Does Intermittent Fasting Help People with Diabetes?

By Brett Goerl

We’ve reviewed the latest research to weigh the benefits and drawbacks of intermittent fasting and time-restricted feeding for people with diabetes.

Living with diabetes means carefully making sure you are eating foods that will keep your blood sugar and insulin levels in balance. However, a recent trend in the diabetes community focuses on when (and when not) to eat. You may have heard friends, family, and healthcare professionals use words like ‘intermittent fasting’ and ‘time-restricted feeding’ to describe this trend. We are going to take a closer look at what these eating strategies entail, how they might affect people with diabetes, and the risks and benefits to consider if you choose to skip a meal.

Intermittent fasting (also called IF) refers to a style of eating in which you alternate through a pattern of consuming little to no calories for a while (could be a few hours or a full day) and a normal eating schedule. Popular IF schedules include alternate day fasting, where one day you consume very little food (500-700 calories or less) and the next day you eat like you normally would – this regimen is usually repeated every other day, though some people make exceptions to this depending on what works best for their lifestyle. Another method is periodic fasting (also called 5:2 fasting), where you reduce your food consumption on certain days of the week (i.e., Tuesdays and Fridays) but then eat regularly on the other days. These patterns aim to reduce the number of calories you are consuming.

Time-restricted feeding (or TRF) is similar to intermittent fasting, but with a key difference: you fast for a specific portion of each day (but not for an entire day). A common approach to TRF is called 16:8 fasting, which restricts the eating window to eight hours a day (for example, you might only eat from noon to 8 PM) while fasting for the remaining 16 hours. Some people with diabetes, like Justine Szafran, have found TRF to be extremely helpful as part of a daily routine for diabetes management.

According to Dr. Mark Mattson, a neuroscientist at Johns Hopkins University who has studied fasting for more than 25 years, our bodies have evolved to handle extended periods of time without food. In modern times, however, many people have access to high calorie food around the clock and can snack all day long – which he believes has contributed to obesity rates and the prevalence of type 2 diabetes. His research suggests that fasting could reverse this trend.

How Does Fasting Affect My Metabolism?

Sugars and fats serve as our body’s main sources of energy. After a meal, sugars are used for energy first. They are broken down and delivered to the bloodstream, while fats are stored for energy later. When we restrict eating, our bodies run out of carbohydrates to use for energy and instead breakdown stored fats, converting them into ketones.

Ketones are our body's alternative to using glucose for energy, and fasting or carbohydrate restriction can make this alternative available for energy use. The change from using sugars to using ketones is known as metabolic switching, a process that scientists believe produces many of the health benefits attributed to IF beyond weight loss. Intermittent fasting and time-restricted feeding allow us to control this process and cause metabolic switching in our own bodies.

However, the excess production of ketones can lead to a dangerous condition known as diabetic ketoacidosis or DKA, especially if you have type 1 diabetes. Because of this risk, you should talk to your healthcare team before starting a diet that includes fasting so that your health can be monitored effectively and your medication doses can be adjusted. If you have type 1 diabetes, you will have to regularly test your ketone levels to avoid the risk of DKA. This risk is especially high if insulin doses are reduced excessively to avoid hypoglycemia (low blood sugar) while fasting.

There is an additional curveball that people with diabetes must consider with a diet that involves fasting – how does fasting affect blood sugar and insulin?

Fasting Affects Blood Sugar Management and Insulin Resistance

What does fasting have to do with insulin? Insulin is the primary hormone our bodies use to regulate blood sugar levels. Maintaining stable blood sugar levels (also called 'glycemic control') is an important goal for people with diabetes and is often measured by Time in Range, A1C, and fasting glucose levels.

Recently, clinical researchers looked at how IF and TRF affect things like blood sugar management and insulin resistance and found promising results.

• In a 2021 literature review, eight studies showed that individuals who practiced intermittent fasting had significant reductions in fasting glucose levels (an average decrease of 4.16 mg/dL; p = 0.003). Other important findings included a reduction in A1C levels, weight loss, and an increase in adiponectin (a natural hormone shown to increase insulin sensitivity and reduce inflammation).

• A study of men with prediabetes found that early morning TRF decreased insulin levels (in people with prediabetes, consistently high insulin levels can lead to insulin resistance, obesity, heart disease, and more), reduced fluctuations in insulin levels, and decreased insulin resistance. This translates to an improvement in insulin levels and insulin sensitivity.

• A research team from the University of South Australia conducted a pilot study and a larger study to look at the 5:2 IF method in people with type 2 diabetes. Both studies found significant reductions in A1C levels and body weight after 12 weeks.

Interestingly, these results did not all necessarily correlate with weight loss – but some studies showed that even if a person’s weight did not decrease, they could still experience improvements in insulin sensitivity. These results align with the growing theory that the effects of IF and TRF on blood sugar and insulin levels may not only be due to weight loss but also to the metabolic switching caused by fasting. 

Is Intermittent Fasting Safe for People with Prediabetes and Type 2 Diabetes?

While studies on intermittent fasting have demonstrated positive improvements in several key measurements of diabetes (including A1C levels, insulin resistance, blood pressure, and weight loss), it is important to know that fasting might have negative side effects for someone with diabetes. In a clinical study on intermittent fasting among 137 people with type 2 diabetes, dizziness, headaches, hunger, and hypoglycemia were the side effects most often reported. This study was conducted using the 5:2 IF approach.

When it comes to blood sugar, “Both IF and TRF will drop a person’s glucose levels, meaning that medication adjustments are necessary to avoid hypoglycemia during the period of calorie restriction,” said Dr. Robert Ratner, a long-time scientific and medical leader in diabetes. “Changes in eating patterns on a daily basis will also require medication adjustments to keep glucose levels in range.”

There is currently no evidence to suggest that TRF would be less safe than IF, in particular because the fasting windows for TRF (when hypoglycemic events are most likely to occur) are shorter. IF and TRF appear to be most safe for people who do not take glucose-lowering medications that could cause hypoglycemia.

Is Intermittent Fasting Safe for People with Type 1 Diabetes?

Many people with type 1 diabetes have found that IF and TRF can help reduce morning blood sugar fluctuations and improve their energy. An observational study on people with type 1 diabetes who participated in various forms of IF/TRF found that these eating schedules can be safe as long as the individual works with their healthcare professional to reduce their usual insulin dose while fasting, and they must monitor their glucose levels carefully to avoid hypoglycemia. Fasting will also increase your ketone levels, so it is important to test your blood ketone levels if you have type 1 diabetes to avoid the risk of ketoacidosis. Remember that people with type 1 diabetes absolutely require insulin to avoid DKA, so insulin dose adjustment to avoid low glucose levels must be done cautiously.

For anyone with diabetes, before trying any type of fasting you should talk to your healthcare professional. For people who take sulfonylureas or insulin, it is important to closely monitor blood sugar levels, especially when you first start a fasting regimen, as dose reductions may be necessary to avoid hypoglycemia. Never stop taking a medication or change its dosage without consulting your healthcare professional first.

Drawbacks to Consider Before Starting Intermittent Fasting

Although IF/TRF may carry health benefits and is generally safe for people with diabetes, there are some drawbacks:

• It is difficult to start a new eating routine! Transitioning from a ‘round-the-clock’ eating schedule to one with planned feeding and fasting times can be stressful, and it might not align well with your daily life. For example, if you exercise in the morning or are an athlete, it might not be possible to participate on an empty stomach.

• Any style of fasting will affect your blood glucose levels and insulin requirements. Especially when starting a fasting regimen, you will need to closely monitor these indicators to make sure that your glucose levels stay in range. If you have type 1 diabetes or are dependent on insulin, you should also test your ketone levels consistently to avoid the risk of ketoacidosis (DKA).

• IF/TRF can have side-effects, such as shakiness, hypoglycemia, irritability, hunger, increased heart rate, sweating, chills, and dizziness.

• Know that IF/TRF can lead to unhealthy eating habits, including disordered eating. If you are concerned that you or someone else with diabetes may be experiencing disordered eating, contact the Diabulimia Helpline or We Are Diabetes; the National Eating Disorders Alliance is a resource for anyone with or without diabetes.

Tips & Tricks for a Healthy Fasting Experience

Dr. Ratner said, “The health benefits of IF and TRF are due to calorie reduction. If people overcompensate when they are eating, the impact is lost.” If you decide you want to pursue some sort of intermittent fasting plan, here’s what you should keep in mind:

• Consult your healthcare team before making any significant changes to your diet or medication. If your healthcare team agrees that it’s safe for you to try a new eating pattern, they should advise you on how often to monitor glucose and whether you’ll need to adjust your insulin dosing or other medications.

• Ask your friends and family who have tried IF/TRF to share their experience with you. Read Justine Szafran’s “Intermittent Fasting: Stabilizing My Morning Blood Sugars.”

• Consider making a meal plan for the days you will be fasting. This will help you make sure you are getting enough food.

• During fasting periods, you can drink water or any unsweetened, zero calorie beverage (i.e., coffee, tea). In fact, you should try to drink extra water while fasting to avoid dehydration.

• 16:8 Time-Restricted Feeding might be easier to incorporate into your daily schedule than Intermittent Fasting – it gives you the chance to build a regular schedule of eating every day.

• It’s okay to feel hungry! You will likely experience more intense food cravings at the beginning of a fasting period, but often your body will get used to the new eating pattern. Some people even experience a boost in energy after they get past the initial hunger.

• When starting a fast, monitor yourself for side effects (i.e., hunger, dizziness, headache, hypoglycemia). Start a journal where you can record your feelings and experiences for future reference and to discuss with your healthcare team.

• When resuming normal feeding, IF/TRF does not require you to adhere to any specific food plan. However, for people with diabetes it is best to consume natural, nutrient-rich whole foods that are not likely to spike your blood sugar.

• It is okay to miss a day of fasting or time-restricted feeding – that doesn’t mean you’ve failed. Similarly, it’s okay if IF/TRF is not for you!

Intermittent fasting (IF) and time-restricted feeding (TRF) are two ways people reduce calorie consumption over time as an alternative to traditional dieting. While research on these forms of fasting is limited in populations with diabetes, some evidence suggests that IF and TRF may offer people with diabetes or prediabetes a means to lose weight, stabilize blood sugar levels, and increase sensitivity to insulin.

While IF and TRF appear safe for people with diabetes, fasting can increase the risk of hypoglycemic events, especially in individuals taking medicine to lower blood sugar levels. Everyone's body is different, so before trying IF or TRF, consult your healthcare professional to help determine if this plan would be safe and helpful for your health and wellbeing.