Normally you might not think twice about, say, nibbling a sample of cheese at the grocery store or finishing the French fries on your child's plate. But if you're recording every morsel that passes between your lips, you're more likely to hesitate. "Oh, if I eat it, I have to write it down. Well, then, forget it. I'm not even hungry."
That self-policing is a huge help, since all those little tastes really add up.