These rats, like Mayer’s mice, are genetically predisposed to get fat. When Zucker rats are put on a calorie-restricted diet from the moment they’re weaned from their mothers’ milk, they don’t end up leaner than their littermates who are allowed to eat as much as they want. They end up fatter. They may weigh a little less, but they have just as much or even more body fat.
As animals starve, and the same is true of humans, they consume their muscles for fuel, and that includes, eventually, the heart muscle. As adults, these obese animals are willing to compromise their organs, even their hearts and their lives, to preserve their fat.