The Swiss research team compared two groups: those who ate white chocolate, and those who ate dark chocolate. All the participants also smoked, a habit known to hasten hardening of the arteries. Blood tests and ultrasounds revealed that the arteries of those who ate dark chocolate became softer two hours after eating 1.5 ounces, and the effect continued for more than eight hours. The white-chocolate group did not have the same results.
Buyer beware, though! There's reason to be attentive of the type of dark chocolate you eat. The rationale behind eating dark chocolate is that it contains antioxidants called flavanols. When the natural cocoa solid is formed into a consumer chocolate bar, sometimes the flavanols (which often are bitter) are removed for a better taste, and the chocolate can just be darkened to mislead. The outside wrapper probably doesn't tell you this information either! So if you want to be in the know, check the labeling for the percentage of cacao (the bean that chocolate is made from) — the higher the percentage, the more flavanols!
Additionally, researchers are quick to warn that chocolate's beneficial effect shouldn't become an excuse to binge on it (or to keep smoking, for that matter!). But it does seem to suggest that the antioxidants found in truly dark chocolate can protect your heart. Just remember, chocolate is also loaded with fat and calories, so eat it in moderation! Want to protect your heart without worrying about weight gain? Try having some tea, onions, or apples — they're all rich in flavonoids, too