Thanks to its association with the Thanksgiving turkey, tryptophan has become probably the most popular amino acid. Whether it’s being blamed for the strong post-meal desire for a nap or being rhymed with “gravy in the pan” in a dancey clothing store commercial, tryptophan is the envy of its 19 peers in the standard amino acid family. But like most scientific crossover stories, public misperception has given turkey’s tryptophan something of a bad rap as a kind of natural sleeping pill – or a convenient excuse to not do dishes.
In fact, research has shown that turkey has no more tryptophan than any other poultry or ground beef; perhaps people should pull out the old “tryptophan” excuse the next time they eat a burger, as well. Simply eating a bunch of carbohydrates, such as the rolls, potatoes, and stuffing on the table next to the turkey, can spike a person’s tryptophan and insulin. The latter, a hormone best known for causing cells to absorb blood sugar, also causes the absorption of amino acids – except tryptophan, which crosses over into the brain. There, it is turned into the neurotransmitters serotonin and melatonin, which, yes, cause sleepiness, but also have a number of other interesting behavioral effects.
The powers of tryptophan are perhaps best depicted by what happens to a person when it’s not there. Tryptophan depletion is a frequently-used tool in psychiatry research, mostly as a shortcut to reducing the levels of serotonin in the brain. Because serotonin has been implicated in depression (many anti-depressant drugs are designed to increase brain serotonin) and impulsivity, some interesting things happen when you remove its key amino acid ingredient from a person’s diet.
In 2009, the laboratory of Emil Coccaro, chair of psychiatry at the University of Chicago Medical Center, used tyrptophan depletion to study an element of intermittent explosive disorder (IED). People with IED tend to have violent temper tantrums that do harm to themselves and others, and show signs of abnormal serotonin signaling, so Coccaro’s group, led by Michael McCloskey, studied the effects of tryptophan depletion in this group. Subjects with IED and controls without the disorder were given a tryptophan-depleting drink: a chocolate-flavored liquid that contained all the other dietary amino acids. Test subjects were then given a test where the punishment for failure was an electric shock; participants were allowed to choose the intensity of their shock.
Independent of the pre-test drink, IED subjects chose a stronger shock punishment than control subjects. But after quaffing the trytophan depletion drink, both IED and control subjects were more likely to choose a higher shock than when they consumed a placebo drink. That result suggests that lower one’s tryptophan, and by extension their serontonin, increases self-aggression and self-injury.
The relationship between tryptophan, serotonin, and self-aggression may be mediated by a connection between low serotonin and impulsive behavior. Impulsivity has itself been linked with drug addiction, as people who are more likely to take risks are more likely to show substance dependence. John Crean in the laboratory of Harriet de Wit, professor of psychiatry at the University of Chicago Medical Center, tested the effect of tryptophan depletion on a group of subjects known to be vulnerable to addiction: people with a family history of alcoholism.
Subjects in this study avoided electric shocks, and instead were asked to complete tasks that measure impulsive behavior through responding to computer signals or playing financial games. Tryptophan depletion before one of the tasks, called a Stop Task, caused the subjects with a family history of alcoholism to make more mistakes than those without family history, suggesting that they have a more sensitive serotonin system in controlling impulsive behavior.
It’s a bit iffy to flip these effects of tryptophan depletion around to speculate on the effects of a Thanksgiving tryptophan overdose. But the extensive tryptophan literature does suggest that the amino acid most commonly associated with turkey comas is in fact responsible for a wider range of behavioral effects. Indeed, the amino acid or derivatives has been studied as an antidepressant, a booster of the immune system, and yes, a sleep promoter. So impress your family this week with a nuanced, scientifically-informed view of tryptophan, an amino acid with the power to shape behavior in more ways than just napping along to a football game.