Tryptophan: What Does It Do?

You've picked the bones of the Thanksgiving bird; you're in an easy-chair nearly napping: "It's the tryptophan!" you say. But it isn't: you've fallen victim to a common myth about an uncommon molecule, tryptophan. Thought to have effects on mood, memory, and sociability, tryptophan is an amino acid with a checkered past, an obscure present and a promising future. Misconceptions and confusion abound, especially in the popular press.

Tryptophan achieves its effects by way of serotonin, one of the key brain chemicals involved in regulating mood. Among other functions, serotonin promotes feelings of calm, relaxation, and sleepiness. Lack of serotonin is associated with depression. Many of today's powerful antidepressant drugs work to increase the level of available serotonin in the brain. Tryptophan is the key ingredient in making serotonin; without it, serotonin won't be produced. Because the body can't make it's own tryptophan, it must be taken in as part of the diet; for this reason tryptophan is known as an "essential" amino acid.

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Many folks believe that turkey will put you to sleep because of its high tryptophan content. Tryptophan can indeed make you sleepy, but turkey doesn't contain substantially more tryptophan than other protein sources. "It's a complete myth," says Simon Young, Ph.D., research psychologist at McGill University.

If anything eating turkey lowers tryptophan. That's because tryptophan uses the same means of transport into the brain as other amino acids, and has to compete against them to cross the blood-brain barrier. As it happens, tryptophan is the least abundant amino acid. Forced to fight for access against the more common amino acids, it's left waiting at the gate: the amount of tryptophan entering the brain decreases.

Why, then, the post-turkey torpor? "Eating a large meal will have a sedative effect," says Young. You've simply stuffed yourself.

For similar reasons, a glass of warm milk at bedtime will not raise the level of tryptophan entering your brain. Neither will walnuts, strawberries or salmon, though many nutrition columns say so. If a glass of warm milk at bedtime helps you fall asleep, it's not because of its tryptophan content.

There is a body of evidence suggesting that eating carbohydrates along with protein can increase the tryptophan available to the brain. When carbohydrates are consumed, the body produces insulin, which diverts other amino acids to body muscles but leaves tryptophan untouched. With fewer competitor amino acids in the bloodstream, tryptophan more freely enters the brain, promoting calm and well-being.

Dr. Young begs to differ. "When you eat a mixture of protein and carbohydrate, you don't need very much protein in there to counteract the carbohydrate effect," he points out. "In any real meal that you'll take in, the effects of protein will predominate, and you'll get a decrease in the ratio of tryptophan to other amino acids."

The only surefire way to increase tryptophan to the brain is with dietary supplements. Taken in pure form, tryptophan works in a qualitatively different way than when obtained from a food source. The concentration of tryptophan becomes high enough to compete on a level playing field against other amino acids; plenty of it gets into the brain.

Tragically, in the late 1980s, over 5,000 people became gravely ill after taking tryptophan supplements made by Japanese manufacturer Showa Denko. Investigation revealed that the preparation contained impurities, one of which was strongly implicated in the illness. It was not clear, however, that tryptophan itself was completely blameless.

As a result, the FDA banned all imports of tryptophan supplements into the U.S. Both consumers and scientists steered clear of tryptophan, and research on the amino acid fell out of fashion. American manufacturers shied away from putting tryptophan on the market for fear of lawsuits.

Still, a handful of recent studies suggest that tryptophan has unexpected benefits. Dr. Young has found that taking tryptophan can affect human social behavior, decreasing aggression, irritability and quarrelsomeness. In animals, increased serotonin has been shown to reduce hostility, increase social affiliativeness and also promote dominance in social hierarchies. Evidence is starting to trickle in that the same applies to humans.

Stay tuned. Interest in tryptophan may be reviving.

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