The chemical in food that may cause people to nod off after dinner also plays a role in maintaining good mood and memory, especially among people with a family history of depression, finds a team of Dutch scientists.
Lead author Dr Wim J. Riedel and his colleagues at the Brain and Behaviour 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, commonly blamed for creating the sluggish after-meal sensations experienced after a large meal, is a metabolic precursor to the chemical messenger known as serotonin. It is found in foods such as milk, bread, cheese and bananas, as well as meats like turkey. 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,” said Riedel.
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 recognise 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 per cent 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 noted. "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 & Behaviour Institute of the University of Maastricht and the University Hospital Research fund. It is published in the journal Brain, Behavior, and Immunity.
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