Americans spend $40 billion a year on books, products, and programs designed to do one thing: help us lose weight. From Atkins to Ornish and Weight Watchers to the Zone, today's dieters have a dizzying array of weight loss programs from which to choose--yet the underlying principles of these diets are often contradictory. Is low fat better than low carb? Is Atkins the answer? And has the USDA Food Pyramid done more harm than good? FRONTLINE examines the great diet debate.