Depletion of body chemical can cause memory, mood changes

November 18, 2002

The chemical in turkey that may cause people to nod off after Thanksgiving dinner also plays a role in maintaining good mood and memory, especially among people with a family history of depression, says new research published in the journal Brain, Behavior, and Immunity.

Lead author Wim J. Riedel, Ph.D., and colleagues at the Brain and Behavior Institute at the University of Maastricht in the Netherlands examined the effects of the body's depletion of an amino acid called tryptophan on mood and cognitive function. They also measured how long the effects of the depletion lasted.

Tryptophan, known for its presence in turkey and commonly blamed in the media for creating the sluggish after-meal sensations experienced by many Thanksgiving diners, is a metabolic precursor to the chemical messenger known as serotonin. In addition to turkey, the chemical is found in foods like milk, bread, cheese and bananas. Tryptophan depletion decreases serotonin levels in the brain, which in turn can lead to depression and other problems. While the study is not definitive and does not offer a solid conclusion that eating more tryptophan will enhance memory or mood, it does indicate a possible connection.

"Experimental lowering of tryptophan, and hence serotonin, appears to impair learning and memory and can cause depressed mood, especially in people who have a family history of depression, Riedel says."

The experiments involved 27 volunteers, 16 of whom had an immediate relative with major depression. Researchers lowered the level of tryptophan in the volunteers' bodies, and memory tests showed impairment in their ability to recall and recognize words they learned during, but not before, the tryptophan depletion time period. However, the volunteers did better on focused attention tasks, concentrated listening tasks and tasks measuring the speed of memory retrieval.

The results also showed that tryptophan depletion induced mood depression in half of the subjects who had a family history of depression but in only 9 percent of those with no family history of depression. The latter finding suggests that people with depression in their families are more vulnerable to changes in serotonin levels. The mood depression effects ended within 24 hours in all of the volunteers, however.

"These findings may have implications for people who have a history of major depression in their families and people whose tryptophan becomes depleted because of dieting," the authors note. "They also may have implications for people whose tryptophan becomes depleted because they are undergoing immunotherapy for cancer."
The study was funded entirely by the Brain & Behavior Institute of the University of Maastricht and the University Hospital Research fund.

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