The American Society of Anesthesiologists has very clear guidelines for deciding when patients should stop any oral intake before surgery. These guidelines were made to prevent patients from aspirating gastric contents upon induction of anesthesia. For years, the rules were no solid foods up until six hours before surgery and no clear liquids two hours before the procedure.
Granted there have always been complaints from patients about how miserable they feel by the time they get to preop from having to fast for such a long period of time. It's especially bad for patients whose procedures take place in the afternoon and were given instructions to be NPO after midnight. Since this is by no means settled science, there is ongoing research to help remedy the situation.
Some have advocated letting patients drink carbohydrate rich fluids to help with the hypoglycemia that makes the patients feel so deprived. However, they still say nothing by mouth for two hours before surgery. Now the British have done something even more bold than Brexit--all you can drink right up to going into the operating room itself.
In a followup to a study first published in the European Journal of Anesthesiology in 2017, researchers in the UK allowed patients to drink as much as they want immediately before surgery. Covering over 30,000 patients, the study showed that the rates of postop nausea was lower for patients who had unrestricted fluid intake, 3.8% compared to 5.2% for patients who could take clear liquids up to two hours before. Postop vomiting was also lower, 2.2% to 2.8%.
The $64,000 questions is what concerns anesthesiologists the most. What are the aspiration risks with unlimited po fluid intake before anesthesia? Surprisingly, they were very good. Only two patients suffered aspiration of gastric contents. Both patients had risk factors including BMI>35 and history of gastric reflux. That compares to a normal aspiration risk of 1 in 8,000 anesthesia patients.
The researchers speculate that patients who were allowed unlimited drinking before surgery had less postop nausea and vomiting because hunger itself can cause nausea. Their conclusion is that the risks of gastric aspiration is low enough that it justifies patients being allowed to drink fluids to prevent the higher likelihood of PONV.
These numbers look very promising on paper. However, in the much more litigious U.S. legal system, I think I will continue to follow the ASA's guidelines instead. People normally go hours during the day without eating or drinking anything. They can certainly do that the day of their operations.