DEAR DR. BLONZ: I want an explanation for the post-meal sensation of tiredness long associated with Thanksgiving. Is it the turkey, or isn’t it? Last year, we had a hearty mealtime discussion about this, with a humorous footnote of all of us sitting around exhausted after the meal. But a number of us only had vegetarian dishes. This year, I wanted to go armed with some facts. What do you say? -- M.Q., Tucson, Arizona
DEAR M.Q.: Turkey does not contain any sleep-inducing chemicals; the connection between Thanksgiving and tiredness relates more to the nature of the feast. It all comes down to the size and makeup of the meal: The greater the amount of protein, fat and carbohydrate consumed, the more pronounced the soporific effect.
Survival equates with sustenance. So as foods are eaten, the body’s priorities shift, and blood flow gets shunted toward the organs that play a role in digestion and absorption. During mealtime, the brain makes shifts in its neurotransmitter controller substances. The body has its own home-brewed relaxation cocktail that encourages us to sit back, relax, and facilitate our ability to take it all in.
And assuming the Thanksgiving feast is your evening meal, all this takes place near the end of the day, when our energy is already on the wane. It’s a perfect scene for post-meal tiredness.
By the time the feast has finished, some of the component parts of the appetizers and the first of the other digested foods will have begun to enter the bloodstream: the great highway by which newly digested nutrients find their way to where they are needed. Carbohydrates break down to simple sugars and cause a release of insulin, the “I am fed” hormone that signals the availability of energy resources in the blood while keeping the blood sugar level from rising too high. Another of insulin’s effects is to encourage the amino acids liberated from the meal’s protein to enter cells to help with repairs or to build new tissue where needed.
Our bloodstream does not flow into the central nervous system, where the brain resides. Our brain is protected by a blood-brain barrier that controls the nature and amount of substances allowed in. Amino acids compete with each other to gain access. Tryptophan, one of the essential amino acids, was thought to be a key player in the turkey-tiredness connection. This makes intuitive sense because, once in the brain, tryptophan can be converted to serotonin -- a neurotransmitter that can elicit relaxation and calmness. Tryptophan is also associated with the production of melatonin, a hormone connected with sleep.
Turkey is a complete protein, and it does indeed contain tryptophan (though not significantly more than other complete proteins). But tryptophan must get to the brain to make you tired, and it is at a disadvantage because it’s found in smaller amounts than other amino acids in most foods. Carbohydrates tilt the odds through their stimulation of insulin, which causes many amino acids to enter other tissue. Tryptophan is less affected by insulin, and, its competition reduced, is better able to enter the brain and have its effect.
The tiredness you feel is due more to the overall volume and types of foods eaten at Thanksgiving. In particular, it’s the carbohydrate foods such as stuffing, candied yams, cranberries or dessert that lead to the fatigued feeling. If you were to have some other meat as the centerpiece, or even have a vegan Thanksgiving feast, the effects would be the same.
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