Association between stimulated plasma C-peptide and age: the Wadena City Health Study.

French L R, Goetz F C, Martinez A M, Boen J R, Bushhouse S A, Sprafka J M

Chronic Disease and Environmental Epidemiology Section, Minnesota Department of Health.

J Am Geriatr Soc. 1992 Apr;40(4):309-15. doi: 10.1111/j.1532-5415.1992.tb02127.x.

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, greater than 60 years of age.

MEASUREMENTS

During a liquid meal of Ensure-Plus (Ensure-Plus 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 cholesterol, 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.