In the dark ages people thought that rats were created out of food scrapes. They saw lots of rats 'suddenly appearing' in the food scrapes. Thus, the rats-come-from-food-scrapes theory appeared and stayed until someone had the courage to debunk the faulty and popular theory.

One day a doctor looked closely at an uncooked piglet sitting in the kitchen, later that night he looked in the mirror and wah loh the fat-comes-from-fat theory was born.

Kidding aside, I think it's the caloric thing, too.