Among type 1 diabetics, the ability to make even a tiny amount of insulin is strongly associated with the ability to get lower A1cs without more severe hypoglycemia.
A majority of Joslin medalists- people who have been on insulin for more than 50 years- are still making a detectable amount of insulin.
Among adolescents who still have measurable levels of c-peptide (and thus, insulin production) one month after diagnosis, more than 90% still have measurable levels of cpeptide two years later.
C-peptide levels at diagnosis of type 1 diabetes are usually younger in younger people.
Decline in c-peptide levels is those type 1 diabetics who are positive for c-peptide is usually most rapid in the first year after diagnosis.
Even among type 2 diabetics, lower c-peptide levels are associated with higher rates of some complications, such as kidney disease.
Some medications used to treat type 2 diabetes (especially the sulfanureas) lower cpeptide.