What to Know About Fasting and Diabetes

By Melinda D. Maryniuk, MEd, RD, CDE

Fasting for people with diabetes is an increasingly popular topic. Fasting has many definitions, but for our purposes, we’ll define it as staying away from almost all food & drink for a specific period of time. While research is still limited, surveys show that about 50% of people with diabetes have done some form of fasting on an occasional basis. We’ll answer common questions about fasting here:

Why do people fast? Are there any benefits?

People fast for many different reasons including for religious and spiritual reasons as well as for their health. Research has linked fasting with diabetes- and weight-related benefits such as promoting weight loss, improving glucose management, reducing insulin resistance, and improving risks for heart disease. For some people, limiting the time and frequency of eating can be a jump start to a healthier and more sustainable eating pattern. A big benefit of fasting that many people report is that it allows time to think about and then break old habits and start with a clean slate for establishing new, better eating patterns.

Fasting is also linked with other benefits including delaying aging and reducing risks of cancer, although it’s important to note that there is very limited research on the benefits of fasting in humans – specifically those with diabetes. Most of the benefits for fasting have been demonstrated in laboratory animals.

What are the risks?

The biggest risk is that we don’t know enough about the potential risks (and benefits) of fasting. There are many risks linked with the frequency & duration of the fast. For example, structuring your day so you allow yourself to eat during an 8 hour window and fast for the other 16 hours, over a period of several weeks, is relatively low risk as compared to fasting for longer times during the day and doing it over a period of months. For people with diabetes who take a glucose lowering medicine (such as insulin or a sulfonylurea medicine), fasting is linked with hypoglycemia or low blood sugar. If you take one of these medicines, it’s extremely important to talk with your healthcare provider about your intention to do a fast and how it might affect your diabetes care plan. Breaking the fast may lead to hyperglycemia. Fasting may also be linked to dehydration. It’s not recommended for certain groups of individuals including pregnant women or those who struggle with disordered eating patterns.

What are the types of fasts?

The most common type of fasting for weight or diabetes management is called “intermittent fasting”. This is an umbrella term for eating patterns that cycle between fasting and not fasting. The two most common types of intermittent fasting patterns are time-restricted fasts (TRF) and day-long fasts

Time restricted fasts: Most of us fast about 12-14 hours a day – the hours between the last time we eat something at night until the first time we eat something in the morning. The principle behind TRF is to gradually increase the amount of time you are fasting and decrease the time you’re eating. For example:

  • 16:8 fast: No food for 16 hours and eat only during an 8 hour window (such as between 11 am – 7pm). 
  • 20:4 fast: Food is only eaten during a 4 hour window each day (a large meal and one to two snacks).

Alternate Day Fasts: There are many variations on this theme. A true “alternate day” (eating and fasting every other day) is the least common approach. More common is the: 

  • 5:2 day fast: For 5 days in the week you eat normally and for 2 days you fast (not consecutive days). During those fast days, you might eat between 500-800 calories. 

Researchers are also exploring the benefits of more prolonged fasts (4-6 days) and seeing benefits in animal models as it relates to anti-aging. However, as these are difficult to follow and not recommended at this point for weight loss or diabetes, they will not be discussed.

What are key points to consider if I want to try an intermittent fast? 

Talk with your healthcare provider about your plans. Many people are afraid to talk about controversial topics like fasting with their doctors as they are afraid they will be immediately dismissed. Let your provider know why you want to try fasting and share your plan of how you want to do it. Seek support from a dietitian who can help guide you in doing it safely.

Discuss if any changes to medicines need to be made. Your provider may want to adjust the dose or type of medicine you take depending on the type of fast you plan to do. This is another reason why including them in your plans is so important.

Choose healthy foods with good staying power. While fasting plans are not as much about what to eat as they are when to eat it – it is helpful to choose foods that will help keep you full. A diet high in low-fiber carbs will mean that you’re hungrier sooner, but choosing foods high in protein, fat and fiber will keep you feeling more satisfied. That means more lean meats, fish, cheese, eggs, nuts, seeds, avocados, as well as high-fiber foods such as vegetables, legumes and whole grains.

Monitor blood glucose more often. If you have diabetes, it is recommended to check blood glucose 4 times a day (before each meal and at bedtime). If you take insulin or use a glucose-lowering medicine such as a sulfonylurea, it is ideal to be wearing a continuous glucose monitor to ensure that any risk of hypoglycemia can be caught and treated early.

Start gradually. If you normally find yourself eating between 7am and 10pm, try decreasing that window from 15 hours for eating – to 14 and then 12. If you want to try an “alternate day” style fast, start with one day a week and limit calories by having 3 meal replacement shakes. (But talk with your doctor and keep checking your glucose!) 

Stay hydrated. If you’re used to drinking a few large glasses of water a day – you may need even more fluid, as you won’t be getting as much water from food during your fasting periods.

The bottom line: if you’re willing, an intermittent fast may work for you as a way to limit caloric intake and restrict food intake to certain times or days. It can be a good launching pad for new and improved eating habits. With your doctor’s approval, try it for a short period of time, see how you feel, and decide if it’s something you can sustain for a few weeks – but not more than 3 months. For most people, following these guidelines will mean lower risk of problems and a great opportunity to re-evaluate and re-set your eating patterns.

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