Can You Fast If You Have Diabetes?

Medically Reviewed by Michael Dansinger, MD on December 12, 2022
4 min read

There’s a lot of buzz about fasting -- that is, not eating for a period of time or cutting back drastically -- for better health. If you have diabetes, is it safe and will it help you lose weight, control your blood sugar, and maybe even need less medicine?

Perhaps. Some studies suggest fasting may be helpful for people with diabetes. But it’s not a mainstream treatment. The American Diabetes Association doesn’t recommend fasting as a technique for diabetes management. The association says lifestyle changes, including medical nutrition therapy and more physical activity, as the cornerstones for weight loss and good diabetes control.

If you’re thinking of trying a fast and you have diabetes, you’ll want to know what the risks are, how to avoid them, and why you should check with your doctor first.

Some fasts don’t allow any food at all. But on intermittent plans, you follow a pattern of fasting and then eat normally.

Some types of intermittent fasting plans include:

Alternate day fasting. You eat your regular diet one day, and then eat fewer than 600 calories the next day, repeating this pattern throughout the week. The popular 5:2 plan is related, in which you eat a regular healthy diet 5 days a week and cut down to about 500 to 800 calories on the other 2 days.

Time-restricted eating. This is when you get all your calories for the day during a specific number of hours. For instance, on an 8-hour plan, you might eat from 10 a.m. to 6 p.m. and then not again until the next day at 10 a.m.

Some people fast for several days or even weeks at a time -- for example, for religious reasons. But not eating for more than 24 hours when you have diabetes can be dangerous.

More research is needed to know if it is safe long-term. Most studies of the effects of intermittent fasting have been done on overweight, middle-aged adults. More research is needed to determine if it is safe for people who are older or younger or people at a healthy weight.

Fasting may have some general health benefits. For example, it could cut down on inflammation, help with weight loss, and lower cholesterol. Fasting may also improve the way your body manages glucose (blood s
The American Diabetes Association notes that if you’re overweight or obese, weight loss can help lower your A1c level (a gauge of your blood sugar control over the last 2-3 months) and lower your risk for heart disease. Fasting isn’t required for weight loss.

Fasting may also affect how much insulin medication you need. In one study, people with type 1 diabetes who stuck with a fasting plan were able to lower their insulin dose.

Some organs that play a role in diabetes may benefit from fasting, too. Your body stores extra glucose in a form called glycogen in your liver. It takes your body about 12 hours to use that glycogen. If you don’t eat, your body begins to burn fat instead of glycogen for energy. That helps with weight loss. It also gives your liver and pancreas (which makes insulin, the hormone that controls blood sugar) a break.

When you fast, you’ll probably be hungry (at least at first). You might also feel drowsy and irritable. Not eating may give you a headache. And if you fast for more than a day or so, your body may not get enough of the nutrients it needs without supplements.

But the biggest danger of fasting if you have diabetes is that your blood sugar levels could go dangerously low (this is called hypoglycemia). That’s especially true if you take medication like insulin to control your diabetes. If you don’t eat, your blood sugar levels are lower and medication may drop them even more, which can lead to hypoglycemia. Hypoglycemia can cause you to feel shaky, pass out, or even go into a coma.

When you “break” your fast by eating, you may also be more likely to develop too-high blood sugar levels. Doctors call this hyperglycemia. This only happens if you eat too many carbohydrates. If fasting prompts you to overeat carbohydrate-rich foods, it may not be the right plan for you.

Talk to your doctor first. If you have type 1 diabetes, other health problems due to diabetes, or have had hypoglycemia, your doctor may recommend you not fast.

If your doctor says it’s OK to try, ask if you need to check your blood sugar more often or adjust your diabetes medication during and after fasting.

Watch for signs of low blood sugar. If you start to feel shaky, sweaty, or confused, your blood sugar may be too low. Stop fasting right away and do what you normally would to treat hypoglycemia. For example, eat a glucose gel or have a sugary drink followed by a small, balanced meal when your blood sugar level is back to normal.

Be careful about what you eat after fasting. Eating too many carbohydrates after fasting can cause your blood sugar levels to become too high. Choose healthy, balanced meals and snacks.

Use caution. Don’t do tough workouts while you’re fasting. Hard exercise can make your blood sugar levels dip, which can lead to hypoglycemia. Ask your doctor what activities are OK to do, or just take a break.

Stay hydrated. Having diabetes puts you at risk of dehydration, which can make your blood sugar harder to manage. Drink lots of water and calorie-free beverages when you fast.