The 7 Types of Intermittent Fasting, and What to Know About Them

Registered dietitians often hear “Tell me what to eat.” Now they might be hearing “Tell me when not to eat.” It’s called intermittent fasting (IF), a dietary approach that involves interspacing planned periods of fasting with regular eating. Proponents say this diet is the key to lasting weight loss, better metabolic health, and a longer life.

When it comes to weight loss, there are two thoughts behind why IF has the potential to work. The first: “Periods of fasting produce a net calorie deficit, so you lose weight,” explains Rekha Kumar, MD, a specialist in endocrinology, diabetes, and metabolism at the Comprehensive Weight Control Center at Weill Cornell Medicine and NewYork–Presbyterian in New York City.

The other concept is more complex: This approach may prevent what’s called the “plateau phenomenon” from happening, Dr. Kumar says.

Though more research is needed on the safety and effectiveness of IF, one of the touted benefits of this approach is that it may prevent this metabolic sputtering. “Most people who try diet and exercise to lose weight tend to fall off the wagon and regain weight,” Kumar says. “Hormones that promote weight regain, like hunger hormones, are kicked into full gear, and the thought is that IF may be a way to prevent this metabolic adaptation from happening.” The idea is that the normal periods of eating in IF “trick” your body into losing weight before the plateau happens.

So, does it actually lead to weight loss? Anecdotal evidence has led proponents of the plan to believe so. “For the people who can adhere to IF, it does work,” Kumar says. But fans of the approach claim there’s so much more to IF than just a lean body. Lori Shemek, PhD, a nutrition and weight loss expert in Dallas and author of How to Fight FATflammation, explains to clients that IF may improve their insulin sensitivity (lowering the risk of type 2 diabetes), reduce inflammation, and “boost longevity by bettering the health of your mitochondria (cell powerhouses),” she says.

Not everyone should (or needs to) try IF. A few groups who shouldn’t: women who are pregnant or trying to become pregnant (extended fasting periods may throw off your menstrual cycle), those taking diabetes medication (blood sugar can drop too far in the absence of food), or anyone who takes multiple medications (food, or lack of it, can affect absorption and dosage), Kumar says. Also, if you have a history of eating disorders, introducing periods where you’re “not allowed” to eat can put you on a dangerous path toward a relapse.

Know that IF has some side effects. You may be cranky — “hanger” is real — during fasting periods because low blood sugar can mess with your mood. You also still need to have a healthy diet when you do eat. “One thought is that it would be difficult to make up a calorie deficit if you fasted for two days, but in our society with access to calorie-dense items, you could probably do it,” Kumar says. Focus on balanced, nutrient-packed choices, like fruits, vegetables, lean meats, legumes, and whole grains (though some experts, like Dr. Shemek, also pair IF with low-carb or keto styles of eating). Expect that for the first couple of weeks you may deal with lower energy, bloating, and cravings until your body adjusts, Shemek says.

There are so many different ways to do IF, and that’s a great thing. If this is something you’re interested in doing, you can find the approach that will work best with your lifestyle, which increases the chances of success. Here are seven:

1. 5:2 Fasting

This is one of the most popular IF methods. The bestselling book The FastDiet introduced it to the mainstream, and it outlines everything you need to know about this approach. The idea is to eat normally for five days (don’t count calories); then on the other two days eat 500 or 600 calories a day, for women and men, respectively. The fasting days are any days of your choosing.

The idea is that short bouts of fasting keep you compliant; should you be hungry on a fasting day, you just have to look forward to the next day, when you can “feast” again. “Some people say, ‘I can do anything for two days, but it’s too much to cut back on what I eat all seven days,’” Kumar says. For those people, a 5:2 approach may work better than cutting calories for the entire week.

That said, the authors of The FastDiet advise against fasting on days that you may be doing a lot of endurance exercise. If you’re prepping for a bike or running race (or run high-mileage weeks), evaluate whether this type of fasting will work with your training plan. Or speak with a sports nutritionist.

2. Time-Restricted Fasting

With this type of IF, you choose an eating window every day, which should ideally leave a 14- to 16-hour fasting period. (Due to hormonal concerns, Shemek recommends that women fast for no more than 14 hours daily.) “Fasting promotes autophagy, the natural ‘cellular housekeeping’ process where the body clears debris and other things that stand in the way of the health of mitochondria, which begins when liver glycogen is depleted,” Shemek says. Doing this may help maximize fat cell metabolism and optimize insulin function, she says.

With this approach, you set your eating window from 9 a.m. to 5 p.m., for instance. It can work especially well for someone with a family who eats an early dinner anyway, Kumar says. Then much of the time spent fasting is time spent sleeping anyway. (You also don’t technically have to “miss” any meals, depending on when you set your window.) But this is dependent on how consistent you can be. If your schedule is frequently changing, or you need or want the freedom to go out to breakfast occasionally, head out for a late date night, or go to happy hour, daily periods of fasting may not be for you.

3. Overnight Fasting

This approach is the simplest of the bunch, and it involves fasting for a 12-hour period every day. For example: Choose to stop eating after dinner by 7 p.m. and then resume eating at 7 a.m. with breakfast the next morning. Autophagy does still happen at the 12-hour mark, though you’ll get more mild cellular benefits, Shemek says. This is the minimum number of fasting hours she recommends.

A benefit of this method is that it’s easy to implement. Also, you don’t have to skip meals; if anything, all you’re doing is eliminating a bedtime snack (if you ate one to begin with). But this method doesn’t maximize the advantages of fasting. If you’re using fasting for weight loss, a smaller fasting window means more time to eat, and it may not help you decrease the number of calories you consume.

4. Eat Stop Eat

This approach was developed by author Brad Pilon in his book Eat Stop Eat: The Shocking Truth That Makes Weight Loss Simple Again. His approach differs from other ones in that he stresses flexibility. Simply put, he emphasizes the idea that fasting is just taking a break from food for a time. You complete one or two 24-hour fasts per week and commit to a resistance training program. “When your fast is over, I want you to pretend that it never happened and eat responsibly. That’s it. Nothing else,” he says on his website.

Eating responsibly refers to going back to a normal way of eating, where you don’t binge because you just fasted, but you also don’t restrict yourself with an extreme diet or eat less than you need. Occasional fasting combined with regular weight training is best for fat loss, Pilon says. By going on one or two 24-hour fasts during the week, you allow yourself to eat a slightly higher number of calories on the other five or six nonfasting days. That, he says, makes it easier and more enjoyable to end the week with a calorie deficit but without feeling as if you had to be on an extreme diet.

5. Whole-Day Fasting

Here, you eat once a day. Some people choose to eat dinner and then not eat again until the next day’s dinner, Shemek explains. With whole-day fasting, the fasting periods are essentially 24 hours (dinner to dinner or lunch to lunch), whereas with 5:2 the fasting period is actually 36 hours. (For example, you eat dinner on Sunday, then “fast” on Monday by eating 500 or 600 calories, and break it with breakfast on Tuesday.)

The advantage of whole-day fasting, if done for weight loss, is that it’s really tough (though not impossible) to eat an entire day’s worth of calories in one sitting. The disadvantage of this approach is that it’s hard to get all the nutrients your body needs to function optimally with just one meal. Not to mention, this approach is tough to stick to. You might get really hungry by the time dinner rolls around, and that can lead you to consume not-so-great, calorie-dense choices. Think about it: When you’re ravenous, you’re not exactly craving broccoli. Many people also drink coffee in excess to get through their hunger, Shemek says, which can have negative effects on your ability to sleep. You may also notice brain fog throughout the day if you’re not eating.

6. Alternate-Day Fasting

7. Choose-Your-Day Fasting

This is more of a choose-your-own-adventure approach to IF. You might do the time-restricted fasting (fast for 16 hours, eat for eight, for instance) every other day or once or twice a week, Shemek says. What that means is that Sunday might be a normal day of eating, where you stop eating by 8 p.m.; then you’d resume eating again on Monday at noon. Essentially, it’s like skipping breakfast a few days a week.

This approach may be easily adaptable to your lifestyle and is more go with the flow, meaning you can make it work even with a schedule that changes from week to week. But a looser approach may mean milder benefits.