1. Oral glucose tolerance test (OGTT): In the case of OGTT, subjects will be given a ready-to-use test solution (TRUTOL® 75, Thermo Scientific, USA) containing 75 g glucose dissolved in 300 mL water, immediately after fasting blood sampling. They will be instructed to drink the test solution within 5 mins. Blood samples will be drawn at 5, 10, 15, 30, 60, 90, and 120 minutes after drinking the ready-to-use test solution. Gastric emptying will be measured by acetaminophen absorption test.