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 What Are the Different Types of Intermittent Fasting and What Should You Know About Them?

"Tell me what to eat," a registered dietitian frequently hears. "Tell me when not to eat," they might be hearing now. It's called intermittent fasting (IF), and it's a dietary strategy that involves interspersing planned fasting periods with regular meals. This diet, according to proponents, is the key to long-term weight loss, improved metabolic health, and a longer life.

Intermittent fasting's Potential Health Benefits

When it comes to weight loss, there are two theories as to why IF could be effective. The first is that "fasting periods produce a net calorie deficit, resulting in weight loss," says Rekha Kumar, MD, a specialist in endocrinology, diabetes, and metabolism at Weill Cornell Medicine and NewYork–Presbyterian in New York City.

The other concept is more complicated: according to Dr. Kumar, this approach may prevent what is known as the "plateau phenomenon."

You may recall the well-known "Biggest Loser" study. After six years, the researchers checked in with the TV show participants and discovered that, despite their initial impressive weight loss, they had regained the majority of their weight and their metabolic rates had slowed, resulting in them burning far fewer calories than would have been expected.

Though more research on the safety and effectiveness of IF is needed, one of its touted benefits is that it may prevent metabolic sputtering. "Most people who try to lose weight through diet and exercise fall off the wagon and gain weight," Kumar says. "Hormones that promote weight gain, such as hunger hormones, are activated in full force, and it's thought that IF could help prevent this metabolic adaptation." The idea is that the regular eating periods in IF "trick" your body into losing weight before it reaches a plateau.

So, does it actually help you lose weight? Proponents of the plan believe this based on anecdotal evidence. "It does work for people who can stick to IF," Kumar says. However, proponents of the method argue that it is about much more than just getting a lean body. Lori Scheme, PhD, a Dallas nutrition and weight loss expert and author of How to Fight FAT flammation, tells clients that IF can improve insulin sensitivity (lowering the risk of type 2 diabetes), reduce inflammation, and "boost longevity by bettering the health of your mitochondria (cell powerhouses)," according to her.

According to studies, IF can result in weight loss of 1 to 8% of one's starting weight, which is comparable to the amount of weight lost on a calorie-restrictive diet. Other aspects of cardio metabolic health, such as blood pressure and insulin resistance, may benefit from IF. arrow pointing up

A separate study looked at 11 IF trials that lasted at least eight weeks and included adults who were obese or overweight.

arrow pointing up Nine of the studies found that an IF program was just as effective at helping people lose weight and body fat as traditional dieting (calorie restriction every day). Finally, another study discovered that while 12 weeks of IF had no effect on cholesterol levels, it did result in weight loss.

It's worth noting, however, that studying human longevity is far more complicated than simply examining weight loss. That's why a lot of the research on whether IF helps people live longer has been done on animals, such as fruit flies. Arrow pointing up According to other research, the metabolic benefit of IF is that it causes your body to enter a state of ketosis (the process involved in the keto diet), in which fat is burned for fuel rather than carbohydrates. Arrow pointing up The idea that ketones may trigger the body's own repair system, ultimately protecting against disease and aging, has been proposed by researchers. Arrow pointing up.

It's still important to keep your expectations in check when it comes to IF. Because much of the research has been done on animals, it's more difficult to apply the findings to humans, who are certainly free-thinking and must contend with the effects of lifestyle issues such as work stress, crazy schedules, emotional eating, and cravings, to name a few — that can affect one's ability to stick to a specific diet. Although IF appears to be promising, it is "no more effective than any other diet." right up arrow

Who Isn't a Good Candidate for Intermittent Fasting?

Not everyone should (or needs to) participate in IF. Women who are pregnant or trying to become pregnant (extended fasting periods may throw off your menstrual cycle), diabetics (blood sugar can drop too low in the absence of food), and anyone who takes multiple medications (food, or lack thereof, can affect absorption and dosage), are among those who shouldn't, according to Kumar. Furthermore, if you have a history of eating disorders, introducing periods when you're "not allowed" to eat can set you on a dangerous path to relapse.

It's important to be aware that IF it has some side effects. Because low blood sugar can mess with your mood, you may be cranky — “hanger” is real — during fasting periods. When you do eat, you must continue to eat a healthy diet. “One thought is that if you fasted for two days, it would be difficult to make up a calorie deficit, but in our society with access to calorie-dense foods, you could probably do it," Kumar says. Concentrate on nutrient-dense foods such as 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 to experience low energy, bloating, and cravings for the first few weeks until your body adjusts.

Consider These 7 Types of Intermittent Fasting

There are a lot of different ways to do IF, which is fantastic. If this is something you want to do, you can figure out what approach will work best for you and your lifestyle, increasing your chances of success. Here are seven of them:

1. Fasting in a 5:2 ratio

This is one of the most widely used IF techniques. The FastDiet, a best-selling book that outlines everything you need to know about this approach, brought it to the attention of the general public. The idea is to eat normally for five days (without counting calories) and then eat 500 or 600 calories per day for the remaining two days, for women and men, respectively. The fasting days can be any days you want.

Short fasting periods are thought to keep you compliant; if you get hungry on a fasting day, just look forward to the next day, when you can "feast" again. "Some people say, 'I can do anything for two days, but cutting back on what I eat for seven days is too much,'" Kumar explains. For those people, a 5:2 diet may be more effective than calorie restriction for the entire week.

However, The FastDiet authors advise against fasting on days when you plan to do a lot of endurance exercise. Assess whether this type of fasting will work with your training plan if you're preparing for a bike or running race (or if you're running high-mileage weeks). Alternatively, you could say

2. Fasting with a Time Limit

You choose an eating window every day with this type of IF, which should ideally leave a 14- to 16-hour fasting period. (Shemek advises women to fast for no more than 14 hours per day due to hormonal concerns.) "When liver glycogen is depleted, fasting promotes autophagy, a natural 'cellular housekeeping' process in which the body clears debris and other things that get in the way of mitochondrial health," Shemek says. According to her, doing so may aid fat cell metabolism and insulin function.

For example, you could set your eating window from 9 a.m. to 5 p.m. with this method. Kumar says it's especially good for someone with a family who already eats a late dinner. Then there's the fact that most of the time spent fasting is spent sleeping. (Depending on when you set your window, you may or may not have to "miss" any meals.) However, this is contingent on your ability to be consistent. Daily periods of fasting may not be for you if your schedule is constantly changing, or if you need or want the freedom to go out to breakfast on occasion, go on a late date night, or go to happy hour.

3. Fasting for an entire night

This method is the most basic of the bunch, and it entails fasting for 12 hours every day. For instance, choose to stop eating after dinner at 7 p.m. and resume eating at 7 a.m. the next morning with breakfast. At the 12-hour mark, autophagy still occurs, but the cellular benefits are milder, according to Shemek. This is the bare minimum of fasting hours she advises.

This method has the advantage of being simple to implement. You also don't have to skip meals; all you're doing is cutting out a bedtime snack (if you ate one to begin with). However, this method does not fully exploit the benefits of fasting. If you're fasting to lose weight, a smaller fasting window means you'll have more time to eat, which may not help you consume fewer calories.

4. Stop eating and start eating again

In his book Eat Stop Eat: The Shocking Truth That Makes Weight Loss Simple Again, author Brad Pilon developed this approach. He takes a different approach than others in that he emphasizes flexibility. Simply put, he emphasizes the concept of fasting as a temporary abstinence from food. You commit to a resistance training program and one or two 24-hour fasts per week. "When your fast is over, I want you to eat responsibly and act as if it never happened." That is all there is to it. "I don't do anything else," he claims on his website.

Eating responsibly entails returning to a normal eating routine, in which you don't binge because you've just fasted, but you also don't starve yourself or eat less than you require. Fat loss is best achieved by combining intermittent fasting with regular weight training, according to Pilon. You can eat a slightly higher number of calories on the other five or six nonfasting days if you go on one or two 24-hour fasts during the week. He claims that this makes it easier and more enjoyable to end the week with a calorie deficit without feeling compelled to go on a strict diet.

5. Fasting for the entire day

You only eat once a day here. Shemek explains that some people choose to eat dinner and then not eat again until the next day's dinner. Fasting periods with whole-day fasting are essentially 24 hours (dinner to dinner or lunch to lunch), whereas fasting periods with 5:2 are actually 36 hours. (For example, you might eat dinner on Sunday, then "fast" on Monday by eating 500 or 600 calories before breaking the fast on Tuesday with breakfast.)

If you're trying to lose weight, the benefit of whole-day fasting is that it's extremely difficult (though not impossible) to consume an entire day's worth of calories in one sitting. The disadvantage of this strategy is that it's difficult to get all of the nutrients your body requires in just one meal. Not to mention, sticking to this strategy is difficult. By the time dinner arrives, you may be ravenous, leading you to consume less-than-healthy, calorie-dense foods. Consider this: When you're hungry, broccoli isn't exactly the first thing that comes to mind. According to Shemek, many people drink too much coffee to satisfy their hunger, which can interfere with their ability to sleep. If you don't eat, you may experience brain fog throughout the day.

6. Fasting on alternate days

Krista Varady, PhD, a nutrition professor at the University of Illinois in Chicago, popularized this approach. People may fast every other day, with a "fast" consisting of 25% of their daily calorie requirements (approximately 500 calories) and nonfasting days being normal eating days. This is a popular weight-loss strategy. In fact, alternate-day fasting has been shown to reduce body mass index, weight, fat mass, and total cholesterol in overweight adults.

On fasting days, you may be concerned about feeling hungry. Dr. Varady and colleagues found that by week two, the side effects of alternate-day fasting (such as hunger) had subsided, and participants began to feel more satisfied on the diet by week four. The downside was that study participants said they were never truly "full" during the eight weeks of the experiment, which can make sticking to this approach difficult.

7. Pick-Your-Own-Day Fasting

This is a more pick-your-own-adventure style of IF. According to Shemek, you can do time-restricted fasting every other day or once or twice a week (fast for 16 hours, eat for eight, for example). That means that Sunday might be a typical day of eating, with you stopping at 8 p.m. and then starting again at noon on Monday. It's the equivalent of skipping breakfast a few times a week.

Keep in mind that the evidence on the impact of skipping breakfast on weight loss is mixed. There isn't much evidence that skipping breakfast has an impact on weight. arrow pointing up However, other studies have found that eating a morning meal can help with weight loss. arrow pointing up Breakfast skipping has also been linked to an increased risk of cardiovascular disease mortality in other studies.

This approach is more adaptable to your lifestyle and more go-with-the-flow, so you can make it work even if your schedule changes week to week. However, a softer approach may result in fewer benefits.

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