C-peptide

C-peptide is a 31-amino-acid chain. It is cut out of proinsulin inside your pancreatic beta cells. One C-peptide is released for every insulin molecule. So your fasting C-peptide directly reflects how much insulin you make yourself. The liver barely clears it, so it lasts longer than insulin (a half-life of about 30 minutes versus 3 to 5 for insulin). That makes it measurable even when you take insulin as a drug. Fasting C-peptide plays two roles. As a gauge of beta-cell reserve, low values flag lost insulin output, as in type 1 and late-stage type 2 diabetes. As a sign of chronic high insulin, high values point to insulin resistance and over-secretion. An NHANES cohort (Patel et al. 2012; 5,153 non-diabetic adults) found fasting C-peptide beat fasting insulin and resistance indexes at predicting heart and all-cause death. A meta-analysis of 23 studies (Ahmadirad et al. 2023) confirmed a 22% higher all-cause death risk (HR 1.22; 95% CI 1.12 to 1.32) and 38% higher heart death (HR 1.38; 95% CI 1.08 to 1.77). In type 1 diabetes, some leftover secretion is protective. In insulin-resistant people, a high value signals metabolic strain. You read it in clinical context (fasting glucose, HbA1c, diabetes status). It tells you most alongside fasting insulin and glucose.

Last reviewed: June 22, 2026

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