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The Different Types Of Intermittent Fasting + Which One Is Right For You

As a medical doctor who focuses on gut health, I suggest intermittent fasting to many of my patients.

I do this because I've repeatedly seen what studies show: Intermittent fasting (IF) has been shown to aid in weight loss, lower inflammation, improve gut health, and reduce their risk for cancer and other diseases. If only finding the right plan proved to be easy for everyone!

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Enthusiastic to give it a go after our first consultation, my 37-year-old patient Joseph did a thorough Google search and quickly became overwhelmed with the numerous IF approaches.

Frustrated by the information overload, he modeled one of his co-workers, who did one 24-hour fast weekly. In other words, this co-worker ate normally six days a week, then completely fasted one day. That worked well for him.

For Joseph—not so much. Joseph’s once-weekly fast quickly backfired and his colleagues and family testified to his snappy behavior.

The foundation of IF is pretty simple: abstain from eating during certain time periods and eat during others. But even that yields some variation. So to cut through the confusion and determine which plan would work best for you, consider these options:

1.

The 12-hour fast

For this plan you might have breakfast at 7 a.m. and wrap up dinner by 7 p.m., creating a 12-hour daily fasting window. This plan is a piece of cake (no pun) since you’ll hopefully be sleeping for about 8 of those 12 hours.

Studies show this easier version of IF works well; one divided mice into four groups, all of which ate the same number of calories. One group could eat whenever they wanted and the other three groups ate during a 9-, 12-, or 15-hour window.

Thirty-eight weeks later the eat-whenever-they-want mice gained weight (no surprise!) but the 9- and 12-hour eaters stayed lean even if they occasionally strayed from the plan.

If you’re a newbie, this 12-hour fast plan is a great option to start with. When you don’t eat for 10 to 16 hours, your body reaches into its fat stores for energy—a desirable effect if your goal is losing weight.

Summary

A 12-hour fast has been said to be a good beginner fasting option and consists of eating for 12 hours and fasting for 12 hours daily. For this plan you might have breakfast at 7 a.m. and wrap up dinner by 7 p.m.
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2.

The 8-hour window

This is one of my favorite ways to transition into IF, especially for patients who need more than 12 hours of fasting daily to get results.

With this plan you might stop eating by 8 p.m. and have lunch at noon, thereby creating a 16-hour daily fasting window. Again, studies confirm the efficacy of this type of fasting; research shows1 an eight-hour eating window (versus eating whenever you want) can have beneficial effects on improving mensuration in women and reducing weight.

Summary

The 8-hour window consists of a daily 16-hour fast and confining your eating to an 8-hour window. With this plan you might stop eating by 8 p.m. and have lunch at noon.
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3.

Alternate-day fasting

Alternate-day fasting (ADF) involves a "fast day," when individuals consume 25 percent of their normal caloric intake, alternated with a "feed day," when they eat whatever they want.

On the 25 percent day, you eat strictly protein, vegetables, and some healthy fats—but no sugar or starches. One study2 on normal weight and overweight adults found that ADF was effective for weight loss and protecting the heart.

In my practice, I’ve found this tricky and definitely for more advanced intermittent fasters. Patients sometimes find that the "feed days" create a slippery slope to eat more.

An alternate tactic would be to completely fast every other day, but this can be hard to sustain over the long term.

Summary

ADF is when you only eat every other day (which essentially works out to a 36-hour fast, followed by a 12-hour eating window).
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4.

The 5:2 plan

A slightly easier variation3 of ADF, the 5:2 plan allows you to eat normally five days every week while eating only 500 to 600 calories on the other two days.

One study4 found that the 5:2 diet produced mild to moderate weight loss and consistent reductions in energy intake, all of which may benefit cardiometabolic health by decreasing blood pressure, insulin resistance, and oxidative stress.

Again, those calorie-restricted days might be more of a challenge, especially if you have a hard time controlling yourself when you eat.

It also may get confusing because not all calories are created equal. On the restricted days, the type of calories consumed is just as important as the number of calories. The focus needs to be on whole foods: protein, healthy fats, and vegetables.

Summary

The 5:2 plan allows you to eat normally for five days every week while eating only 500 to 600 calories on the other two days.
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5.

Erratic "anything goes" hunger-centered fasting

This becomes a broad category open to massive interpretation, unless you really learn to listen to your body. Basically, you eat when you’re hungry and abstain when you’re not. You don’t just eat because it’s lunchtime, for example.

Proponents of this type of fasting suggest5 eating sensibly most of the time, eating nothing for an extended period every now and then, and indulging occasionally.

This "anything goes" IF perspective works best when a health care professional custom-designs a plan for you rather than you whimsically deciding when to eat or not.

Its flexibility, however, becomes ideal during vacations and other occasions when you know you’ll eat less-than-healthy foods.

Summary

This type of fasting is when you eat when you’re hungry and abstain when you’re not and works well when a health care professional custom-designs a plan for you.

The takeaway

When it comes to IF, there is no one plan that work for everyone. Figuring out what works for you takes some trial-and-error, and a professional can help you tweak and troubleshoot.

If you go it alone, I suggest starting with a 12-hour daily fast and easing into a 16- to 18-hour fast. As Joseph quickly discovered, immediately leaping into a more challenging plan could have unpleasant side effects.

Regardless of which plan you choose, there are also a few caveats. Fasting for longer periods of time when your body isn’t prepared can yield serious consequences.

If you feel lightheaded or weak, eat something. Fasting isn’t a magical "cure-all"; it's simply another tool in your weight-loss and optimal-health arsenal.

Vincent M. Pedre, M.D.
Vincent M. Pedre, M.D.

Vincent M. Pedre, M.D., medical director of Pedre Integrative Health and president of Dr. Pedre Wellness, is a board-certified internist in private practice in New York City since 2004. He completed his bachelor’s degree in Biology at Cornell University before attending the University of Miami School of Medicine and completed his residency in Internal Medicine at the Mount Sinai School of Medicine. He has appeared on the Martha Stewart Show and ABC and is the author of Happy Gut: The Cleansing Program to Help You Lose Weight, Gain Energy, and Eliminate Pain. Dr. Pedre is a clinical instructor in medicine at the Mount Sinai School of Medicine and is certified in yoga and medical acupuncture.

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