Why Must Insulin Be Injected into Patients with Diabetes Mellitus

Why must insulin be injected into patients with diabetes mellitus type 1 as opposed to oral administration?

A) Insulin is such a small molecule that it easily passes through the highly vascularized tissue of the oral mucosa and enters the blood stream too quickly, causing a hypoglycemic crash.
B) Since the absorbed contents of the GI tract first passes through the liver before moving on into the general circulation, it creates a first- pass effect which weakens its effectiveness.
C) Insulin is a hormone, so it must be injected directly into the bloodstream to be effective.
D) Insulin is a peptide that would be degraded in the gastrointestinal tract and would therefore only be absorbed as individual amino acids.