It's true. What you put in your body has a direct impact on how your body functions and how you feel. Your body converts food into energy during the digestion process and in turn burns calories. How efficiently you burn calories is measured by your metabolism, which is your body's ability to convert food into energy. Both short term and long term health are determined, in part, by how you eat. When you eat a lot of junk or fast food, your energy levels drop significantly. There just aren't enough nutrients in these foods to keep you going.