Diabetes Forecast

Fasting for Surgery: What If I Have a Low?

I have diabetes and will be getting an operation. I am not supposed to eat or drink after midnight, but after four hours, my blood glucose drops. What can I take to bring it up?

Belinda Childs, APRN, BC-ADM, CDE, responds

Preparing for a test or surgical procedure that requires fasting (avoiding food and drink for more than eight hours) is difficult for anyone with diabetes. How you prepare and manage before the test will depend on what diabetes drugs you use. Medications may need to be adjusted prior to the procedure—especially if they pose a risk for low blood glucose.

What to Know

One of the normal functions of the liver is to produce as well as store glucose to fuel our bodies when we are fasting. These liver stores of glucose are built up with good nutrition and regular meals and snacks. But daily activities can alter these stores: Intense exercise uses the stored glucose, and ingesting alcohol prevents the liver from making new glucose—which is why it’s best not to imbibe the day before a medical procedure that requires fasting.

Learn More

Eating a snack before midnight that includes protein and fat will help prevent overnight lows. For the snack, avoid rapidly absorbed forms of carbohydrate (such as sugar-sweetened drinks and candy). If you’re still producing insulin, these simple carbs can cause your body to overrespond by producing too much insulin, which may result in low blood glucose.

While fasting as instructed before a medical procedure, keep close watch on your blood glucose by checking it often. Talk with your medical provider to learn at what glucose level you should take action—the general rule of thumb is when it hits 70 mg/dl or below, but you may need to treat at higher levels during a fast, especially if you take insulin or sulfonylureas, which can cause lows. Also ask what treatment is recommended. In most cases, taking 15 to 20 grams of glucose in the form of tablets or gel will increase your glucose level while allowing you to proceed with your scheduled procedure. But be sure to tell the medical team that you experienced and treated hypoglycemia.


Follow a healthy eating plan the day before your fast starts and drink plenty of fluids. Ask your health care provider how to handle low blood glucose should it occur during fasting.



Take the Type 2
Diabetes Risk Test