C-Peptide
The optimal range for C-Peptide (C-Peptide) is commonly cited as 0.9–2.5 ng/mL (fasting) in metabolically healthy adults, tighter than the standard lab reference range of 0.8–3.5 ng/mL (fasting). Endogenous insulin production marker - cleaner than fasting insulin for measuring what your pancreas is actually making.
How C-Peptide ranges relate
The standard lab range vs the optimization-focused target. Illustrative trajectory shows what a 12-week improvement pattern looks like — not real user data.
What C-Peptide Measures
C-peptide is cleaved from proinsulin in a 1:1 ratio with insulin, but unlike insulin it is not cleared by the liver on first-pass. This makes it a more stable and honest reflection of pancreatic beta-cell output. In patients on exogenous insulin or GLP-1s, C-peptide shows endogenous production separately from exogenous hormone.
It is the gold standard for distinguishing T1D (low C-peptide) from T2D (normal-to-high C-peptide, reflecting insulin resistance).
What Affects This Biomarker
C-peptide is influenced by: beta-cell mass and function, insulin resistance (higher output compensating), post-prandial stimulation (rises after meals), renal function (accumulates in CKD), and GLP-1 therapy (raises glucose-dependent insulin secretion during meals, usually lowers fasting values over time via improved insulin sensitivity).
In the Context of Peptide Protocols
Order when fasting insulin is equivocal, to confirm endogenous production on GLP-1 therapy, or to distinguish early autoimmune diabetes from insulin resistance. Not a routine repeat - baseline and then again if clinical picture changes.
Peptides That Commonly Move C-Peptide
Frequently Asked Questions
What does C-Peptide measure?
C-peptide is cleaved from proinsulin in a 1:1 ratio with insulin, but unlike insulin it is not cleared by the liver on first-pass. This makes it a more stable and honest reflection of pancreatic beta-cell output. In patients on exogenous insulin or GLP-1s, C-peptide shows endogenous production separately from exogenous hormone.
What is the optimal range for C-Peptide?
The optimal range for C-Peptide is commonly cited as 0.9–2.5 ng/mL (fasting) in metabolically healthy adults. The standard lab reference range is 0.8–3.5 ng/mL (fasting).
What affects C-Peptide levels?
C-peptide is influenced by: beta-cell mass and function, insulin resistance (higher output compensating), post-prandial stimulation (rises after meals), renal function (accumulates in CKD), and GLP-1 therapy (raises glucose-dependent insulin secretion during meals, usually lowers fasting values over time via improved insulin sensitivity).
Where This Fits in Your Panel
C-Peptide is one marker on a fuller panel. The Peptide Blood Work Checklist lays out the complete baseline panel, what to add by protocol type, and when to retest.
See the full blood work checklist →Upload any lab PDF and MyProtocolStack maps your values to C-Peptide and 40+ other biomarkers. StackAI interprets the trend in context of your protocol.
Start tracking →Last reviewed: June 2026
Informational only - not medical advice. Reference ranges vary by lab and individual context. Work with a licensed provider to interpret your specific results.