In 1946, obstetrician and cardiologist Curtis Lester Mendelson discovered a disturbing phenomenon: He found that some women who had anesthesia in labor were vomiting and aspirating on their stomach contents during delivery. As reported in his landmark study on surgical aspiration, Mendelson discovered 66 such cases in more than 44,000 pregnancies. This condition, aspiration pneumonitis, occurs because the laryngeal reflexes do not work under general anesthetic, making it possible to draw the contents of the stomach into the lungs. In his study, he wrote about two women who had died from the condition, their airways obstructed by undigested solid food. (Those who had aspirated liquid suffered from shortness of breath, blue discoloration [cyanosis], and a faster heartbeat than normal.) It was coined Mendelson’s syndrome after its author, and patients were advised to fast for prolonged periods prior to operations to avoid contracting the asthmalike syndrome.

By the 1960s the term nil by mouth (or its Latin variant NPO, nil per os) after midnight had become the widely accepted guideline for all surgical patients. If you recently had an elective procedure, you might know that it has not changed much since—fasting before surgery, meaning no food and no water, is still advice routinely given to preoperative patients. Yet the evidence—and medical practice, and even the recommendations—have evolved since Mendelson. Medical practice has yet to catch up.

For one thing, anesthesiologists no longer use ether, a substance known to make patients nauseated. They also employ endotracheal tubes, which protect the airways from the aspiration of stomach contents. Knowledge about digestion has increased to the point where the rate of calories leaving the stomach is predictable: A spate of studies on gastric emptying found that patients who consume clear fluids two hours prior to an operation do not have higher gastric volumes than those who fast for longer. In 1999, the tide of mounting evidence pushed the American Society of Anesthesiologists to amend its preoperative fasting guidelines: Patients are now instructed to have a light meal six hours before a procedure and clear fluids—drinks that you can see through, such as pulp-free juices, black coffee, or tea without milk and cream—until two hours prior to the operation. Guidelines in other countries were similarly amended.

Nevertheless, most patients appear to still be getting outdated advice and arrive to surgery thirsty and irritable. A presentation at the 2016 Anesthesiology Annual Meeting found that only 25 percent of hospitals in Michigan adhered to the new guidelines. A 2016 study of oral and maxillofacial surgeons found that 99.1 percent of them did not adopt ASA guidelines, and a worrying analysis of pediatric practices discovered that most children were fasting longer than necessary before their medical procedures, leading to negative experiences. Prolonged fasting can be associated with dehydration, hypoglycemia, and electrolyte imbalance. Some patients experience headaches and nausea before surgery.

Still the NPO-after-midnight approach seems to be ossified in surgical practice. “It’s still a fallback position for a lot of people. ‘Let’s be on the safe side and make sure we do this,’ so they stick with that old standby routine,” said Craig Palmer, a professor of anesthesiology at the University of Arizona College of Medicine. One of the barriers preventing practice change is the structure of surgical lists. Surgeries are scheduled throughout the day but are sometimes changed or adjusted. It’s easier to make scheduling shifts if all patients have received the same blanket advice to fast from midnight onward. “There’s an efficiency imperative at work with a lot of hospitals,” Palmer said.

But this reasoning overestimates how frequently patients are taken into surgery early. Anesthesiologist Joyce Wahr, the director of the University of Minnesota Health Preoperative Assessment Center, researched this phenomenon and found that patients were taken in ahead of time in just 5 to 6 percent of cases. In her own hospital she tracked data and discovered that even fewer patients—just 1 percent—went to surgery earlier than expected.

“So what we are doing in the U.S. is dehydrating a patient 95 percent of the time, in order to preserve this unrealistic hope that a case could go earlier,” she said.

Even hospitals that did adopt the new guidelines had problems. At Uppsala University’s Children’s Hospital in Sweden, children would stop drinking liquids two hours before their scheduled surgeries, but then the surgeries might be delayed, causing further dehydration, said Peter Frykholm, an associate professor at the hospital. “We didn’t want children going to surgery thirsty or hypoglycemic,” he said.

So, in 2000, the hospital decided to do away with the fluid fast altogether. Frykholm’s team analyzed physiology and gastric emptying rates and found that a half-hour safety margin after drinking clear fluids was enough to circumvent rates of aspiration. Conveniently, this was exactly how much time elapses between being summoned for an operation and being anesthetized. “We just let children drink water, apple juice, lemonade, and clear ice blocks until they are called to theater,” Frykholm says.

Frykholm and colleagues compiled data from their practice for an almost six-year period and found that aspiration occurred in just 0.3 percent of patients, none of whom died due to the condition.

Another reason why hospitals might not be implementing the new guidelines is that they don’t see extra fasting as harmful, Palmer said. Instead, they just think of it as taking an extra precaution. But research says that there are definite clinical consequences. For one thing, from the perspective of staff, patients are more irritable after fasting. “Thirst is such a primitive drive. When you aren’t allowed to have a drink of water, you are really miserable,” Wahr said. Anesthetists also find that dehydrated patients can present to surgery more anxious and in some cases require more drugs. And putting an IV into a patient is much easier when he or she is hydrated. The physical pressure a surgery puts on a body is much like a marathon, so it’s baffling to think that patients—especially elderly ones—would commence such an event by dehydrating themselves.

In 2014, the Annals of the Royal College of Surgeons of England investigated the effects of administering a complex carbohydrate drink two hours before an operation. Carbohydrate-loading was found to significantly improve insulin resistance and several categories of patient comfort, “especially hunger, thirst, malaise, anxiety and nausea.” No adverse effects to do with aspiration were reported. A recent article in the British Journal of Anesthesia touted the benefits of preoperative carb-loading and credited it for reducing length of hospital stays and improving postoperative muscle function.

Wahr said she’d be interested in examining the evidence and costs and benefits of complex carbohydrate drinks, but that at this stage she would be happy if patients did the bare minimum and drank water rather than fasting in the two hours before their operation times. If you have an operation scheduled and a physician advises you to fast for longer than two hours beforehand, refer her to the ASA guidelines and ask if she really feels a prolonged fast is necessary. As Wahr said, “It’s an education issue. Today the data is so good. Not following the guidelines is just cruel.”

Update, Sept. 8, 2017: This story’s headlines have been updated to more accurately characterize the type of fasting discussed in the story.