They tested the protection of epicatechin on heart infarct size in mice. Epicatechin is a flavinoid and a major component of dark chocolate. It has antioxydant effects associated with a lower risk of stroke and heart failure. Epicatechin can bind to opiod receptors that can induce heart protection, moreover it can induce cardiac protection from ischemia-reperfusion injury with a heart attack.
What they did was treat mice with epicatechin and naloxone alone and a combination of both. Naloxone is an opiod antagonist. Infarct size was significantly reduced in the epicatechin group, this effect was attenuated when administered together with naloxone. These data suggest that the protective efect of epicatechin is mediated by the opiod receptor.
This is the first demonstration of a receptor-mediated mechanism for epicatechin-induced cardiac protection.
And for our ongoing discussion on dark chocolate dosage:
Studies show that 6.3 g (30 kCal) per day of dark chocolate containing 30 mg of polyphenols are sufficient to reduce blood pressure in hypertensive patients
Panneerselvam, M., Tsutsumi, Y., Bonds, J., Horikawa, Y., Saldana, M., Dalton, N., Head, B., Patel, P., Roth, D., & Patel, H. (2010). Dark chocolate receptors: epicatechin-induced cardiac protection is dependent on -opioid receptor stimulation AJP: Heart and Circulatory Physiology, 299 (5) DOI: 10.1152/ajpheart.00073.2010