Just a little message from the book Paleo Manifesto by John Durant:
"We Americans would have saved ourselves a lot of trouble if we had never discovered the (overly) simple fact that a gram of fat contains more calories than a gram of protein or carbohydrate, which seems to suggest that, all things equal, any overweight animal should eat more carbohydrate and less fat to lose weight. Yet carb-heavy grains are exactly what farmers feed to livestock to fatten them up as quickly and as cheaply as possible. Let me repeat that, just for good measure: grains -- the base of the USDA food pyramid for humans -- are what American farmers use to fatten up their livestock." (my emphasis)
For more, read Chris Kresser's review on this book.