Why is C-peptide a better measure of insulin secretion?
This is because of the following reasons:
a) The amount of C-peptide that is released is equal to the amount of insulin release
b) C-peptide is NOT extracted by the liver, whereas the majority of endogenous insulin is extracted by the liver (remember insulin is released into the portal circulation)
c) C-peptide clearance occurs via the kidney - but this is constant. That means at any given point in time, the amount cleared by the kidney is constant to the amount which is being produced
d) Exogenous insulin does not impact the measured of c-peptide; hence c-peptide is a measure of endogenous insulin production