There i preliminary evidence that dark chocolate increase the function of the endothelial cell wall of blood vessels. The effect lasts about three hours. Moderation on the amount is needed. In the study the amount was 3.5 ounces. It also effects seratonin in the brain to elevate mood.
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http://www.thebostonchannel.com/health/3690805/detail.html?treets=bos&tml=bos_health&ts=T&tmi=bos_health_1_12150108302004