C-Peptide: What Type of Type 1 Diabetes Do You Have?

Why C-peptide matters

Not all type 1 diabetes is the same biologically. The amount of residual beta-cell function a person retains after diagnosis — measured through C-peptide — shapes insulin requirements, glucose stability, and long-term complication risk in ways that standard clinical review rarely addresses.

John shares his own experience: diagnosed at 27, he retains 220 pmol/L of C-peptide after ten years and uses only 30 units of insulin per day. He reflects openly on the bias and privilege this biological difference confers, and on what it means for people who face greater biological burdens.

What is C-peptide?

When the pancreas makes insulin, it produces a molecule called pro-insulin, which splits into insulin and C-peptide in equal amounts. C-peptide circulates in the bloodstream and can be measured in blood or urine. The urinary C-peptide to creatinine ratio is now the most practical test — it requires no fasting, can be done in clinic, and gives a reliable picture of residual beta-cell output.

How C-peptide levels are classified

C-peptide values in type 1 diabetes are typically grouped as follows:

• High: above 200 pmol/L
• Intermediate: 30 to 200 pmol/L
• Low: below 30 pmol/L

For context, the non-diabetic range is approximately 1,000 to 3,000 pmol/L. Even the “high” category in type 1 diabetes represents a fraction of normal beta-cell output.

Who tends to have what?

People diagnosed under the age of 20 tend to have very low residual function after 15 or more years. Nearly all children diagnosed young lose almost all residual insulin production over time. Adults diagnosed later in life are more likely to retain meaningful C-peptide for longer.

What the evidence shows

DCCT and long-term outcomes

In the landmark DCCT and follow-up EDIC studies, higher C-peptide was associated with better HbA1c, lower fasting glucose, lower insulin dose, and fewer complications. The relationship between residual insulin production and complication risk follows a dose-response pattern: more C-peptide, proportionally lower risk.

Time in range

A Newcastle study examining C-peptide and time in range found clear differences across all contexts:

• Overnight TIR: 76% in people with high C-peptide versus 50% with low
• Post-meal TIR: 68% versus 50%
• Post-exercise TIR: 73% versus 40%

The post-exercise difference is particularly striking. The absence of residual insulin function removes a key physiological buffer during and after activity.

Why residual function helps

Two mechanisms account for much of the benefit. First, portal insulin delivery: endogenous insulin released from beta cells enters the portal vein and reaches the liver directly, clearing glucose more efficiently than subcutaneous insulin. Second, glucagon suppression: functioning beta cells help suppress glucagon after meals, reducing the liver’s glucose output and blunting post-meal spikes. When these mechanisms are absent, the challenge of glucose management rises substantially.

Clinical and practical implications

Using C-peptide to prioritise technology

In resource-limited settings, John’s team uses C-peptide to prioritise access to hybrid closed-loop systems, GLP-1 therapies, and amylin analogues for the people most likely to benefit.

Case study: an athlete with low C-peptide

John describes a young triathlete with C-peptide below 10 pmol/L. Without residual function, exercise management is substantially harder. An adapted strategy was built around lower carbohydrate intake before activity, higher intake afterwards, careful CGM use, and prioritisation for hybrid closed-loop technology.

Therapy considerations

GLP-1 therapies may be especially worth exploring for people with low C-peptide, given the absence of endogenous glucagon suppression. Future options including amylin analogues, SGLT-2 inhibitors, and pioglitazone are discussed in the GNL insulin resistance guide. All pharmacological decisions require individual clinical input.

Empathy and realistic targets

John’s central message is that type 1 diabetes is not the same for everyone. Low C-peptide means a greater biological burden — and that burden deserves recognition, not judgment. Standard targets such as 70% time in range may not be a fair or meaningful benchmark for people with very low C-peptide.

For healthcare professionals and people with diabetes alike, understanding C-peptide shifts the conversation from “why isn’t this working” to “what are we actually working with.”