From the point of view of the body, the contents of the digestive system are alien, and their processing and absorption into the blood stream is tightly controlled by first denaturing the contents and sterilising the bugs. By injecting anything directly into your bloodstream, you are bypassing those controls. In terms of functioning, the contents of the digestive system are actually outside the body... if that makes sense.... the digestive system tube is surrounded by protective mechanisms acting as gatekeepers.