Decreased plasma C-peptide to insulin molar ratio after oral glucose in elderly subjects

To evaluate the effect of aging on hepatic extraction of insulin, peripheral plasma insulin and C-peptide levels after an oral glucose ingestion were measured in 39 non-obese healthy subjects with normal glucose tolerance test. The subjects were divided into two groups. One age group was below 30 years (n = 17) and another was above 50 years (n = 22). Blood glucose level was significantly higher in elderly subjects than in young at 30 and 60 min, and insulin level was also significantly higher in elderly subjects than in young subjects at 60 min after glucose ingestion. C-peptide response after glucose ingestion was similar in both groups. The molar ratio of C-peptide to insulin after glucose ingestion in elderly subjects was slightly lower than that in young subjects, and the difference between the incremental areas of C-peptide and insulin divided by the incremental area of C-peptide were significantly lower in elderly subjects. These results suggest that hepatic insulin extraction after glucose loading is decreased in elderly subjects and this may partly be responsible for the slightly altered glucose metabolism in elderly subjects with normal glucose tolerance test.