Can You Fast If You Have Diabetes?

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.

Intermittent Fasting

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.

Benefits

Much of the research on fasting has been done in lab animals. Scientists are studying the effects in people, including those with diabetes. Though early findings are promising, they’re not certain.

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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 sugar) and cut down on insulin resistance.

One very small study included three men who had had type 2 diabetes for 10-25 years. With medical supervision, the men fasted every other day or 3 days a week. Within a month, all of the men were able to stop taking insulin. And in less than a year, they were able to cut down on or stop other diabetes medications. In another small study, 10 obese men with type 2 diabetes followed a time-restricted eating plan. They improved their fasting glucose and lost weight over 6 weeks.

Bigger studies are needed to confirm those findings and to see how long the results last. It’s also not clear which fasting plan is best or how often you’d have to do it.
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.

Risks

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.

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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.

Before You Try Fasting

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.

WebMD Medical Reference Reviewed by Michael Dansinger, MD on December 27, 2018

Sources

SOURCES:

Kathryn A. Boling, MD, Mercy Medical Center.

CMAJ: “Intermittent fasting: the next big weight loss fad.”

Annual Review of Nutrition: “Metabolic Effects of Intermittent Fasting."

The Ohio State University Wexner Medical Center: “Does Intermittent Fasting Work?” 

Nutrition and Healthy Aging: “Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: A pilot study."

Johns Hopkins Health Review: “Are There Any Proven Benefits to Fasting?” 

Harvard Medical School: “Intermittent Fasting: Surprising Update.” 

Diabetes Medicine: “Type 1 Diabetes and Prolonged Fasting."

J BI Database of Systematic Reviews and Implementation Reports:  Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis."

Cell Metabolism: “Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes."

BMJ Case Reports: “Therapeutic use of intermittent fasting for people with type 2 diabetes as an alternative to insulin."

American Diabetes Association, “Hypoglycemia (Low Blood Glucose).”

Joslin Diabetes Center: “Yom Kippur and Diabetes.” 

Mayo Clinic: “Diabetes and exercise: When to monitor your blood sugar.”

BMJ Open Diabetes Research & Care: “Recommendations for management of diabetes during Ramadan: update 2015."

CDC: “Managing Diabetes in the Heat.”

William T. Cefalu, MD, chief scientific, medical & mission officer, American Diabetes Association.

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