Q. Can turkey really make you sleepy?

A. Another Thanksgiving has passed and the last of the leftover turkey has disappeared. Many of you will eat turkey again as part of your Christmas dinner, so this question deserves an answer.

It is common to feel tired after a holiday meal. Many people mistakenly believe it is the fault of the turkey. They blame it on an amino acid contained in the turkey called tryptophan.

This belief is fostered by the fact that once tryptophan enters the brain, it is converted to serotonin, a brain chemical that elicits relaxation and calmness.

However, for tryptophan to have a sedative effect, it must be taken on an empty stomach. It loses its sleep-promoting abilities when eaten with other foods. Since turkey is usually eaten with gravy, stuffing, sweet potatoes and other treats, its tryptophan would be rendered useless.

In order for tryptophan to make you tired, it needs to get into the brain. When other foods are eaten in the same meal as the turkey, the amino acids in these foods compete with the tryptophan for entry into the brain. Tryptophan is at a disadvantage because it is found in smaller amounts than other amino acids in most foods. Not enough tryptophan reaches the brain to have a sedative effect.

Many other foods have tryptophan: Salmon, cheese, beef and soybeans contain even more tryptophan than turkey. No one complains about being tired after eating these foods.

Then why are you sleepy after a holiday meal?

Overeating is the most likely explanation. Feeling sleepy after consuming large quantities of food is normal. Many of the foods consumed during the holiday meal are heavy, containing a lot of fat. These types of foods take a long time to digest, therefore inducing sluggishness.

Alcohol consumption during the meal can also have a sedative effect.

To avoid feeling tired after a holiday meal, watch your portions. Rather than gobbling up your food, eat slowly. This helps you recognize when you have had enough to eat. Stop eating when you are approximately 80 percent full. The carbohydrates in your meal will expand in your stomach, and later you will feel 100 percent full.

It is OK to leave some food on your plate. You can always have leftovers later.

Amy Tousman is a registered dietitian and diabetes educator. Hawai'i experts in traditional medicine, naturopathic medicine and diet take turns writing the Prescriptions column. Send questions to: Prescriptions, Island Life, The Advertiser, P.O. Box 3110, Honolulu, HI 96802; islandlife@honoluluadvertiser.com; or fax 535-8170. This column is not intended to provide medical advice.