If you’ve been looking for an eating plan to help you lose weight and keep your glucose in check, you’ve no doubt come across many different options. One approach that’s been increasing in popularity recently is intermittent fasting (IF) — you may have heard about this and wondered what it’s all about and if it’s right for you.
What are fasting and intermittent fasting?
Fasting is totally avoiding foods and sometimes beverages for a certain period of time. People may fast for a variety of reasons, including for health and religious purposes. IF is an eating plan that alternates between fasting and feasting periods. There are three different categories of IF.
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This approach includes periods of refraining from food and caloric beverages, followed by days of eating. For example, the 5:2 fasting method involves five days where you can eat and two nonconsecutive days of fasting. During fasting days, you can drink water, tea, coffee, and sugar-free beverages. There are no calorie restrictions during the feasting days.
In this approach, practitioners can eat as usual for five days and consume a reduced number of calories (typically 400 to 600) during two sequential or alternate days. There is no standard amount of calories allowed during fast days in a modified fasting plan, but consumption may be anywhere between roughly 25% and 40% of usual calorie intake. (For example, someone who eats 1,800 calories in a typical day would eat between 450 to 700 calories daily during the fasting period.)
In this method, there are a number of hours of fasting, followed by a window where you can consume calories. The most popular types are the following (with the first number representing the hours of fasting and the second number representing the eating window): 16:8 (For example, eating between 11 a.m. and 7 p.m.) 18:6 (For example, eating between 11 a.m. and 5 p.m.) 20:4 (For example, eating between 12 p.m. and 4 p.m.)
What are the metabolic benefits of intermittent fasting?
In studies, the metabolic benefits of intermittent fasting have been found to be comparable to those of standard low-calorie diets. There appear to be similar reductions in total blood cholesterol levels, low-density lipoprotein (LDL, or “bad”) cholesterol, triglycerides (a type of blood fat), fasting blood glucose, and insulin. Three studies showed that participants who followed the IF 5:2 plan (five days of eating and two alternate fasting days) showed improvements in insulin resistance. And the metabolic benefits were found to be similar in people with type 2 diabetes: According to seven studies that tested the benefits of IF in participants with type 2, there were improvements in A1C (a measure of glucose control over the previous two to three months), fasting glucose, and blood pressure.
Is intermittent fasting better than a low-calorie diet?
Eleven studies that compared intermittent fasting with a low-calorie diet showed that participants with overweight or obesity lost a similar amount of weight with both approaches. The studies also showed that weight regain was similar in both groups. (There do not appear to be any studies that compared time-restricted feeding with low-calorie diets.)
Patients may find IF easier to follow compared to a low-calorie diet. For example, Marie L., one of my patients who is not a breakfast person, likes the 16:8 time-restricted program: “I have terribly busy days at work, and it is impossible to count calories,” she says. “Intermittent fasting is perfect for me. It eliminates the burden of weighing and measuring. I have my first meal at 11 a.m. and my last meal by 7 p.m. I drink unsweetened beverages after 7 p.m.”
Jim W., another of my patients who has type 1 diabetes, hit a stall in his weight loss. He tried intermittent fasting two days a week (changing his insulin regimen to avoid lows on these days), and it broke the stall. He follows a low-carb meal plan for the rest of the week.
Is it safe to fast with diabetes? Can diabetics do intermittent fasting?
If you have diabetes and are thinking of trying intermittent fasting, speak with your health care team. Hypoglycemia (low blood glucose), hyperglycemia (high blood glucose), dehydration, and diabetic ketoacidosis are among the potential complications. The Association of Diabetes Care & Education Specialists (ADCES) has published four risk categories of fasting for people who have diabetes (see sidebar below).
change the time, duration and/or intensity of my physical activity routine? Medication: Do I need to change the dose and timing of my diabetes medications? Diet: How many calories should I eat during the fasting period? How much fluid should I drink? Risk reduction: When should I stop the fast?
Risk categories of fasting
Very High Risk (One or more of the following)
- Type 1 diabetes — not in target range
- Low glucose levels
- Severe low glucose levels within the past three months
- History of frequent episodes of low blood glucose
- Hypoglycemia unawareness
- History of hyperosmolar hyperglycemic coma within the past three months
- Acute illness
- Engages in extremely intense physical labor
- Advanced chronic kidney disease
- On hemodialysis
- History of disordered eating patterns
- Type 2 diabetes — not in target range
- Type 2 diabetes — taking basal-bolus insulin or mixed insulin
- Type 1 diabetes
- Hyperglycemic — average glucose 150mg/dl-300mg/dl
- Lives alone
- Advanced age or frail
- Advanced macrovascular complications
- Microvascular complications
- Renal insufficiency
- Able to manage glucose using rapid acting, short acting or meglitinides
- A person with diabetes who manages diabetes with lifestyle (meal planning, activity, monitoring, no medications)
- Manages diabetes with lifestyle and medications that alone do not usually cause low glucose levels, such as metformin, thiazolidinedione, DPP-4i, GLP-1ra
Questions to ask your diabetes health care team before fasting
If you’re considering trying intermittent fasting, be sure to ask your healthc are team the following questions (along with any others you may have):
How often should I check my glucose levels, and what are my target fasting and postmeal targets?
Do I need to change the time, duration and/or intensity of my physical activity routine?
Do I need to change the dose and timing of my diabetes medications?
How many calories should I eat during the fasting period? How much fluid should I drink?
When should I stop the fast?
What’s the secret behind intermittent fasting?
The circadian rhythm is a 24-hour body clock that regulates our sleep-wake cycle, along with other physiological processes. For example, insulin sensitivity decreases throughout the day and into the night. Lipase, an enzyme that transports fat and aids in weight loss, is lower in the morning and higher at night. In an animal study, mice fed a high-fat diet during their resting phase (which for mice occurs during the day) were heavier than mice fed a high-fat diet during their active phase (at night). In humans, the time we eat may also affect how effectively we burn calories. Late-night eating may reset our internal clocks and disrupt normal metabolic processes, leading to weight gain. In fact, night-shift workers are more likely to have metabolic syndrome and are at increased risk (29%) of becoming overweight or obese. Intermittent fasting may help better align our eating times with our circadian rhythm.
What should you eat during your eating window?
Joy Pape, RN, CDCES, says that it is usually the feasting, not the fasting, that gets people in trouble. It’s vital to choose healthy food options. Patient Patricia F. chose a high-protein diet as the preferred approach for her eight-hour feasting period. “My doctor recommended I start the day having 25 grams to 30 grams of protein within 45 minutes of waking up,” she says. Catherine M. combines intermittent fasting with a low-carb diet and eats two meals during her eating window: “It was difficult at first. It may not work for everyone, but it certainly works for me!” Schedule an appointment with a Registered Dietitian Nutritionist who specializes in diabetes care and education to find the right plan for you.
Nutrition tips for a healthy feasting period
Drink water and unsweetened and caffeine-free beverages to avoid dehydration.
Select fresh, frozen, or canned fruits. Buy canned fruit in its own juice.
Select a variety of colorful, fresh vegetables. Frozen and low-sodium canned vegetables are other good options.
Cook with whole grains, such as brown rice, corn, oats, whole wheat, barley groats, millet, and others. Whole grains may also help you better manage blood glucose levels.
Animal protein foods may be high in saturated fat, increasing the risk of heart disease and high cholesterol levels. Choose plant proteins such as legumes and nuts, along with very lean meats.
Fruits are great dessert options. When you crave rich desserts, be mindful of the portions.
Use unsaturated oils like olive oil and avocado.
Nuts and seeds
Select low-fat dairy products or plant-based milks.
Herbs and spices contribute flavor that can help reduce the amount of salt, sugar, and fat added to the diet. Be bold!
Have a plan
Intermittent fasting is a popular approach that can help you manage your weight and blood glucose levels. There are many variations of these programs, and your health care team can help you find the right one for you. Remember that hypoglycemia, hyperglycemia, dehydration, and diabetic ketoacidosis are among the potential risks of fasting. Work with your health care team and have a plan to assess your risk and, if IF is appropriate for you, determine how best to fast and feast to ensure your safety and well-being.