C-peptide is the 31-amino-acid peptide cut out of proinsulin as insulin is processed. It is released from the pancreatic beta cell alongside insulin in roughly equimolar amounts, and for most of the last fifty years it was treated as an inactive by-product of insulin biosynthesis. Measuring it is routine clinical practice: the C-peptide assay is an established way to estimate endogenous beta-cell function and to distinguish insulin the body has made from insulin that has been injected.
Whether the peptide itself is a therapeutic candidate is a separate and much more contested question. The research literature reports signals in diabetic peripheral neuropathy and in diabetic kidney disease, drawn from a small set of short, older trials conducted almost entirely in type 1 diabetes. This article sets out what those trials found, where they fell short, and what the UK position is. Nothing in it is guidance for human use.
What C-peptide is
Human C-peptide is a 31-residue peptide that connects the A and B chains of proinsulin. Prohormone convertases cleave it out during insulin maturation, and the beta cell then co-secretes insulin and C-peptide together. Because C-peptide is not extracted by the liver to anything like the degree insulin is, circulating C-peptide is a comparatively stable index of beta-cell output — which is why the assay is used clinically to separate endogenous insulin secretion from injected insulin.
That diagnostic role is well established and uncontroversial. The contested claim is that C-peptide is itself bioactive: that its deficiency in type 1 diabetes contributes to complications, and that replacing it might alter their course.
The mechanistic case
The mechanistic literature has moved C-peptide from 'inert' towards 'weakly signalling' over three decades. In vitro and animal work reports that it binds cell-membrane surface signalling molecules and activates downstream pathways, with anti-oxidative, anti-apoptotic and inflammation-regulating effects. Endothelial studies report reduced reactive oxygen species through restoration of mitochondrial electron-transport-chain activity and reduced NADPH oxidase activity; at least one line reports AMPK-alpha activation. In nerve tissue, C-peptide has been reported to stimulate Na/K-ATPase and endothelial nitric oxide synthase — the proposed route to improved nerve blood flow.
Two caveats belong here. Most of this work is preclinical, and the receptor or receptors through which C-peptide acts have not been settled. A plausible mechanism is not the same thing as a confirmed one.
Diabetic peripheral neuropathy: the human trials
The human record is small and consists largely of short trials in type 1 diabetes.
A double-blind, placebo-controlled study of 46 people with type 1 diabetes — average age 29, about ten years of diabetes duration, reduced sensory and motor nerve conduction velocities but no other neuropathy signs — gave C-peptide at 1.8 mg per day or placebo for three months alongside each participant's usual insulin. Sensory nerve conduction velocity and vibrotactile sensation improved. Temperature and motor nerve conduction velocity did not. The dose is reported here as a trial parameter; it is not guidance of any kind.
That work was extended into a six-month randomised, double-blind, placebo-controlled multicentre study of 161 people with type 1 diabetes and confirmed distal symmetrical polyneuropathy (mean age 44; mean diabetes duration 29 years; baseline sural sensory conduction velocity about 2.6 standard deviations below normal). After six months of C-peptide replacement, the C-peptide group showed a statistically significant improvement in sural sensory nerve conduction velocity: 0.48 +/- 0.19 m/s assessed as peak velocities, and 0.93 +/- 0.29 m/s on the study's second velocity measure.
A smaller autonomic study examined heart-rate variability during deep breathing, a reproducible index of vagal function. Baseline variability was reduced to 13 +/- 1% against a normal reference of 24%. During a three-hour C-peptide infusion that restored plasma levels to the physiological range, variability improved to 20 +/- 2%; no change was seen with saline infusion (p < 0.001).
The largest neuropathy study is also the least supportive of a clean effect. A 2016 trial in 250 people with diabetic peripheral neuropathy gave long-acting C-peptide weekly for 52 weeks. Vibration perception threshold improved significantly, but sural nerve conduction velocity, other electrophysiological variables and the modified Toronto Clinical Neuropathy Score did not differ significantly from placebo. The doses used raised plasma C-peptide well above the physiological range — approximately 1.8-2.2 nmol/L at the lower dose and 5.6-6.8 nmol/L at the higher — a difference the review authors single out as worth weighing.
A six-month double-blind, placebo-controlled, dose-finding multicentre trial registered as NCT00278980 tested C-peptide in type 1 diabetes with peripheral sensory neuropathy, with change in sensory nerve conduction velocity as the primary outcome.
Diabetic kidney disease
The renal literature follows a similar pattern. In diabetic rodents, C-peptide reduces glomerular hyperfiltration and albuminuria. Cohort studies of people who received combined islet and kidney transplants suggest that maintained C-peptide secretion is associated with better renal graft function. Short-term studies in type 1 diabetes report a lowered hyperfiltration rate and reduced microalbuminuria.
A 2017 review of C-peptide and diabetic kidney disease concluded that the available information justifies clinical trials in diabetic nephropathy and described them as urgently required — which is itself an acknowledgement that the definitive trial has not been run. Other reviews are blunter: because the studies are small, short and lack large-scale randomised data, the effect on estimated glomerular filtration rate remains contentious and the evidence base for replacement therapy stays limited.
Where the evidence sits
Read together, the C-peptide literature is better than anecdotal and short of strong. It contains randomised, placebo-controlled human trials with a broadly reproducible direction of effect on nerve conduction measures, but the studies are small, decades old and inconsistent: the largest neuropathy trial met one endpoint and missed the others, and no adequately powered modern trial has been run. On Peptide Data's four-tier scale, the human evidence for C-peptide in diabetic complications sits at Limited — a real human signal beside a small, mixed and unreplicated body of trial data. The 'dual hormone deficiency' framing of type 1 diabetes remains a hypothesis, not a settled finding.
The UK position
No C-peptide product holds a UK marketing authorisation, and C-peptide is not a licensed medicine in the UK. Its established clinical use is as a laboratory analyte — a diagnostic measurement — which is a different thing from a therapy.
Under the Medicines Act 1968 and the Human Medicines Regulations 2012, a product intended for human use that is presented as treating, preventing or modifying a physiological function is likely to be a medicine and needs a marketing authorisation from the MHRA before it can be supplied. The MHRA decides that boundary case by case, and it has said plainly that it disregards claims that a product is for 'research purposes' where those claims are being used to avoid medicines regulation. A research-use-only label does not convert a product carrying a medicinal claim into something outside the legislation.
That matters here because C-peptide is sold into the grey market on the strength of the same neuropathy and kidney studies summarised above. Buying a research chemical carries no assurance of identity, purity or sterility, and no supplier can lawfully market C-peptide for human use in the UK without authorisation.
Research use only
C-peptide is a research compound. This article describes the published research record and the UK legal position; it is not consumption advice, it contains no dosing guidance, and nothing in it should be read as a recommendation to use C-peptide or any other peptide. Describing what a trial administered is not the same as recommending it: the amounts quoted above are trial parameters reported in the literature, and they are reported for accuracy, not for use.
This article is AI-researched and editorially reviewed. It is provided for research and educational purposes only and is not medical advice. Research peptides are not licensed for human consumption in the UK.