 At time zero 0 min, a 50 glucose bolus will be injected during 1 min to increase PG to 15mM. The glucose bolus will be calculated as:15mM-FPG×35 mg glucose × body weight kg. PG will be measured bedside every 5 min and maintained at 15mM by an adjustable continuous 20 glucose infusion. After 90min, PG will be lowered down to 7mM for the islet cells to rest, then the subject will be instructed to consume 75g glucose solution orally in 5min. 40min after the oral glucose consumption, the 90min-hyperglycaemic clamp experiment wil be restarted. The oral period of hyperglycaemic clamp process is the same as whats done in fasting period. Blood samples will be collected in -2h, 0min, 10min, 90min in both hyperglycaemic clamp experimental process for the measurement of insulin, c-peptide, glucagon, active GLP-1 and other incretin hormones. 