The precursor to insulin produced by the pancreas’s beta cells is a peptide chain known as proinsulin. Made up of amino acids bound into a u-shape by a connecting polypeptide, proinsulin is stored in beta cells until a glucose load demands the release of insulin. At this point, the connecting molecule is broken off the bottom of the “u”-its shape earning it the moniker C-peptide-freeing the insulin molecule for secretion.
Although the process also means the release of trace amounts of proinsulin and other molecular debris for the body to dispose of, one constant always remains true: for every molecule of insulin the body releases into the blood, one C-peptide molecule is released as well.
It follows, therefore, that a measurement of the blood’s C-peptide level will reflect the amount of insulin in the bloodstream.