By Pei-shan Zhao, MD
To answer this question, let’s start with some basic knowledge. Fifty percent of the water we drink passes through the stomach in 10 minutes and the stomach is almost empty in one hour. Even caloric fluids like sugar water will all be passed through the stomach after 90 minutes. In contrast, approximately 50 percent of solid food is passed through the duodenum (the intestine immediately connected to the stomach) roughly two hours after a meal. It takes much longer for the stomach to empty solid food.
A lot of things affect gastric emptying. It takes longer for the body to digest fried and high caloric fatty food. Gastric emptying is slower in females and the elderly than in males. Gastric emptying times for solids are delayed in smokers, but not in nicotine patch users.
Recreational abuse of cannabinoids, opioid treatments and heavy consumption of alcohol also slow gastric emptying, while some systemic diseases, such as diabetes mellitus, slow down digestion.
For mature neonates and infants, gastric emptying of human milk takes more than two hours and less than five, respectively. Premature babies have a longer gastric emptying time, while cow’s milk passes through our system much more slowly than human milk does. Gastric emptying time for formulas vary with the content of the formula.
Obese patients have a similar gastric emptying time to nonobese patients. Pregnant women have a normal gastric emptying rate, except during labor, when gastric emptying slows down until at least two hours after delivery.
The reason I have talked so much about gastric emptying is that it is related to residual gastric volume. If the gastric emptying time is slowed, the residual gastric volume will increase, which in turn increases the risk of regurgitation of stomach acid into the throat under general anesthesia. This regurgitation then increases the risk of aspiration of stomach acid into the lungs, resulting in “aspiration pneumonia.” Aspiration pneumonia, although rarely, may cause patient death.
General anesthesia is always the backup plan, no matter what kind of anesthesia the patient will have for surgery. This is why anesthesia doctors tell patients not to eat or drink after midnight before surgery. In actuality, you may have clear liquids two hours before surgery because clear liquids are emptied from the stomach faster than solids. Clear liquids include water, clear tea or black coffee without milk and sugar, as well as fruit juices without pulp, such as apple juice, not orange juice. Although beers and hard liquors are clear, they are not included in “clear liquids” here. The American Society of Anesthesiologists (ASA) has published clinical guidelines for fasting before surgery:
Ingested Material Minimum Fasting Period
Clear liquids 2 h
Breast milk 4 h
Infant formula 6 h
Nonhuman milk 6 h
Light meal 6 h
These recommendations apply to healthy patients who are undergoing elective procedures. Regular meals normally have high calories. In order to secure emptying of solids, longer fasting is needed. Most anesthesiologists will ask patients to fast more than eight hours before surgery.
Sometimes, the patient decides not to follow these instructions. Chinese patients, for example, like to have noodles or congee the morning of surgery. Unfortunately for them, the doctors then have to cancel or postpone the surgery. So, for your own safety, please follow the doctor’s instruction and tell the doctor truthfully if you have had something to eat or drink before the surgery.
Pei-shan Zhao is an attending anesthesiologist and director of the Asian Community Outreach Program in the department of anesthesiology, Tufts Medical Center. He is a graduate of Peking University’s School of Medicine, and trained at Peking University and Caritas St. Elizabeth’s Medical Center in Boston.