Why aren’t we allowed to eat before going in for surgery?
So you don’t get sick all over the anesthesiologist’s stuff.
Here’s an old link: http://boards.straightdope.com/sdmb/showthread.php?threadid=6941
There is a greater risk than staining the anethestist’s outfit.
When you’re anesthetized, your reflexes are dulled including those that normally operate to prevent you from inhaling your saliva and anything else in your mouth. So, if you’ve got a full stomach and vomit because of the anesthesia, you’re at risk of inhaling the contents of a full stomach - bad news (those pieces of meat and potatoes not only inflame the lung but block the airpipes). Conversely, if you’ve got only water in your stomach, and happen to vomit and inhale it, the chance of a serious problem is lessened (but by no means eliminated).
People who need emergency surgery may have full stomachs, of course. That is one more reason why emergency surgery is risky.
All those things, plus I puked all over the gurney after I came to because the nurse was rolling me backwards.