Preoperative fasting is used to reduce the risk of pulmonary aspiration. From the ASA guidelines on preoperative fasting, meta-analysis of randomized controlled trials demonstrated in adult patients given clear liquids 2-4 hours before a procedure versus 4 hours had smaller gastric volumes and higher pH values. In children who received clear liquids 2-4 hours prior to a procedure versus fasting for >4 hours , meta-analysis of randomized controlled trials demonstrated only higher gastric pH values, but was equivocal regarding gastric volume.
In contrast, observational studies with ingestion of breast milk and infant formula 4 hours prior to a procedure were equivocal regarding higher gastric volumes or lower gastric pH.
American Society of Anesthesiologists Committee Practice guidelines for preoperative fasting and the use of pharmacologic agents to reduce the risk of pulmonary aspiration: application to healthy patients undergoing elective procedures: an updated report by the American Society of Anesthesiologists Committee on Standards and Practice Parameters. Anesthesiology: 2011, 114(3);495-511