C-Peptide

What it isWhen the pancreas makes insulin, it cleaves a precursor protein and releases insulin plus a "connecting peptide" in a one-to-one ratio. C-peptide is cleared slowly by the kidneys, giving a more stable readout of native pancreatic insulin output than insulin itself.

DoesActs as a fingerprint of the pancreas's actual insulin production, unaffected by injected insulin.

GoodMid-low C-peptide in someone with normal glucose suggests the pancreas isn't being driven hard — a hallmark of insulin sensitivity.

BadHigh C-peptide signals hyperinsulinemia and pancreatic strain, predicting diabetes, fatty liver, and cardiovascular disease. Very low C-peptide in a diabetic indicates type 1 / beta-cell failure.