In a manner of speaking, the food we eat never really enters our bodies.
Now you’re probably shocked and appalled, but there’s actually very sound reasoning behind this.
Physiologically, the contents of the gastrointestinal tract are considered as outside the body, because the tract itself is continuous with the external world. It’s essentially a long ‘tube’ that runs from the mouth to the anus, and until nutrients are absorbed through the intestinal lining, they aren’t considered as inside the body.
Furthermore, that intestinal lining, known as the mucosa, is actually the external boundary of the tract, protecting the body from toxins or other possible invaders that might try to sneak past the mucosa.
Think of it as a sock with the toes chopped off, turned inside out, and stuck inside a larger sock with its toes chopped off, too. If you were to pass a pencil through that inner sock, it would come out the other end without touching the outer sock.
Of course, if the inner sock was coated in enzymes that could break down the pencil so it seeps through to the outer sock… well, you get the picture.