On Nutrition by Ed Blonz

Carbs, Not Turkey, More Likely to Cause Sleepiness

DEAR DR. BLONZ: In a lecture at my community college, there was a statement that the turkey we have at Thanksgiving contains specific enzymes that make us feel sleepy after we eat. I had always thought that this was a myth, but was reluctant to challenge the professor. Is that statement valid? If so, what is in turkey that makes us feel sleepy after we eat it? -- M.M., Phoenix

DEAR M.M.: As a prelude to every holiday season, we hear the commonly held belief that turkey causes tiredness, but I had never heard it phrased as “specific enzymes.” Since this was an academic setting, you should not have been reluctant to ask more about these enzymes and how they supposedly cause tiredness. This is a new one, and I am always interested in learning the rationale behind such beliefs. I must confess, I am quite puzzled, given that the cooking of the turkey would destroy any enzymes in the meat.

Let’s put some other aspects of the turkey-tiredness legend in perspective. One key is tryptophan, one of the amino acid building blocks of protein. When this amino acid gets in the brain, it can be converted into serotonin, a compound that elicits relaxation and calmness. (One reason that a certain class of anti-anxiety drugs works is that it helps to maintain serotonin in the brain.) Turkey, as a complete protein, will indeed contain tryptophan, but it doesn’t contain that much more than other complete proteins, such as chicken or other meat.

Why, then, would turkey have the reputation for causing sleepiness, and not other proteins? It turns out that the tiredness you feel is more likely due to the carbohydrates often consumed along with turkey at the holiday feast.

To explain this, you should know that there is an important barrier between the blood that flows in the general circulation of the body and that which flows around the central nervous system. Called the blood-brain barrier, it helps protect the brain and control the compounds allowed to circulate in and around delicate brain tissues.

After a protein-based meal, the enzymes in our digestive system help break down the proteins into their constituent amino acids building blocks. Amino acids can pass through the blood-brain barrier, but they compete with each other to cross. Tryptophan is at a disadvantage because it’s found in smaller amounts in foods relative to other amino acids. Carbohydrates, however, can tilt the odds in tryptophan’s favor. Carbohydrates cause insulin to be released, and insulin causes amino acids to enter muscles and other body tissues, but tryptophan to a lesser extent. With its competition thus reduced, tryptophan is better able to enter the brain, turn into serotonin, and bring about the relaxation. 

Thanksgiving turkey tends to be paired with carbohydrate-rich foods such as cranberries, potatoes, stuffing and pumpkin pie, all of which raise our blood sugar level and ease the way for tryptophan to do its thing. Such logic would also explain why those who partake of other main courses -- such as ham, duck or even a vegetarian dish -- might feel that same post-feast tiredness.

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