Yes, in theory. But what about the "biggest loser" contestant who eats 800 calories a day, and still struggles with his weight? The calories in-calories out model is quite limited. From personal experience I can tell you it isn't possible to sustain "eating less." Hunger wins out. Low-carb diets show much more success than low-fat diets; there are numerous studies supporting this. Thus, what you eat does have an impact.