One square of milk chocolate contains the same amount of phenols as a glass of red wine, and dark chocolate contains even more. It is believed that phenols protect against heart disease by acting as an anti-oxidant.
The chemical process of oxidation is now thought to damage substances in the blood known as low-density lipoproteins or LDLs. This action eventually leads to a fatty buildup -- called plaque -- clogging the arteries. Such clogging and also clotting in human arteries are major causes of often fatal heart attacks. Phenols prevent the oxidation of low density lipoproteins into the bad fats reducings the risk of heart disease.
Phenolics also prevent the fat in chocolate from becoming rancid.