reaches your small intestine, they have been broken down into glucose monomer units.
The higher glucose concentration in your bloodstream triggers your pancreas to release a hormone called insulin. Insulin attaches itself to insulin receptors found on the cell membranes of muscle, liver, and fat cells. In turn, this causes glucose transporters also found on cell membranes to open up. Glucose is allowed to enter the cells, providing them with energy.
Holiday meal digestion also involves breaking down proteins from the turkey into amino acids. Once again, insulin regulates amino acid uptake into cells, but in this case it is very picky about which ones it lets through: only branch-chained amino acids (BCAAs) are allowed. But tryptophan has a ring structure, so it is forced to remain in the bloodstream. The higher concentration of tryptophan in the blood means that there is a higher chance of it latching on to another type of transporter that will ferry if from your bloodstream into your cerebrospinal fluid through the blood-brain barrier.
Did you know? Melatonin is a hormone naturally produced in your body under dark conditions that helps regulate your circadian rhythms.Once in your cerebrospinal fluid, tryptophan is converted into serotonin, a neurotransmitter responsible for that relaxed, content, and overall satisfied feeling. Furthermore, in the pineal gland found below the base of your brain, serotonin is converted to melatonin, which is what makes you feel sleepy.
The overall effect of eating a large meal is called post-prandial somnolence and it can happen after any large, rich meal—high in carbohydrates and sugars, but not necessarily high in tryptophan. So this year, try not to blame the turkey for your impending snooze. Instead, just be thankful that it's there on the table for you and your loved ones to eat.
Definition: Insulin (2014)
Lahle Wolfe, Pre-diabetes.com
Brief introduction to insulin, including its role in moving blood glucose out of the blood stream and into cells.
Melatonin and Sleep (2014)
National Sleep Foundation
Brief introduction to melatonin and its role.
R. J. Wurtman and J. J. Wurtman JJ. (1986). Carbohydrate Craving, Obesity, and Brain Serotonin. Appetite 7 (Suppl.), 99-103.
Article on the effect of tryptophan on the brain. An abstract is available on the publisher’s website. However, a subscription is required to view the full text.