ASSOCIATION BETWEEN STIMULATED PLASMA C-PEPTIDE AND AGE - THE WADENA-CITY-HEALTH-STUDY

Objective: To assess age-related changes in stimulated plasma C-peptide in a population-based sample of adults. Design: Cross-sectional study. Setting: Wadena, Minnesota, a city of 4,699 residents (1980 census) in west central Minnesota, approximately 150 miles from Minneapolis/St. Paul. Study Subjects: 344 non-diabetic subjects (NDDG standards) from a stratified random sample of the total adult population of Wadena, MN. The six-study strata were men and women from three age groups: young, 20-39 years of age; middle-aged, 40-59; and older, > 60 years of age. Measurements: During a liquid meal of Ensure-Plus (Ensure-Plus(R) challenge test; EPCT; Ross Laboratories), blood samples were taken for glucose, free fatty acids, creatinine, and C-peptide. Plasma C-peptide taken 90 minutes after the EPCT was used as a surrogate measure for insulin. Clinical tests included one-time samples for hemoglobin, glycosylated hemoglobin, plasma cholestercol, triglycerides, and lipoproteins. Physical measurements included height, weight, and blood pressure. Urine was assayed for C-peptide and creatinine. Assays of urine and plasma C-peptide used antibody M1221 (from Novo; Copenhagen, Denmark). Main Results: No differences were observed for the relationship between age and C-peptide within each of the three age groups for men and the three age groups for women. However, the levels of plasma C-peptide for older men or women were statistically significantly higher than levels for the young age groups of the same sex; fasting plasma glucose also was higher for older groups of both sexes, and postmeal glucose was significantly higher for older women. There were decreases with age in urine C-peptide clearance for women and men; the decline for women was statistically significant. In multiple regression models for men alone and women alone, that controlled for age, post-meal plasma glucose best explained plasma C-peptide levels, For young men, plasma glucose alone provided the best prediction of plasma C-peptide levels; body mass index (BMI) and plasma glucose provided the best prediction for young women. For older men and both middle-aged and older women, a combination of urine C-peptide clearance and plasma glucose best predicted plasma C-peptide levels; for middle-aged men, BMI also contributed to the prediction. Conclusions: Secretion of insulin in response to an orally administered mixed meal is undiminished with age in non-diabetic adults.