A Pilot Study to Evaluate the Glucagon-Like Peptide 1 Response to the Challenge of Mixed Meals in Chinese Subjects

Glucagon-like peptide 1 (GLP-1) is a 30-amino acid peptide produced in the intestinal epithelial endocrine L-cells. It stimulates insulin and suppresses glucagons secretion, inhibits gastric emptying, and reduces appetite and food intake. In the fasting state, the plasma concentrations of GLP-1 are very low. Levels of circulating GLP-1 rise rapidly after food intake. The GLP-1 meal response depends on ingested nutrients. Carbohydrates are strong stimuli of GLP-1 release. Protein stimulates GLP-1 release, even more than carbohydrates. GLP-1 concentrations also increase after intake of fat, although the elevation is delayed compared to the stimulation of carbohydrates. Dietary fibers may also modify the postprandial GLP-1 response. Therefore, it is essential for us to characterize nutrient compositions of test meals for further studies of meal response of GLP-1.

Other sampling conditions of the meal tests are also needed to be taking care of. GLP-1 is rapidly degraded by the enzyme dipeptidyl peptidase IV (DPP-4). During the meal tests, it is important adding enzyme inhibitors into sampling tubes to avoid hormone degradation. Lugari et al. collected blood samples into tubes containing EDTA and aprotinin during a meal test. They could obtain samples from the same tube for both glucagons and GLP-1 assays. Other study groups reported that samples for GLP-1 needed to be collected into Vacutainer tubes prepared with EDTA and DPP-4 inhibitor for preventing degradation of GLP-1. Another aim of the present study is to compare the differences in GLP-1 concentrations in samples collected by EDTA tubes containing aprotinin or a DPP-4 inhibitor during the meal tests.

Racial differences in GLP-1 levels during oral glucose challenge have been demonstrated recently that severely obese African Americans exhibited lower GLP-1 concentrations than Caucasians. It is worthwhile to examine the meal response of GLP-1 across ethnicities. The current study proposes to observe the GLP-1 response during standardized meal tests in a group of Chinese subjects.

The purpose of the study is to standardize a meal test to facilitate future studies of the postprandial GLP-1 response in Chinese subjects. Eighteen healthy Chinese subjects, aged between 20-65 years old, without history of diabetes, will be recruited for the study. Each subject will receive two mixed meal tests for postprandial GLP-1 excursion at random order: 60 % carbohydrate (CHO)/20 % fat vs. 50 % CHO/30 % fat. The postprandial glucose, insulin, glucagon and other related hormones responses will also be measured. Through the study, we hope to build up a platform for the study of the postprandial GLP-1 response and insulin secretion.