In individuals with long-duration type 1 diabetes (T1D), the ability to secrete proinsulin persists, even in those with undetectable serum C-peptide, according to a recent study. C-peptide and proinsulin were measured in fasting and stimulated sera from 319 subjects with long-standing T1D (≥3 years) and 12 control subjects without diabetes. Researchers considered 3 categories of stimulated C-peptide: 1) C-peptide positive, with high stimulated values ≥0.2 nmol/L; 2) C-peptide positive, with low stimulated values ≥0.017 but <0.2 nmol/L; and 3) C-peptide <0.017 nmol/L. Longitudinal samples were analyzed from C-peptide–positive subjects with diabetes after 1, 2, and 4 years. They found:
- Of individuals with long-standing T1D, 95.9% had detectable serum proinsulin (>3.1 pmol/L), while 89.9% of participants with stimulated C-peptide values below the limit of detection (<0.017 nmol/L; n=99) had measurable proinsulin.
- Proinsulin levels remained stable over 4 years of follow-up, while C-peptide decreased slowly during longitudinal analysis.
- Correlations between proinsulin with C-peptide and mixed-meal stimulation of proinsulin were found only in subjects with high stimulated C-peptide values (≥0.2 nmol/L).
Sims EK, Bahnson HT, Nyalwidhe J, et al. Proinsulin secretion is persistent feature of type 1 diabetes. [Published online ahead of print December 10, 2018]. Diabetes Care. doi:10.2337/dc17-2625.