Turkey doesn't make you sleepy; the potatoes and stuffing probably do
Blaming the Thanksgiving turkey for the post-dinner slump is a myth: turkey holds about as much tryptophan as any other poultry. The drowsiness more likely comes from a carbohydrate-heavy meal, which sets off a chain of insulin, amino acid traffic and brain chemistry that nudges you toward sleep.
Tryptophan is one of the amino acids the body strings together into proteins, and humans cannot make it, so it must come from food. It shows up in most protein sources, from oats, milk and eggs to chickpeas, peanuts and pumpkin seeds. Its name comes from trypsin, the digestive enzyme used when Frederick Hopkins first isolated it from milk protein in 1901, and its single-letter code, W, was chosen because its bulky double ring looks rather like the letter.
The body uses it as raw material for more than protein. It is the starting point for serotonin, which the brain in turn converts into melatonin, the hormone that helps set daily rhythms, and for vitamin B3. Over 95 percent of it, though, is broken down along a different route into kynurenine, a pathway that behaves differently in conditions such as schizophrenia and bipolar disorder.
Here is the turkey mechanism. A starchy meal releases insulin, which pulls competing branched-chain amino acids into muscle but leaves tryptophan in the blood. With less competition at the transporter that carries both across the blood-brain barrier, more tryptophan gets into the brain, where it becomes serotonin and then melatonin. The carbohydrates, not the bird, tip the balance. Eating tryptophan-rich food does not raise brain serotonin the way a purified dose does.
Purified tryptophan has a chequered history. In 1989 an outbreak of eosinophilia-myalgia syndrome in the United States produced more than 1,500 reported cases and at least 37 deaths. Investigators linked it to contaminated batches from one Japanese maker, Showa Denko, which used genetically engineered bacteria, though some evidence suggests tryptophan itself may have contributed. The FDA recalled supplements and banned most sales in 1990, easing limits in 2001. A 2001 Cochrane review found only two of 108 studies on depression rigorous enough to include, and called the evidence too weak to be conclusive.
Source: Tryptophan
