C-peptide was discovered in 1967. The first actual use of a C-peptide test was in 1972. In more recent times, C-peptide has been found to have an effect on microvascular blood flow and tissue health.
Type I diabetics typically have very low levels of C-peptide because most of their beta cells have been destroyed. This has been seen in the development of long-term complications such as peripheral and autonomic neuropathy. I have experienced both kinds of neuropathies, but neither type has proved to be particularly problematic after 66 years of type 1.
C-peptide has been shown to significantly improve nerve and kidney function. It has also been reported to have anti-inflammatory effects as well as aid repair of smooth muscle cells.
"Several physiological effects have been observed in several Phase 1 and exploratory Phase 2 studies in almost 300 type 1 diabetes patients, who lacked endogenous C-peptide. Improvements were seen on diabetic peripheral neuropathy, nephropathy and other decrements associated with long-term complications of type I diabetes. So far, dosing with C-peptide has shown to be safe and there were no effects of C-peptide demonstrated in healthy subjects (who make their own C-peptide)."
"Newly diagnosed diabetes patients often get their C-peptide levels measured as a means of distinguishing between type 1 diabetes, Maturity Onset Diabetes of the Young (MODY), Latent Autoimmune Diabetes of Adults (LADA) and type 2 diabetes."
The pancreas of a type 1 diabetic does not usually produce adequate insulin, so a decreased level of C-peptide is expected. Some secretion can still be present during the honeymoon stage for type 1 diabetics. C-peptide levels in type 2 diabetics are normal, or even higher than normal. Determining the amount of C-peptide in people who inject or pump synthetic insulin "can help to determine how much of their own natural insulin these patients are still producing, or if they produce any at all."
The results of a C-Peptide test can vary from lab to lab. So different labs may have different "normal" ranges. Doctors will sometimes evaluate your results based on your health and other factors. A C-Peptide value that falls outside the normal range may still be normal for you. The C-Peptide and blood glucose levels are measured at the same time to give a better evaluation.
A normal C-peptide level may be indicated as – Fasting: 0.51-2.72 nanograms per milliliter (ng/mL) or 0.17-0.90 nanomoles per liter (nmol/L)
The 725+ participants in the Joslin Medalist Study were tested for their C-Peptide levels. That test along with a glucose tolerance test showed that more than 66% of these long term diabetics do still produce some of their own insulin. My C-Peptide level was less than 0.1, showing that my good health after 66 years of diabetes has to be explained by factors other than insulin secretion.
"Persons with LADA typically have low, although sometimes moderate, levels of C-peptide as the disease progresses and high blood glucose levels.
The most common MODY syndrome may also have normal fasting C-peptide results because the flaw in this case is in the secretion of insulin in response to rising glucose and fasting secretion is still near normal. Their postprandial C-peptide however is below normal with elevated blood glucose.
Low levels of both C-peptide and blood glucose are found in liver disease, a severe infection, Addison’s disease, or insulin therapy."
I have believed for several years that C-Peptide present in the animal insulins I used for 50 years may have protected me against certain complications. It may help explain the longevity and good health of all the Joslin Medalists. Today's synthetic insulins do not contain any C-Peptide. I have experienced several minor complications since starting synthetic insulins, but none of them have been particularly problematic. I asked Dr. King, head of the Joslin Medalist Study, about this. He said he does not think that C-Peptide is a factor in explaining the longevity and good health of the medalists. Maybe he will have changed his mind when the study concludes.
The link below is my reference. It discusses C-Peptide in much more detail.
There is a company that has produced an injectable form of C-Peptide. Stage 2 trials have been held, and human subjects have been helped by these injections.