What to Know About Fasting and Diabetes

By Melinda D. Maryniuk, MEd, RD, CDE

Fasting is an increasingly popular topic among those with diabetes. Fasting encompasses various interpretations, though for our purpose, we’ll define it as abstaining from nearly all food for a designated period of time.

Why do people fast? Are there any benefits?

People fast for many different reasons, including religious, spiritual, and health purposes. Research has linked fasting with various benefits such as improved weight management, better glucose management, reduced insulin resistance, and lowered risks of heart disease. For some people, limiting the time and frequency of eating can be a jumpstart to a healthier and more sustainable eating pattern. Many people report that fasting pushes them to analyze their eating patterns and potentially  break old habits.

There is very limited scientific research, though, on the benefits of fasting – specifically for those with diabetes. Most of the benefits for fasting have been demonstrated in laboratory animals.

What are the risks?

There is not enough research done on fasting to know all of its potential risks. However, we know there are many risks linked with long frequencies & durations of a fast. For example, eating during an 8 hour time period and fasting for the remaining 16 hours (for just a few weeks) is relatively low-risk. In comparison, fasting for longer times during the day and doing it over a period of months has much higher risks. 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 could negatively affect your diabetes. Breaking a fast may lead to hyperglycemia. Fasting can also be linked with dehydration.

Fasting is NOT recommended for certain groups of people, 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 naturally fast about 12-14 hours a day – the hours between the last time we eat at night, until the first time we eat in the morning. The principle behind this 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; eat only during an 8 hour window of the day.

Alternate Day Fasts

There are many variations of this. Most common: 

  • 5:2 day fast: For 5 days of the week, you eat normally. For 2 days, you fast (not consecutive days). During the fasting days, you may eat around 500-800 calories.

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

Talk with your healthcare provider about it. 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.

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 couple weeks – but not more than 3 months.

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The medical information on Diabetes – What To Know’s website is provided as an information resource only. The content is not in any way intended to be nor should you rely on it as a substitute for professional medical evaluation, diagnosis, advice and treatment.

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