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A Guide to Fasting Before Surgery 
(2021 update)

by Hoang Lam Tran

Medically reviewed by Dr David Reiner, B.Med, F.A.N.Z.C.A, PG Dip Echo


If you have been told to fast before a surgical procedure, you are not alone. Many people are confused about what they can eat and drink the night before a medical procedure. In addition to finding out about how long you should fast before surgery, it is also good to understand why fasting is required.


This article aims to answer some frequently asked questions including:

What is preoperative fasting?

Preoperative fasting is a common safety measure before any surgery. Fasting or NPO (nil by mouth) status helps reduce risks of pulmonary aspiration or the regurgitation of stomach contents into the lungs. Not only does it help lower the risk, but it may also help decrease the amount of required anaesthesia and shorten recovery time. However, what happens when there is delayed gastric emptying?

What is preoperative fasting?

Why do we fast before surgery?

1.   Aspiration

Pulmonary aspiration is described as the impediment of solid content in the airway occurring after induction of anaesthesia. Aspiration can cause fatal risks for patients by suffocating during the surgery. Hence, preoperative fasting manages the safety of patients beyond the surgical risks 2.

2.   Ruining your bowel preparation

Some gastrointestinal surgeries require bowel preparation as a process of completely emptying your digestion tract to protect you from infection due to food. Eating or drinking before surgery can ruin the effort of bowel preparation and put you at risk 3.

3.   Postoperative nausea and vomiting (PONV)

Postoperative nausea and vomiting are common distressing symptoms of surgery and the following anaesthesia. PONV can cause dehydration, aspiration, infections and reduce the effectiveness of recovery after surgery. Fasting is the easiest way to prevent PONV compared to other medications 4.

4.   Diabetes

Patients with diabetes have a higher incidence of surgery in terms of morbidity and mortality. The complication of diabetes may alter the outcome of surgery and reduce recovery. Talking with your anaesthetist or surgeon to handle your low blood sugar issues to ensure your safe fasting 5.

Why do we fast before surger
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Principle guideline for preoperative fasting

  • Preoperative assessment:

Prior to fasting, a preoperative assessment will be performed to review and evaluate your pertinent medical records and physical examination, and patient survey. You will be informed of fasting requirements and the reasons for you sufficiently in advance of your procedures and verify patients’ compliance with fasting requirements. Following the fasting guidelines, the proceeding will be compared its risks and benefits with consideration about the amount or type of the solid and liquid ingested 6.

  • Fasting recommendations:

Principle guidelinefor preoperative fasting

What are the negative side effects of fasting?

Pre-operative fasting helps prevent food and fluids from going down the wrong pipe but extending this period too much can have a negative impact on metabolic effects and worsen postoperative outcomes. Examples follow:

  • Headache, nausea, and vomiting

  • Dehydration

  • Lowered blood sugar

  • Irritability, hunger, and thirst

  • Decreasing patient safety

What are the negative side effects of fasting?

Can I drink water or other fluids before surgery? 

You are encouraged to drink the clear liquid that is consented to be ingested safely up to 2 hours before surgery. Since this diet is a staple of many medical protocols, it's important to learn and follow the protocol properly to not get into trouble. The following foods and drinks are considered clear liquids 7:


  • Water

  • Fruit juice (without containing pulp)

  • Black coffee, clear tea without any milk or cream

  • Soup broth (only liquid)

  • Sport drink (Gatorade)

  • Pedialyte (electrolyte drinks for sick children)

  • Carbohydrate drinks (Dex) 

Can I drink water or other fluids before surgery? 

What are carbohydrates drinks? 

Carbohydrate drinks are complex carbohydrate-rich drinks used for preoperative dietary management before surgery


What is Dex?

Dex overview:

Dex is a pre-op alkaline complex carbohydrate drink without artificial flavour, additional sugars, or colours that ensure the safety of pre-operative fasting before surgery and anaesthesia. As ready-to-drink meals providing energy, Dex is designed to meet strict fasting guidelines that recommend clear fluid preoperatively and support postoperative recovery processes for patients.

What are carbohydrates drinks? 
What is Dex?

How does Dex help patients optimize their surgery recovery?


  • Appreciative patients – less anxiety, hunger, thirst, headache, nausea, lightheadedness

  • Appreciative nurses and secretaries - look forward to dealing with your patients who are in a better physical and mental space (less irritable).

  • Nurses have something positive to offer patients who have their surgery delayed



  • Stimulates anabolic insulin hormones which counteract the catabolic endocrine and metabolic stress response from the trauma of surgery

  • Intravenous cannulation is facilitated – less vasoconstriction

  • Improved stability on the induction by keeping patients hydrated



  • Prevents insulin resistance

  • Facilitates immune response by minimizing postoperative hyperglycemia. Excess blood sugar inhibits phagocytosis and chemotaxis

  • Less postoperative nausea and vomiting has been demonstrated

How does Dex help patients optimize their surgery recovery?

When to stop eating and drinking before surgery?

The decision to fast before surgery should be made with thorough research and consideration given to all of your treatment options and based on your ages 8:


  •  Adults and teenagers over 12

Adults and teenagers over 12 are allowed to eat solid food or drink until 8 hours prior to their arrival time at the hospital for surgery. Please notice that you are encouraged to drink only clear liquid until 2 hours before the arrival time. Here is the timing schedule for when to stop eating or drinking 8.

When to stop eating and drinking before surgery?
  • Children 6 months-12 years old

Children are encouraged to follow special preoperative fasting instructions due to their safety. Similar to the former group, children 3-12 years old may eat solid food and drink mild or daily products until 8 hours prior to the arrival time of their surgery. Please offer your children only to drink clear liquid until 2 hours before the scheduled time 8.

  • Infants less than 6 months old

For this special group consideration, you may offer your baby formula or breast milk until 4 hours before arrival time for the scheduled surgery. Please offer your babies only to drink clear liquid until 2 hours before the surgery 8.


What are the guidelines for special patient groups? 

There are some patient groups that need to have special fasting requirements and monitor them throughout the fasting period 9 :

  • Diabetic patients

It is widely known that published guidelines do not recommend no-carbohydrate drinks before surgery for people with diabetes. However, it's hard to keep track of instructions due to the difficulty in standardizing the timing of the diet order, and delivery and consumption of carbohydrate-containing fluids.

Therefore, preoperative carbohydrates containing fluids are not allowed in the Preoperative Fluids for Diabetes diet because carbohydrates can elevate blood sugar, and that can lead to poor surgical outcomes. 

The best way to manage diabetes and surgery is to discuss it with your endocrinologist, surgeon, and anaesthetist. Some patients suffering from diabetes may require small amounts of carbohydrate-containing oral fluids to correct hypoglycemia. The amount required should be determined on a case-by-case basis following local protocols 9.


  • Older patients

Older patients, particularly those who have presented to the hospital with an acute medical condition and high risk of malnutrition, and may need additional nutrition support are also considered as the special group 9.

  • Children

Children are the vulnerable group that may have a higher risk of malnutrition and dehydration and need additional nutrition and hydration support 9.

What are the guidelines for special patient groups? 
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