The relationship between plasma C-peptide and the 6-year incidence and progression of diabetic retinopathy was examined in a population-based study in Wisconsin. Individuals with younger-onset (n = 548) and older-onset (n = 459) diabetes were included. C-peptide was measured by radioimmunoassay with Heeling's M1230 antiserum. Retinopathy was determined from stereoscopic fundus photographs. Younger- and older-onset insulinusing individuals with undetectable or low plasma Cpeptide (<0.3 nmol/L) at baseline had the highest incidence and rates of progression of retinopathy, whereas older-onset individuals with C-peptides >0.3 nmol/1 had the lowest incidence and rates of progression of retinopathy. However, within each group (youngeronset using insulin, older-onset using insulin, and olderonset not using insulin), after we controlled for other characteristics associated with retinopathy, there was no relationship between higher levels of C-peptide at baseline and lower 6-year incidence or progression of retinopathy. These data suggest that glycemic control, and not C-peptide, is related to the incidence and progression of diabetic retinopathy.

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