C-peptide

Mechanism

C-peptide is generated endogenously rather than synthesised against a defined receptor target. Proinsulin, the single-chain precursor, is processed by prohormone convertases PC1/3 and PC2 and carboxypeptidase E into mature insulin and the C-peptide fragment, which are packaged together in secretory granules and released on glucose stimulation. In humans the fragment is 31 amino acids long, and its sequence is conserved across several mammalian species — one reason animal findings have been carried into human studies.

Proposed mechanisms

Several mechanisms have been proposed for the effects reported in the research literature. These include a G-protein-coupled receptor-mediated signalling pathway, activation of Na+/K+-ATPase, effects on endothelial nitric oxide synthase, and anti-inflammatory and anti-apoptotic actions in nerve and kidney tissue. Much of this mechanistic work is preclinical, conducted in cell and animal models, and the receptor or receptors that would mediate a physiological effect in humans remain incompletely characterised. That gap — a plausible mechanism without a confirmed human target — is the central uncertainty in the C-peptide literature.

Evidence base

The human evidence for C-peptide is small, dated in places, and inconsistent between endpoints. An early short trial in type 1 diabetes reported improved sensory nerve conduction velocity and vibrotactile thresholds after three months, with no significant change in motor conduction or temperature perception. A subsequent randomised, double-blind multicentre trial of 161 adults with type 1 diabetes and a long disease duration reported a modest improvement in sural sensory nerve conduction velocity and an increase in a heart-rate-variability measure of autonomic function over six months.

A larger later trial of 250 people with diabetic peripheral neuropathy, using a long-acting formulation over 52 weeks, found an improvement in vibration perception threshold but no significant difference from placebo in sensory nerve conduction velocity, other electrophysiological measures, or the trial's composite symptom score. Plasma C-peptide concentrations in that study rose above the physiological range, which the authors noted as a limitation on how the result should be read.

For kidney endpoints, a 2017 review of the field collated animal models, transplant-cohort observations and short-term human signals, and concluded that the evidence was insufficient to support a clinical recommendation, calling for adequately powered trials. Reported effects on estimated glomerular filtration rate are contested between studies.

Taken together, the human record is best graded Limited: a real but small and non-replicating set of signals, without a large confirmatory trial. This profile states that grade plainly rather than implying a therapeutic effect.

UK legal status

C-peptide has no UK marketing authorisation as a medicine. It is not a controlled drug under the Misuse of Drugs Act 1971. Its regulatory position turns on the medicines boundary: under the Medicines Act 1968 and the Human Medicines Regulations 2012, a product is a medicine if it is presented for treating or preventing disease, or if it can be administered to restore, correct or modify physiological function. The MHRA assesses that boundary case by case, and it does not accept a 'research purposes' or 'not for human consumption' label as a way to place a product outside medicines regulation where the substance and its presentation indicate a medicinal use.

In a laboratory or diagnostic context, C-peptide is a legitimate analyte and reference material. Outside that context, supplying C-peptide for human use without a marketing authorisation would engage the unlicensed-medicines provisions. Nothing in this profile states that C-peptide is licensed, and nothing here is consumption or dosing guidance; it is a research reference.