Diabetes and Intermittent Fasting

Suprising Research on the Safety and Efficacy of Fasting

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Intermittent fasting (IF) is a type of eating plan that involves limiting the time period when you eat. There are lots of different ways to do that.

Some people don't eat at all on certain days of the week. Others limit the number of calories they eat during a fasting period. Some IF diets restrict eating after a certain time of day. Other diets rotate between normal days and fasting days.

This article explores the science behind these eating plans, especially the effects they could have on people with diabetes.

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History of Diabetes and Intermittent Fasting

Fasting has been around for a long time. It's been a historical part of some spiritual traditions stretching back centuries. More recently, IF has been used as part of a healthy diet for weight loss, as a "detoxifying" strategy, and more. 

There's been some debate about whether fasting is healthy for those with diabetes. A growing body of evidence suggests that some IF diets could benefit people with diabetes. Scientists note that when a person fasts may be just as important as the diet itself.

Glucose Metabolism

To understand the benefits of intermittent fasting and diabetes, it’s good to know a little about how your body processes glucose and insulin.

Insulin is a hormone that enables glucose (sugar) to enter muscle, fat, and liver cells, where it's used for energy. When blood glucose rises, the pancreas releases insulin. The insulin lowers blood sugar by "unlocking" cells so that they absorb sugar from the bloodstream. That's how your body keeps blood sugar at a healthy level.

Insulin Resistance

Sometimes muscle, fat, and liver cells don't respond normally to insulin. Glucose builds up in the blood because it can't enter the cells. This is called insulin resistance. The cells resist the effects of insulin.

The pancreas responds by making more insulin. The extra insulin will keep the blood sugar level in normal range—until the pancreas can no longer make enough insulin to overcome the insulin resistance of the cells.

Prediabetes

Prediabetes means your blood glucose levels are higher than normal, but not high enough to qualify as diabetes. You may have prediabetes if you have insulin resistance. Prediabetes can also happen if your pancreas doesn’t make enough insulin to keep your blood sugar within the normal range.

Over time, prediabetes often progresses into type 2 diabetes.

How Intermittent Fasting Works

The primary goal of intermittent fasting for weight loss is to get insulin levels low enough so your body burns stored fat for energy instead of sugar.

Here is how it works: When your body breaks down the food you eat, it ends up as molecules in your blood stream. One such molecule is glucose. It comes from the breakdown of carbohydrates. 

Your body makes insulin so your cells can use that glucose. If you have more blood glucose than your body can use, it gets stored as fat for future use.  

When you're not eating meals or snacks, insulin levels drop. When insulin levels are low, fat cells release some of the stored fat so it can be used for energy. That results in weight loss.

Benefits of Intermittent Fasting

A few small studies have shown that intermittent fasting can have health benefits for people with diabetes. The benefits include:

  • Weight loss
  • Lowering insulin requirements

A long-term study published in the New England Journal of Medicine showed that fasting can:

  • Lessen inflammation
  • Lower insulin levels
  • Improve a wide range of illnesses such as asthma, arthritis, and more
  • Detox the body
  • Help the body rid itself of damaged cells, which may lower the risk of cancer

Other reported benefits of fasting include:

A 2019 study published in the journal Nutrients found that more studies need to be done to fully explain the long-term benefits of fasting for humans. That includes the possibility that fasting could cut the risk of heart disease.

Side Effects

Fasting has a few downsides. Side effects of IF diets can include:

  • Bad breath (which often results from low-carb diets)
  • Trouble concentrating
  • Excessive hunger
  • Irritability
  • Insomnia
  • Dehydration
  • Daytime sleepiness
  • Low energy levels that may impact your ability to exercise, which is important for people with diabetes
  • A much higher risk of low blood sugar (hypoglycemia)
  • High blood sugar (hyperglycemia) which can happen if the liver responds to fasting by releasing stored glucose

More research is needed to understand how common and how severe these side effects could be.

Some side effects, like hyperglycemia, could be dangerous for people with diabetes. Talk to your healthcare provider before you start any type of IF diet.

Types of IF Diets for Diabetes

Several IF diets have proven safe and effective for people with diabetes. Here's a look at the research.

The 5:2 Diet

The popular 5:2 diet was introduced in Dr. Jason Fung’s best-selling book "The Obesity Code" in 2016. It involves eating a recommended amount of calories five days per week, with two non-consecutive days of eating a reduced-calorie diet. On fasting days, you don't stop eating altogether. You just cut the number of calories.

If you have diabetes and want to try the 5:2 diet, speak to your healthcare provider or diabetic team. They'll help you set a target calorie intake for the fasting and non-fasting days.

Studies on the 5:2 Diet

Studies have shown that the 5:2 diet may lower insulin resistance. It may also help with weight loss in people with type 2 diabetes or prediabetes.

The first long-term study of the 5:2 diet was published in 2018 by the Journal of the American Medical Association (JAMA). It found that fasting could be effective for those with diabetes who have trouble sticking to a long-term, daily diet regimen.

The study tracked 137 people with type 2 diabetes. Half of them followed the 5:2 diet. The other half ate 1200 to 1500 calories per day. The group that fasted for two non-consecutive days per week took in 500 to 600 calories on the fasting days. They ate normally on the other five days.

The study found that those who followed the 5:2 diet were just as likely to control their blood sugar levels as those on the daily restricted-calorie diet. Researchers said the 5:2 diet "may be superior to continuous energy restriction for weight reduction.”

Safety of the 5:2 Diet

Some experts question the safety of the 5:2 diet for people with diabetes. But a long-term study published in 2018 reported that “fasting is safe for those with diet-controlled type 2 diabetes."

Study authors concluded that those who take insulin or oral diabetes medications such as glyburide or metformin need very close monitoring. They may need to adjust their dosages. This is because fasting can cause hypoglycemia if you're taking medications that lower your blood sugar.

This study found the 5:2 diet safe for those with diabetes. Still, it’s important to talk to your healthcare provider before fasting or starting any other type of diet.

Early Time-Restricted Feeding Diet (eTRF)

With an early time-restricted feeding (eTRF) diet, you fit all your meals into a specific period of time each day.

The eTRF diet plan may be an eight-hour, 10-hour, or even a six-hour plan. On the eight-hour plan, if you begin eating at 7:00 a.m., the last meal or snack for the day would be at 3:00 p.m. An example of the 12-hour early time-restricted feeding plan would be when you eat the first meal of the day at 7:00 a.m. and the last meal or snack no later than 7:00 p.m.

How the eRFD Diet Works

The eTRF diet may work with your circadian rhythm. Circadian rhythm refers to the body’s internal clock, which controls the timing of sleep, waking, and metabolism. That's why some think the diet could aid weight loss.

If you stop eating earlier in the evening, you will extend your overnight fast. According to a study published in the New England Journal of Medicine, fasting triggers some important cell functions, such as lowering blood sugar and boosting metabolism.

Benefits of the eTRF

Benefits of the eTRF diet include:

  • Less appetite
  • Weight loss
  • More fat loss via oxidation (burning of fat)
  • Lower blood pressure

Study on the eTRF Diet for Diabetes

In a 2018 study, the eight-hour eTRF diet was compared with the 12-hour diet. The study found that the eight-hour group had dramatically lower insulin levels than the 12-hour group. Both groups maintained their weight. And both groups lowered insulin and blood pressure.

Tips for Intermittent Fasting When You Have Diabetes

Diabetes UK recommends that you follow these tips if you have diabetes and plan to start an IF diet:

  • Consult with your diabetes team about whether the diet is a good choice for you. Follow their advice on any changes to your medication schedule. You want to be sure to keep a healthy blood glucose level.
  • Maintain a balanced diet during the fasting and non-fasting periods. Include foods from all of the food groups. Don't overeat during the non-fasting periods.
  • Just before you fast, eat foods that are more slowly absorbed. That includes those that are lower on the glycemic index scale. They're often high in fiber and are digested slowly.
  • Be sure to eat foods that fill you up and keep your blood sugar steady during the fast. Examples are fruits, vegetables, and fresh salads.
  • When you break the fast, limit the amount of fatty or sugary foods you eat. Try grilling or baking instead of frying.
  • Drink lots of fluids during the fast to avoid dehydration. Avoid sugary drinks.
  • Test your blood glucose levels often.
  • If you have symptoms of hypoglycemia, break the fast immediately. Use your action plan, such as taking glucose tablets followed by a snack. Talk to your healthcare provider before resuming the fast.
  • If you have type 1 diabetes, watch for signs of hyperglycemia when you fast. These include fatigue, extreme thirst, and frequent urination. Contact your healthcare provider right away if you have these symptoms or your blood sugar level stays high.

The safety of fasting for those with type 1 diabetes has not been fully established. If you have type 1 diabetes, you should never fast without first discussing it with your healthcare provider.

Summary

Intermittent fasting is an eating plan that's become popular for weight loss and detox purposes. There are questions about whether fasting is a good idea for people with diabetes. More research is needed, but some studies show there could be benefits. This is especially true for people with type 2 diabetes.

The 5:2 IF diet allows normal eating five days per week. On two non-consecutive days, you eat fewer calories. Studies show this plan is safe for people with type 2 diabetes that's controlled with diet. If you take medications to keep type 2 diabetes in check, you will need to monitor your blood sugar closely to make sure it doesn't drop too low.

Another popular fasting plan is early time-restricted feeding. This plan works by limiting the number of hours you can eat within a day. Eight-hour and 12-hour plans can lower insulin and blood pressure and help maintain weight.

If you have diabetes and want to try an IF diet, it's vital that you work with your diabetes team to do it safely.

Frequently Asked Questions

  • What are normal glucose levels for an adult?

    Normal blood glucose levels are 70 to 99 mg/dL. When numbers are higher, it may mean prediabetes or diabetes.

  • How does exercise work with intermittent fasting?

    You can exercise with IF diets. You may need to adjust your workouts to match your energy level, age, fitness, and goals. Cardiovascular exercise and strength training are part of a healthy lifestyle. Do include them, but lower the intensity as your body adjusts to fasting. You may also want to refuel after a workout.

  • Is intermittent fasting safe?

    Yes, intermittent fasting is safe for most people. It isn't right for everyone, though. Check with your doctor before making major changes to your diet if you have diabetes. IF diets may not be a good idea for people who are pregnant or breastfeeding, under age 18, have a history of eating disorders, or have diabetes. Speak with your doctor to be sure.

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14 Sources
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  1. National Institute of Diabetes and Digestive and Kidney Diseases. Insulin resistance and prediabetes.

  2. American Diabetes Association. 2. Classification and diagnosis of diabetes: Standards of medical care in diabetes-2020. Diabetes Care. 2020;43(Suppl 1):S14-S31. doi:10.2337/dc20-S002

  3. Tello M. Intermittent fasting: Surprising update. Harvard Health Publishing. Harvard Medical School.

  4. Diabetes.co.uk. 5:2 fasting diet.

  5. De Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541-2551. doi:10.1056/NEJMra1905136  

  6. Grajower MM, Horne BD. Clinical management of intermittent fasting in patients with diabetes mellitus. Nutrients. 2019;11(4). doi:10.3390/nu11040873 

  7. Diabetes UK. Fasting and diabetes.

  8. Carter S, Clifton PM, Keogh JB. Effect of intermittent compared with continuous energy restricted diet on glycemic control in patients with type 2 diabetes: A randomized noninferiority trial. JAMA Netw Open. 2018;1(3):e180756. doi:10.1001/jamanetworkopen.2018.0756

  9. Eckel-Mahan K, Sassone-Corsi P. Metabolism and the circadian clock converge. Physiol Rev. 2013;93(1):107-35. doi:10.1152/physrev.00016.2012  

  10. Ravussin E, Beyl RA, Poggiogalle E, Hsia DS, Peterson CM. Early time-restricted feeding reduces appetite and increases fat oxidation but does not affect energy expenditure in humans. Obesity (Silver Spring). 2019;27(8):1244-1254. doi:10.1002/oby.22518  

  11. Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metab. 2018;27(6):1212-1221.e3. doi:10.1016/j.cmet.2018.04.010

  12. Cleveland Clinic. Blood glucose test.

  13. Prospect Medical. Working out while intermittent fasting.

  14. Johns Hopkins Medicine. Intermittent fasting: What is it, and how does it work?