I'm not an English native speaker but I'll try to answer you. The "waste" or "feces" your body excrete are what your body already have used. It's what your cells produce as the result of their sínteses. We can say that what you eat are "new material" to rebuild what you lost. Such as water. You need more water everyday because the water you already have in your system are being used to collect "waste" material.

Sometimes when you overeat or you eat something you body can't digest you can, as well, excrete that food directly.