Gastric Volume After Ingestion of Carbohydrate Drink vs Water in Obese Volunteers

* Group C: will receive a carbohydrate drink of 400 ml (50 g of glucose prepared by nutrition division, same appearance and containing bottles of water) * Group W: will receive water 400 ml (prepared by nutrition division, Songklanagarind hospital, same appearance and containing bottles of carbohydrate drink) In both groups, the received 400-mL drink was consumed within 5 minutes. * Before drinking, baseline data (gastric residual volume, POCT glucose, degree of hunger, and thirst) will be evaluated. * Gastric ultrasound will be performed by the same Radiologist on each participant at 0, 30, 60, 90, 120 minutes * after drinking to evaluate the gastric residual volume and time to empty gastric antrum. * POCT glucose and degree of hunger and thirst are also assessed again at 120 minutes after drinking. * After completing one type of drinking, participants will be appointed again after 1 week later for doing the same protocol with another drinking.