Is Intermittent Fasting Healthy or Just a Fad Diet?

Weight-loss strategies abound, from prioritizing protein to mindful eating and beyond. If you’ve been researching different diets, you may have heard of intermittent fasting, in which you alternate between periods of eating normally and periods of fasting, aka consuming fewer calories.

While there is little evidence that intermittent fasting is better than other diets that try to cut calories, it may work for you. 

So, who should try intermittent fasting? And how does it work? We’ve got answers to these questions and more, below. 

What is intermittent fasting?  

Intermittent fasting involves alternating between eating normally and deliberately limiting the number of calories you consume. Examples of this include:  

• The 5:2 Diet — In this diet, you eat your normal diet five days a week, then consume approximately a quarter of your recommended intake one or two days per week.
• The Eat-Stop-Eat Diet — In this diet, you fast for a full 24 hours once or twice a week.  

Intermittent fasting isn’t the same as time-restricted eating, another diet in which you limit your eating to a dedicated daily window. An example of time-restricted eating is the “16-8 rule,” or eating during eight hours of the day and fasting during the other 16. This is different from intermittent fasting because it involves eating during a specific period rather than eating less on certain days.

The key to intermittent fasting is to eat the amount of food you usually do, and not any more, on the days when you aren’t fasting. For some people, eating fewer calories one or two days per week can take the pressure off having to plan out every meal and snack on the other days. On the other hand, consuming a quarter of what you normally eat a couple times a week may feel too constricting. Or you may feel that you lack the energy you require to do your job and remain physically active on days when you eat less. The most important thing is to notice the effects of fasting and to listen to your body. 

“Any time we try a new diet the hope is that it resets the brain a bit and helps us become more mindful of our eating,” says Karen Munger, MS, RD, a certified diabetes educator and registered dietitian at the University of Washington Medical Center - Northwest. 

Is intermittent fasting effective?

It isn’t clear whether intermittent fasting works long-term, and what works for one individual may not work for another.

“If there was one diet that worked for everyone, there would be one diet book in the bookstore,” Munger says. “Unfortunately, that’s not the case.”  

Intermittent fasting isn't recommended during pregnancy or while you're lactating, for example, and for people with a number of medical conditions, so it's always wise to check with your doctor before starting a new diet.

Can people with diabetes benefit from intermittent fasting?  

First off, if you are on medications to control Type I or Type II diabetes, intermittent fasting is probably not for you as it can increase the risk of dangerous side effects, particularly dangerously low blood sugar levels. If you’re managing your diabetes without medicine, you should still talk with your doctor before you try intermittent fasting. They might suggest wearing a continuous glucose monitor (CGM), which can give you real-time information about your blood sugar throughout the day. There are over-the-counter versions of glucose monitors available.

Proponents of intermittent fasting say this type of diet can help those with diabetes to have more stable blood sugar. The thinking goes like this: If your blood sugar goes up when you eat, fasting should help you regulate your blood sugar, since you aren’t taking in food. Right?  

It’s not always that simple, Munger says.  

Here’s the catch: Our bodies continually make sugar to give us energy. If you have diabetes, your body might produce an excess of glucose when you’re fasting. Then, when you do eat, your blood sugar may spike. This doesn’t mean that you can’t try intermittent fasting if your doctor thinks it’s OK. But you do need to carefully monitor your glucose. Plan to use a CGM or finger pricks throughout the day to determine how you’re able to regulate your blood sugar while trying this diet.  

Bottom line: What you want to avoid is hitting a very low blood sugar (hypoglycemia), which can lead to loss of coordination, seizures, and even death. If intermittent fasting causes your blood sugar to repeatedly spike and plummet, it’s probably not a good idea to keep going with it.  

How to try intermittent fasting  

If you’re interested in trying intermittent fasting, have a conversation with your doctor or dietitian to see if you’re a good candidate. Once you get the green light, you’ll learn the most through trial and error and observing the results firsthand. Here are some things to keep in mind.  

Make a plan

Research has found that sticking with intermittent fasting is difficult for many people, so it’s important to think through how you’ll approach your diet. On what days will you restrict the number of calories you consume? How will you adjust your lifestyle and activity level accordingly?  

It’s also important to plan for the first “normal” meal you eat after fasting. This meal should have fresh fruits and vegetables and a balance of carbohydrates, fats and protein, Munger says. Something like a not-too-spicy stir fry with rice could do the trick.  

Finally, get your favorite water bottle or tumbler out, because staying hydrated is key while fasting.  

Practice and observe  

Try the diet for a few weeks to get the hang of it. Note what’s working and what’s not. How do you feel on normal eating days? How do you feel on fasting days?  

Remember, what works for one person may not work for the next. If you lack energy or brain power on days when you’re cutting calories, you may need to consider another eating pattern that better complements your lifestyle. Strategize with your doctor and fine tune your diet to meet your needs.

What should you do if intermittent fasting doesn’t work?

Time-restricted eating, or only eating during a limited window between breakfast and dinner, is a mellower alternative to intermittent fasting that can work for lots of people, including people with insulin resistance and diabetes.

To try time-restricted eating, consider closing the kitchen at a dedicated time after dinner each night (say, 7 or 8 p.m.). After this time, you don’t reach for snacks or other foods — you simply wait until the morning to eat your next meal. Munger likes time-restricted eating as an option, particularly for becoming more mindful of the calories we’re consuming at night, when we might be more likely to eat a bag of chips or down a pint of ice cream while watching a show.

Neither of these methods are a silver bullet, but they might be worth experimenting with — after having a conversation with your doctor, of course.  

“Any tool that helps you lose weight in a healthy and sustainable way is important for preventing a lot of health conditions, including diabetes, cancer, heart disease and fatty liver disease,” Munger says.