Preoperative fasting is the practice of a patient abstaining from oral food and fluid intake for a time before an operation is performed. This is intended to prevent pulmonary aspiration of stomach contents during general anesthesia.
Fasting guidelines have been relaxed in recent years, but it's not uncommon for patients to be given the traditional after-midnight order. While it's always best to follow your doctor's advice, it's perfectly reasonable to ask about relaxing the fasting requirements—especially if you're scheduled for an afternoon procedure. In that case, you might be asked to go without food for more than 12 hours! Doctors and anesthesiologists are often willing to accommodate your wishes.
The after-midnight order has been the norm for decades. It's a precautionary measure to prevent pulmonary aspiration, which occurs when stomach contents enter the lungs, potentially blocking airflow and putting patients at risk for serious infections like pneumonia. However, modern anesthesia techniques make pulmonary aspiration much less likely. And when it does happen, it almost never results in long-term complications or death.
What's more, research has demonstrated that the stomach empties much faster than previously believed, and a long fasting period probably won't reduce aspiration any better than a short fast.
A long fast may add to discomfort during recovery. Fasting can lead to headaches, nausea, dizziness and dehydration. Dehydration can be serious and makes it difficult for nurses to draw blood for necessary tests.