What Happens to Your Blood Sugar When You Fast

Many factors affect your blood sugar levels, including how often you eat. Fasting may lower your blood sugar, but it’s best to check with your healthcare provider before starting a fasting routine.

How Fasting Affects Blood Sugar

During fasting, blood sugar and insulin levels drop, prompting the pancreas to release glucagon, according to Kimberly Prado, DNP, APN, a clinical associate professor at Rutgers University School of Nursing.

“This hormone signals the liver to break down stored sugar, or glycogen, and as fasting continues, the body shifts to burning fat for fuel, producing ketones, which are an efficient energy source for the brain and muscles,” Prado told Verywell.

However, not all fasting diets are structured the same way, and some may not be as beneficial as others when managing your blood sugar.

“There is some research that intermittent fasting—eating in an eight-hour window and fasting for 16 hours or eating normally for five days and significantly restricting calories for two days—can improve blood sugar management,” Abigail Collen, MS, RD, CDN, a dietitian at Mount Sinai, told Verywell.

How Long You Fast Matters

The specific benefits of fasting depend on how long it lasts.

“Within the first eight to 12 hours, blood sugar levels decline as glycogen, or stored sugar, is tapped for energy. By 12 to 18 hours, fat burning intensifies, ketones rise, and cognitive clarity may increase, and some report appetite suppression due to the activation of ketones,” Prado said.

“After about 24 hours, cellular cleaning begins, as autophagy kicks in, which aids in reducing inflammation and boosting immune health. As fasting stretches to 36 to 48 hours, growth hormone levels surge, promoting muscle repair and recovery,” she explained.

After 72 hours, stem cell regeneration may occur, “offering an immune system reset and potential protection against certain diseases,” Prado explained.

Other Health Benefits of Fasting

Prado said it may be helpful to think of household chores to understand the relationship between fasting and blood sugar.

“Fasting is like giving your body a deep spring clean. It starts with sweeping out the surface, easy-to-reach areas, as it burns through sugar reserves,” she explained.

“Then it moves into deeper zones, breaking down fat and clearing out old and damaged cells. The process of fasting can improve insulin sensitivity, reduce inflammation, promote weight loss, sharpen mental clarity, and boost immune function.”

Other potential benefits of fasting, Prado added, include cellular repair and metabolic reset.

Are There Risks to Fasting?

There are known risks associated with fasting, particularly for people with certain metabolic diseases.

“The potential downsides include low energy, disrupted eating patterns, inability to comfortably eat with their social unit, and, sometimes, unpredictable blood sugar shifts,” Yasi Ansari, MS, RDN, CSSD, senior dietitian at UCLA Health in Santa Monica, told Verywell.

Fasting isn’t advised for people with a history of eating disorders, pregnant people, people who are very physically active, and anyone else who requires “more specialized nutrition and consistent eating patterns,” Ansari said.

Can People With Diabetes Fast to Manage Blood Sugar?

Some research has suggested that certain fasting diets—when supervised by medical doctors—may help people manage type 2 diabetes. Specifically, fasting could promote weight loss and improve glycemic control.

Prado recommends taking these extra steps to make sure you’re fasting in the safest possible way:

If you have type 2 diabetes and you’re taking medication, you should talk to a doctor before fasting, since the dosage of your medication may need to be adjusted while you fast.
If you have type 1 diabetes, intermittent fasting is more complex due to insulin dependence. Because of this, you will need to stay in communication with your medical provider and continuously monitor your glucose. If you don’t, you could put yourself at risk for hypoglycemia (low blood sugar) or ketoacidosis.
Anyone else, including people with pre-diabetes, should speak with a doctor before fasting and pay close attention to how they’re feeling when they try a new diet.