1.  Americans are surrounded by temptations to consume calorically dense foods.

2.  Humans, some researchers maintain, are hard-wired to desire calorically dense foods.

3.  It is a consequence of eating too much, especially too many calorically dense foods like fats and sweets, and exercising too little.

4.  Throughout most of human evolution, calorically dense foods were in relatively short supply, which kept the species lean.