Your car is designed to run on gasoline. When you put diesel fuel into its tank, the results are disastrous for the engine. The same principle is true for us: We are designed to run best on the wild plant and animal foods that all humans gathered and hunted just 500 generations ago. The staples of today’s diet—cereals, dairy product, refined sugars, fatty meats, and salted processed foods—are like diesel fuel to our body’s metabolic machinery. These foods clog our engines, make us fat, and cause disease and ill health.
Loren Cordain in The Paleo Diet