Longitudinal Changes in Fasting and Glucose-Stimulated GLP-1 and GIP in Healthy Older Subjects

Context: It is not known whether glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) levels correlate within individuals, nor whether levels change with age. Previous studies have all been cross-sectional in design. Objective: To evaluate longitudinal changes in fasting and glucose-stimulated incretin hormone concentrations in healthy older subjects. Patients and Design: Forty-one healthy older subjects had measurements of plasma GLP-1 and GIP while fasting and after a 75-g oral glucose load on two occasions separated by 5.9 +/- 0.1 years [mean age at the initial study: 71.2 +/- 3.8 (SD) years]. Breath samples were collected to calculate the gastric 50% emptying time (T50). Results: For GLP-1, both fasting concentrations (P < 0.001) and area under the curve 0 to 120 minutes (P = 0.001) were decreased at followup. Fasting GIP was also lower (P = 0.03) at follow up, but there was no change in the area under the curve 0 to 120 minutes (P = 0.26). The gastric emptying T50 was slower at followup (P = 0.008). Neither the change in T50 nor the body mass index at the initial study was a determinant of the change in incretin responses. Between the two study days, fasting GIP (r=0.72, P<0.001) correlated well, but not fasting GLP-1 (r=0.23, P=0.18). However, both glucose-stimulated GLP-1 (r = 0.50, P = 0.002) and GIP (r = 0.60, P < 0.001) showed correlations between the initial and follow-up studies. Conclusions: Fasting GIP and glucose-stimulated GLP-1 and GIP concentrations correlate within individuals over a follow-up period of similar to 5.9 years. Aging is associated with reductions in fasting GLP1 and GIP, and glucose-stimulated GLP-1, which may predispose to the development of glucose intolerance and type 2 diabetes.