The relationship between plasma C-peptide and the frequency and severity of diabetic retinopathy was examined in a population-based study in Wisconsin in 1984–1986. Individuals with younger- (n = 835) and older- (n = 940) onset diabetes were included. C-peptide was measured by radioimmunoassay with Heding's M1230 antiserum. Retinopathy was determined from stereoscopic fundus photographs. The highest frequencies and most severe retinopathy were found in insulin-using individuals with undetectable or low plasma C-peptide (<0.3 nM), whereas the lowest frequencies of retinopathy were found in older-onset overweight individuals not using insulin. In olderonset individuals using insulin, having no detectable C-peptide was significantly associated with the presence of proliferative retinopathy. Otherwise, within each group (younger onset using insulin, older onset using insulin, and older onset not using insulin), after controlling for other characteristics associated with retinopathy, there was no relationship between higher levels of C-peptide and lower frequency of or less severe retinopathy.

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