Nil by mouth, they say. But your tongue is so dry it sticks to the roof of your mouth. Your throat is sore from having a breathing tube down it for a couple of hours. You'd give anything for a soothing sip of water or an ice chip.

All this, the anesthesia's wearing off, and you've got a urinary catheter. It's rough. But a small price to pay for safe recovery from surgery, right?

The problem is, the misery of thirst and not choosing what and when to start eating doesn't contribute to safer recovery after most operations. A long fasting period is more likely to slow recovery down. Restrictive "nil by mouth" policies are being challenged by research on early fluids, food...and chewing gum. More on that soon. But to understand why the tradition of routine fasting is clinging on in so many hospitals, we need to go back to why it started.

Surgery developed very quickly once anesthesia with gases like ether, chloroform, and nitrous oxide started around 1840. Surgery is still a trauma, though, and our bodies react to it in complicated ways we don't entirely understand.

Much more happens than the impact of wounding and blood loss. Fluid redistributes internally, hormones react, and anesthesia and pain relief after surgery affect bodily functions - especially the gastrointestinal tract. Bowel function doesn't resume straight away, a condition called postoperative ileus (or bowel obstruction).

Things are usually back to normal within 2 to 3 days. If it takes longer than that, it's called paralytic postoperative ileus. That's a very serious condition, causing symptoms like cramping, nausea, vomiting, and abdominal pain. It can be excruciating and dangerous. No wonder it strikes fear in healthcare professionals.

Some people can't keep food or drink down soon after surgery. That helped form the impression that the body couldn't handle digestion at first. The whole digestive system seems to have shut down. The return of bowel sounds was regarded as the signal that the gastrointestinal tract was recovering - and often the requirement for "allowing" people to eat again.

But the lower gastrointestinal tract kicks into gear when you start to eat or drink. So withholding food and drink actually delays the return of bowel activity rather than helping get back to normal. And for some types of surgery, especially on the gastrointestinal system itself, it can be 2 to 5 days before even a liquid diet is started.

Controlling hydration and nutrition intravenously just seemed safer for everyone - even though there was never rigorous evidence that this specifically was helping. Other things were contributing to bowel problems, too - like opioids and lying in bed for too long.

Surgical traditions are hard to change, though. For example, by the 1880s it was clear that fluid therapy was important for hemorrhage and shock. But during the 1940s and '50s it became standard practice to restrict fluids (and saline in the fluids) after surgery. It took more than two decades to correct course.

Another example is the policy of "nil by mouth" for everyone from the midnight before surgery. That seems to have begun in the 1960s. It was believed necessary to prevent complications while under anesthesia. But despite clinical trials since the early 1990s showing this routine was unnecessary and harmful, the policy still hasn't completely disappeared.

In the early 1970s, the push began to encourage less draconian policies about drinking and eating for many patients after surgery, too. Clinical trials started to address these questions, providing evidence against this traditional surgical practice - especially for abdominal surgery.

A push for "fast track surgery" to reduce harm and get people back to normal more quickly was gathering momentum in the late 1990s, and that included diet and preventing serious bowel obstruction. But it's still hard to get healthcare professionals to accept that "nil by mouth" right after surgery is not necessary for everyone. You don't have to look far to find examples of hospitals or individual practitioners who still restrict all their patients from drinking and eating for a day or more. (See for example this spread of medical advice on the question.)

Enter the chewing gum controversy. Whether it's before surgery or after, a compromise for some professionals was to allow people to suck on a bit of candy or chew some gum for comfort. In 2002, the first clinical trial of chewing gum after surgery arrived.

On the basis of research suggesting that chewing gum was a kind of "sham feeding" that could activate the bowel, Takayuki Asao and colleagues from Maebashi in Japan recruited 19 people with colorectal cancer in a clinical trial. The 10 people randomly assigned to chewing gum chewed gum 3 times a day until they were allowed to eat: their first bowel motion came more than 2 days sooner than the average in the control group.

That was more dramatic than the results from later trials, but it encouraged more doctors to experiment with, and advocate, chewing gum in the early hours after surgery. It might not be for everyone, but most people apparently like it - the extra salivation stops their mouths and throats being so dry.

That Japanese trial has been joined by many others: 28 small trials in abdominal surgery, including colorectal surgery and cesarean section, have been analyzed in recent systematic reviews (listed below). And there are some trials for other types of surgery, too.

Chewing gum does not have a big impact on things like preventing paralytic postoperative ileus or getting out of hospital more quickly when there isn't a delay of days till a normal diet is introduced. It doesn't seem to be any better than sipping water or ice chips in the hours after surgery, for example. But it doesn't seem to cause vomiting or other common problems, either. So what's stopping its widespread use for the time water is not allowed?

Partly it's because many doctors, nurses, and midwives wouldn't be following the evidence on this - or it's hard for them to accept all their restrictions have not been scientifically valid.

Partly it seems to be because there is only a modest impact on outcomes like prevention of serious bowel obstruction - the misery of many patients isn't being taken seriously. And partly, I believe, it's because of hospital convenience and power. "Nil by mouth" after midnight, for example, enabled hospitals to change surgery schedules without worrying about exactly when particular patients had their most recent meals.

It's hard to watch someone begging for a sip of water and being denied it. Controlling what people can wear, and when they can eat, drink, and go to the toilet: these are some of the ultimate exercises of power over others. Taking those liberties away from people weakens them in fundamental ways. There needs to be a compelling reason to do it. If there isn't, it's a misuse of power over others.

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Recent systematic reviews on chewing gum after surgery:

The cartoon that starts this post is my own (Creative Commons License): more at Statistically Funny.

Morton's ether inhaler is from the National Museum of American History in Washington D.C. Photo by Daderot in Wikimedia Commons.

Operating theater image from the History of Medicine Division of the U.S. National Library of Medicine (NLM) at the National Institutes of Health (NIH). (Click on the image for details.)

The certificate with the image of Florence Nightingale is from the Wellcome collection, via Wikimedia Commons.

* The thoughts Hilda Bastian expresses here at Absolutely Maybe are personal, and do not necessarily reflect the views of the National Institutes of Health or the U.S. Department of Health and Human Services.