organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l -stereoisomer appears in mammalian protein. It is one of several essential amino acids needed in the diet; human beings cannot synthesize it from simpler metabolites. Young adults require about 7 mg of this amino acid per day per kg (3 mg per lb) of body weight. Nicotinic acid (niacin), a vitamin of the B complex, can be made from tryptophan in the body, but evidently the rate of transformation is insufficient for the demands of normal growth and maintenance, and hence nicotinic acid must be supplied in the diet. Deficiency of tryptophan in the diet enhances the progress of the vitamin-deficiency disease pellagra , which is treated by restoring nicotinic acid to the diet, usually supplemented with tryptophan. Bacteria in the intestine break tryptophan down to compounds such as skatole and indole, which to a great extent are responsible for the unpleasant odor of feces. Tryptophan contributes to the structure of proteins into which it has been incorporated by the tendency of its side chain to participate in hydrophobic interactions (see isoleucine ). The amino acid was isolated from casein (milk protein) in 1901, and its structure was established in 1907.

Turkey does have the makings of a natural sedative in it, an amino acid called tryptophan. Tryptophan is an essential amino acid, meaning that the body cannot manufacture it. The body has to get tryptophan and other essential amino acids from food. Tryptophan helps the body produce the B-vitamin niacin, which, in turn, helps the body produce serotonin, a remarkable chemical that acts as a calming agent in the brain and plays a role in sleep. So you might think that if you eat a lot of turkey, your body would produce more serotonin and you would feel calm and want a nap.

That was the conclusion that led many people to begin taking a dietary supplement of tryptophan in the 1980s as a way to treat insomnia, but the U.S. Food and Drug Administration banned tryptophan supplements in 1990 because of an outbreak of eosinophilia-myalgia, a syndrome that causes muscle pain and even death. The FDA said contaminated tryptophan supplements caused the outbreak.

But nutritionists and other experts say that the tryptophan in turkey probably won't trigger the body to produce more serotonin because tryptophan works best on an empty stomach. The tryptophan in a Thanksgiving turkey has to vie with all the other amino acids that the body is trying to use. So only part of the tryptophan makes it to the brain to help produce serotonin.

It is the whole traditional Thanksgiving meal that can produce that after-dinner lethargy. The meal is quite often heavy and high in carbohydrates -- from mashed potatoes, bread, stuffing and pie -- and your body is working hard to digest that food. Also, if you drink alcohol with your dinner, you will likely feel its sedative effect, too.

There is a way to take advantage of the tryptophan in turkey. If you have trouble getting to sleep one night while there is still leftover turkey in the refrigerator, you could have a late snack of turkey and that, nutritionists say, might be the right amount of tryptophan on an empty stomach to help produce some serotonin.