More endurance athletes than you'd think use phenol-rich cacao as an effective performance enhancer. A Japanese animal study, published recently in PLoS One, shows how cacao works. The flavan-3-ols in cacao boost the activity of nitrogen monoxide or NO in the blood vessels, and as a result improve the blood's ability to transport oxygen and nutrients to the muscles. The ergogenic effect of cacao probably reaches its maximum 1-2 hours after intake.
Source: How cacao helps heart and blood vessels function better
Other references:
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