C-Peptide (Connecting Peptide)
Glucose Metabolism marker
C-Peptide
C-Peptide (Connecting Peptide)
Fragment cleaved from proinsulin and released into the blood in equal (equimolar) amounts with endogenous insulin. Reflects how much insulin the pancreatic beta cells are actually producing and, unlike injected insulin, is not present in pharmaceutical insulin.
PED Notes
Strong PED relevance. Because injected (exogenous) insulin contains no C-peptide, this test separates the body's own insulin output from injected insulin: a bodybuilder using exogenous insulin will show high blood insulin but low or suppressed C-peptide, whereas insulin resistance from GH or MK-677 drives high endogenous insulin AND high C-peptide. C-peptide is the better gauge of true beta-cell output and is more stable than insulin (longer half-life, no first-pass liver clearance). Use it alongside fasting insulin, glucose, and HOMA-IR when screening for the insulin resistance that accompanies growth hormone, MK-677, and high-calorie growth phases.
When high
When HIGH:
- High C-peptide means the pancreas is secreting a lot of its own insulin, usually because of insulin resistance. In enhanced athletes the common drivers are growth hormone, MK-677 (which raises GH/IGF-1 and blunts insulin sensitivity), large caloric surpluses, and high body fat
- High C-peptide with high glucose indicates the beta cells are working hard but not keeping up; this is the early picture of type 2 diabetes and warrants intervention
- Action: improve insulin sensitivity (lose visceral fat, reduce refined carbohydrate, add zone 2 and resistance training), reassess GH and MK-677 dosing, and pair with HbA1c and fasting insulin. Berberine, metformin (physician-prescribed), and adequate sleep all help
- Rarely, very high C-peptide with low glucose can indicate an insulin-secreting tumour (insulinoma); persistent fasting hypoglycaemia with high C-peptide needs medical work-up
When low
When LOW:
- Low C-peptide means low endogenous insulin output. Combined with high measured insulin and high glucose, this is the fingerprint of exogenous insulin use, since injected insulin suppresses the body's own secretion and contributes no C-peptide
- Low C-peptide with high glucose and no insulin use can indicate beta-cell failure (advanced type 2 or type 1 diabetes) and needs medical assessment
- Low C-peptide is expected and not concerning during a fast or when blood glucose is genuinely low
Clinical context:
- C-peptide is released 1:1 with insulin but cleared more slowly and not extracted by the liver on first pass, so it is a steadier marker of beta-cell function than a single insulin value
- Always interpret with a simultaneous glucose; a C-peptide value means little without knowing the glucose at the time of the draw
- Fasting samples are standard; reference ranges are assay dependent (fasting adult roughly 260-1730 pmol/L)
History Chart
Reading History
Frequently Asked Questions
Reference Ranges
Standard Range
VitalMetrics Range