The phrase " You are what you eat" is a simple way to tell people to be mindful of what they're eating. Everything you consume basically stays in your body becoming something else, whether it's energy, stored, or excreted, it's still inside of you for a considerable amount of time and so you really are what you eat. For example if you were to eat 5 slices of pizza and a 32 oz soda, your digestive system breaks it down into things like fat, protein and glucose but your body probably isn't going to use all of the fat and sugar, but it has to go somewhere, so it's stored for later. The pizza slice obviously isn't a whole piece stored in you, but what it was broken down into is. So when you're eating healthy, you're body isn't storing a whole bunch of extra fat and sugar, therefore, you'll weigh less. But you'll also have to be either taking in the same amount of energy you're putting out, or taking in less energy than you're putting out to either maintain weight, or loose it.