An evolutionary perspective predicts that most diets and fitness programs will fail, as they do, because we still don’t know how to counter once-adaptive primal instincts to eat donuts and take the elevator. Further, because the body is a complex jumble of adaptations, all of which have costs and benefits, and some of which conflict with one another, there is no such thing as a perfect, optimal diet or fitness program. Our bodies are full of compromises.