A CALORIE is a calorie. This truism has been the foundation of nutritional wisdom and our beliefs about obesity since the 1960s. What it means is that a calorie of protein will generate the same energy when metabolized in a living organism as a calorie of fat or carbohydrate. When talking about obesity or why we get fat, evoking the phrase “a calorie is a calorie” is almost invariably used to imply that what we eat is relatively unimportant. We get fat because we take in more calories than we expend; we get lean if we do the opposite. Anyone...