Fasting is hardly new. People have been fasting for religious reasons — Christians, Jews, and Muslims, among others — for centuries.
But fasting for weight loss is a relatively recent phenomenon, one that has become increasingly popular, in part because it seems to work, at least for some people. Intermittent fasting (IF) has also gained a lot of attention recently because celebrities have endorsed the plan, and there have been recent releases of new IF diet books.
Studies have shown that periodic sessions of IF — in which a person limits their meals to a certain window of time, with a fixed period of eating little or nothing — can boost weight loss, reduce waist circumference, and lower blood pressure, blood sugar, and total cholesterol, according to a review of research published in September 2021 in the journal Nutrients.
Given that weight, blood pressure, blood sugar, and total cholesterol levels are all important in people with type 2 diabetes, IF is worth exploring if you have the condition. Still, there are factors to consider before giving it a try.
What Is Intermittent Fasting and How Is It Done?
Even if you’ve never tried IF, you’ve likely fasted before, without even thinking about it. Fasting is often required for blood tests, medical procedures, or surgery, for example.
“When you are fasting, you naturally get fewer overall calories,” says Vandana Sheth, CDCES, a dietitian and nutritionist who specializes in diabetes management in her own practice in Torrance, California. “The fasting state also causes an increase in growth hormone levels, increase in norepinephrine, and decrease in insulin levels, and these changes in the hormones also cause an increase in our metabolic rate. All of these factors of intermittent fasting help with weight loss.”
There are different ways to do IF, including skipping meals and eating only during a certain time period, or restricting calories on certain days of the week and eating normally on other days, according to the Association of Diabetes Care & Education Specialists. The most common IF diets consist of a 16-hour daily fast, a 24-hour fast on alternate days, or a two-day-a-week fast on nonconsecutive days, according to the authors of the Nutrients analysis.
Whatever plan you choose — after consulting your care team, including a dietitian who specializes in diabetes — should be tailored to your lifestyle, type 2 diabetes symptoms, and nutritional needs.
Potential Benefit of Intermittent Fasting for Diabetes: It May Boost Weight Loss
In previous years, dietitians and scientists thought of IF as a negative practice, so there isn’t a wealth of high-quality clinical research on how it may affect people with diabetes, says Jason Fung, MD, a nephrologist in Toronto and coauthor of The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting. But attitudes have begun to change, and some newer studies suggest the approach may have benefits, including for people with diabetes.
For example, a small study published in August 2021 in the journal Hormone and Metabolic Research found that IF lowered insulin resistance in 13 adults with type 2 diabetes. Insulin is a hormone that helps shuttle glucose (sugar) out of the bloodstream; people with type 2 diabetes are resistant to insulin, which results in higher blood sugar levels.
It’s thought that losing weight through IF can lead to improvements in insulin sensitivity, too, says Michael Mosley, MD, a science journalist and coauthor of The FastDiet.
In addition, a review of existing research published in February 2021 in the journal Clinical Diabetes and Endocrinology included several studies that found IF reduced fasting glucose levels, weight, and post-meal blood sugar levels in people with type 2 diabetes. Even with these studies, though, further research needs to be conducted to see whether IF is truly safe for people with type 2 diabetes as a whole, experts say.
Potential Drawback of Intermittent Fasting: It May Harm Blood Sugar Control
On the other hand, some experts say that IF carries risks, especially for people who need to keep their blood sugar levels stable. For starters, skipping whole meals can result in poorer blood glucose control, not to mention issues such as fatigue and reduced energy—and, thus, an increased risk of injury.
Skipping meals may also cause people to make poor diet choices, which can have the opposite effect on their waistline and blood sugar. If you haven’t eaten for hours, you may be more inclined to reach for a carb-heavy pastry or plate of pasta, for instance.
“Blood sugar management can be a real concern with intermittent fasting for people with type 2 diabetes,” Sheth says. “That’s why it’s important to discuss intermittent fasting with your physician, monitor your blood sugar closely, and know that your medications may need to be adjusted as well. Your dietitian and diabetes care and education specialist can help guide and support you through this process.”
A plan that severely restricts a person’s calories or asks them to skip meals can be hard to stick with long term, too, says Ruth S. Pupo, CDCES, who practices at Adventist Health White Memorial in Los Angeles. While losing weight can be beneficial for people with diabetes, because it increases insulin sensitivity, putting weight back on can have the opposite effect, increasing the risk for diabetes complications.
Another risk of IF for people with diabetes is hypoglycemia, or low blood sugar. A study published in February 2018 in the journal Diabetic Medicine found that IF doubles the risk for hypoglycemia in people with type 2 diabetes. People who are on specific diabetes medications — sulfonylureas and insulin, in particular — may be at increased risk of this complication, which can be life threatening.
Certain people, such as those who are pregnant or breastfeeding or have an underlying disease or medical condition, should also avoid IF, Pupo says. “Anytime you have a higher demand for more nutrition, you don’t want to do a fast,” she says, explaining that people who are pregnant or breastfeeding require extra calories for themselves and their babies, and fasting can cause them to run out of glucose and burn fat, tissue, and muscle. What’s more, if a pregnant person overproduces ketones (a compound that is produced when the body uses fat for fuel), the effect can be harmful to the fetus.
People with diabetes also run the risk of developing diabetic ketoacidosis, or DKA, which occurs when your body doesn’t have enough insulin to move blood sugar into your cells for use as energy, according to the Centers for Disease Control and Prevention (CDC). When that happens, your liver responds by producing too many ketones, which can build up in the body and damage the kidneys as well as cause brain swelling, Pupo says. DKA may even lead to a diabetic coma or death, she adds.
A few case studies, such as a study published in October 2020 in the Journal of the Endocrine Society, and another published in November 2019 in American Journal of Case Reports have shown that there may be a link between IF, used in conjunction with a keto diet, and DKA.
For anyone, regardless of whether you have diabetes, cutting out meals and restricting entire food groups can cause nutritional deficiencies. Without proper nutrition, particularly protein, there’s also a risk for muscle mass loss. “When you really deprive the body of nutrients, your body not only breaks down fat but muscle as well. And your heart is a muscle,” Pupo says.
Steps to Take Before You Try the Popular Diet Plan
Here are a few things to keep in mind before trying IF.
Talk to your doctor. Before starting IF, talk to your doctor to come up with a safe approach and adjust any of your medication doses as needed. People on insulin should be particularly careful: If you’re on it and restrict your eating, you may be at a greater risk of low blood sugar, which can lead to dizziness and confusion as well as life-threatening symptoms such as seizures, or loss of consciousness, according to the CDC. If left untreated in people with diabetes, low blood sugar (hypoglycemia) can be fatal, the American Diabetes Association reports.
Find a plan that works for you. Plans for IF vary. There are those that restrict calories two or three days a week or limit eating to certain periods of the day. Stricter plans include fasting for up to 36 hours at a time for 7 or 14 days, though the latter aren’t typically recommended for people with type 2 diabetes, according to Sheth. The key is to find a plan that you can stick with long term.
Be prepared for side effects. It’s common to have headaches, cramps, constipation, or diarrhea, at least initially. But, says Dr. Fung, “If you’re feeling very unwell, stop. You might be hungry, but you shouldn’t be lethargic or throwing up.”
Eat healthfully. Even if you’re eating less food, you should still stick to a healthy diet that consists of whole, unprocessed foods, including non-starchy vegetables, protein, and healthy fats, as well as a multivitamin and plenty of water to prevent dehydration and headaches. A healthy diet will help you lose or manage your weight and keep your blood sugar steady.
Keep your expectations in check. IF doesn’t work for everybody, and your medical team may not feel that it’s a good fit for you. It’s important to consult professionals before giving IF a try, as going for long periods without eating when you have diabetes can be dangerous — or, at the very least, not give you the results you want.
Get support. If you do get the all clear to try IF, following the plan with a friend or joining an online community or social support network may help motivate you to stick with it.
Intermittent Fasting and Diabetes: The Bottom Line
Due to the risk of blood sugar swings, full-blown IF may not be for you, especially if your diabetes is not well controlled. Instead, you can try other strategies, such as decreasing your portion sizes, increasing your physical activity between meals, and making healthy food swaps, all of which align with IF. If your doctor has given you the OK, though, IF may help you manage diabetes, provided it helps lower your weight and body fat.