A chemical in turkey called tryptophan has received a lot of attention for its ability to make you feel sleepy, but Dutch researchers say tryptophan may also affect your mood and memory.
In a study in the current issue of Brain, Behavior and Immunity the researchers, from the University of Maastricht in The Netherlands, found that tryptophan depletion altered people's moods and cognitive function, particularly in those with a family history of depression.
Tryptophan is an amino acid, and a precursor to the brain'schemical messenger, serotonin, which is known to play a role in depression. Tryptophan is present in many foods, including turkey, milk, bread, cheese, and bananas. But many of the foods high in tryptophan can actually deplete the levels of this chemical in your body because they are so high in other amino acids that cancel out the effects of tryptophan, says Wim Riedel, lead author of the study and an associate professor at the University of Maastricht. When you eat foods high in carbohydrates, tissues in the body are able to pull amino acids from the blood. However, tryptophan is not absorbed this way and stays in the bloodstream, where it becomes available to the brain.
But during digestion tryptophan competes with other amino acids to cross the blood-brain barrier. If there are many other amino acids present, tryptophan gets crowded out, according to Joy Short, assistant professor of nutrition and dietetics at Saint Louis University in St. Louis. Short was not involved in the current research.
For the current study, the scientists lowered tryptophan levels in 27 volunteers, 16 of whom had a family history of depression, by having them ingest a drink full of these competing amino acids.
Tryptophan depletion reached its peak six hours after the volunteers drank the amino acid mixture. The researchers noticed behavioral changes in the subjects at this time: half of the volunteers with a family history of depression reported feeling blue, while 9 percent of those with no family history of depression noticed a change in their mood. These changes didn't last, however. Tryptophan levels and the volunteer's moods were back to normal by the next day.
The researchers also found that memory was affected during tryptophan depletion. The study participants had trouble recalling and recognizing words that they learned during the tryptophan depletion, but had no trouble recalling words they had learned when their tryptophan levels were normal. Interestingly, recall of old memories, verbal fluency, and listening ability were all improved during the tryptophan depletion period.
The authors suggest that these findings may have implications for people who have a family history of depression, and therefore might be more vulnerable to fluctuations of tryptophan levels. The findings may also affect people who diet frequently and those undergoing immunotherapy for cancer because these can affect tryptophan levels.
"It has been shown in other research that protein-rich, carbohydrate-poor meals lead to relatively lower tryptophan levels whereas meals with the opposite composition slightly increase tryptophan levels," says Riedel. "Apart from that, dieting lowers tryptophan as well as certain metabolic diseases such as irritable bowel syndrome. Interferon treatment for cancer also lowers tryptophan and, finally, chronically elevated levels of cortisol may lower tryptophan levels."
Does that mean we should all eat carbohydrate-rich diets if we want to be happy? No, says Short. "Tryptophan is not a magic bullet for treating depression," says Short. "The effects of administering tryptophan through diet are small when compared to the drugs available to treat depression,"she explains.
But, she says, this study provides another piece of the puzzle on the food-mood connection, and may give depression researchers a reason to possibly try developing therapies with tryptophan.
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