C-Peptide

C-peptide reflects how much insulin the pancreas is making because it is released when proinsulin is split into insulin and C-peptide. Measuring C-peptide assesses beta-cell function and endogenous insulin production, helps distinguish type 1 (low) from type 2 diabetes (normal/high), detects insulinoma or factitious insulin use (low with injected insulin), and guides treatment and prognosis.

C-peptide is often normal or elevated in early type 2 diabetes because insulin resistance drives higher endogenous insulin secretion. Over time, progressive beta-cell dysfunction can reduce C-peptide levels, so advanced or long-standing type 2 disease may show low or falling C-peptide. Measurement helps distinguish insulin deficiency from insulin resistance and guides treatment decisions.

In type 1 diabetes, C‑peptide is typically low or undetectable because autoimmune destruction of pancreatic beta cells reduces endogenous insulin production. Soon after onset some residual C‑peptide may be present, but levels usually decline over time. Measuring fasting or stimulated C‑peptide helps distinguish type 1 from type 2 diabetes and assess remaining beta‑cell function.

Foods that raise C‑peptide (i.e., stimulate insulin secretion) are primarily carbohydrate‑rich, especially high‑glycemic/refined carbs and sugary drinks (white bread, sweets, white rice). Protein‑rich foods—dairy, eggs, meat, and fish—can also boost insulin because certain amino acids (arginine, leucine) stimulate secretion. Mixed high‑carb/protein meals provoke larger C‑peptide responses than low‑GI, high‑fiber or plant‑based meals, which blunt spikes.

High C‑peptide indicates increased endogenous insulin production and can be seen with insulinoma (insulin‑secreting tumor), type 2 diabetes and insulin resistance (obesity, metabolic syndrome), treatment with insulin secretagogues (sulfonylureas/meglitinides), conditions causing severe insulin resistance (Cushing’s, acromegaly, pregnancy), postprandial states, and reduced renal clearance (chronic kidney disease). Exogenous insulin does not raise C‑peptide.

C‑peptide is not a general tumor marker. It reflects endogenous insulin secretion and is used to distinguish endogenous from exogenous insulin, assess beta‑cell function, and help diagnose insulin‑secreting tumors (insulinomas) or factitious hypoglycemia. Elevated C‑peptide may suggest insulinoma but must be interpreted with glucose, insulin assays and imaging—it's a functional marker, not a broad cancer screening test.