Mcdonalds has a hybrid synthetic form of coke and methamphetamine designed to trigger an intense "happy buzz" for the consumer, centering around warm emotions for the food. ie. "i'm lovin it"
KFC has been found to contain a small amount of low alkaloid morphine content, which allows the consumer to indulge absent mindedly, increasing their ability to consume large quantities of fatty foods. Thus engaging the eater in an addiction to the drug and the presence of fat/oil.