Linette Lopez writes:

In one study, rats who were given junk food food (like sausages, bacon and chocolate) for a long period of time weren't interested in healthy food such as fruits and vegetables.

From Scientific American:

The new study showed that after eating a diet full of sausage and sweets for 40 days—even though regular lab rat chow was available—the obese rats had little interest in reverting to the more healthful diet when the tasty stuff was taken away. In fact, after depriving the high-fat habituated rats of their human junk foods, the rats would refuse to eat their standard chow for an average of 14 days. "I was really shocked at the magnitude of the effect," Kenny says. "They basically don't eat anything. If that translates over to us as a species, that's a major problem."