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According to the study by Rose et al,1 chocolate consumption was associated with higher depression ratings. Whether causality exists between the 2 variables is subject to further research. However, other than what was mentioned in the article by Rose et al, there is a plausible physiological mechanism to explain the relation between chocolate and mood.
The ingestion of carbohydrates stimulates the release of insulin which, along with its anabolic effects, promotes amino acids in the blood to enter muscle cells, except for tryptophan.2 This will cause a relative increase of tryptophan over other amino acids, which compete for passing the blood-brain barrier. As a consequence, more tryptophan enters the brain, and there is an increase in the synthesis of serotonin,3 a neurotransmitter postulated to have a major role in mood disorders and a target of many psychopharmaceuticals.
Silva NR. Chocolate Consumption and Effects on Serotonin Synthesis. Arch Intern Med. 2010;170(17):1608-1609. doi:10.1001/archinternmed.2010.331