Low fasting C-peptide indicates reduced beta-cell secretory capacity.

Fasting C-peptide serves as a highly reliable clinical proxy for endogenous insulin production, providing critical insights into the secretory capacity of pancreatic beta cells.

Clinical evidence for beta-cell function

C-peptide is a standard biomarker used to quantify the remaining function of pancreatic beta cells. Because C-peptide is co-secreted with insulin in a 1:1 molar ratio but avoids the extensive "first-pass" metabolism by the liver that insulin undergoes, it provides a more stable and accurate measurement of systemic insulin output.

• Secretion Ratios: For every molecule of insulin produced, one molecule of C-peptide is released into the bloodstream, making it a direct reflection of total secretory activity.
• Predictive Thresholds: In clinical settings, fasting C-peptide levels below 0.6 ng/mL (0.2 nmol/L) are generally indicative of severe beta-cell failure, often necessitating exogenous insulin therapy.
• Correlation with Disease Progression: Studies have shown that a decline in C-peptide area under the curve (AUC) during mixed-meal tolerance tests correlates strongly with the loss of functional beta-cell mass in both Type 1 and advanced Type 2 diabetes.

Mechanistic explanations

The biological basis for using C-peptide lies in the processing of proinsulin within the beta-cell secretory granules.

• Proinsulin Cleavage: Inside the beta cell, proinsulin is cleaved by endopeptidases into one molecule of insulin and one molecule of C-peptide.
• Pharmacokinetics: Insulin has a short half-life of approximately 3–5 minutes due to rapid hepatic clearance. In contrast, C-peptide has a longer half-life of 20–30 minutes and is primarily cleared by the kidneys, resulting in plasma concentrations that are 5–10 times higher than insulin and less prone to rapid fluctuations.
• Dysfunction Indicators: Beta-cell dysfunction occurs when the pancreatic islets can no longer synthesize or release adequate insulin to maintain glucose homeostasis. Low fasting C-peptide levels quantitatively represent this reduction in the biosynthetic and secretory machinery of the pancreas.

Bottom line

Fasting C-peptide is an accurate and essential marker for assessing endogenous insulin production; low levels consistently indicate reduced beta-cell secretory capacity and dysfunction.