Intermittent fasting—or fasting for 12 to 36 hours on regular intervals—is all the rage right now. But when you pass the 36-hour mark, you leave intermittent fasting behind and enter the realm of extended fasting. 

Extended fasting, also called periodic fasting, is nothing new. Humans have gone long stretches without food since time immemorial; during fasts, our ancestors burned body fat to power their hunting and gathering.

Today, a growing body of research suggests that longer fasts are safe and beneficial for most people. However, this doesn’t mean that extended fasting is for everyone. Or that it’s easy. 

In this article, you’ll learn the basics of extended fasting: how it works, why it’s beneficial, and how to do an extended fast. Keep reading. 

How Extended Fasting Works

An extended fast refers to a long period of time—typically over 36 hours—without eating; during this time, water, electrolytes, and non-caloric beverages are allowed. Think of extended fasting as a supercharged version of intermittent fasting. The effects are more or less the same, but amplified due to longer periods without food. 

The main effect of fasting is on your metabolism, or how you use energy. Fasting flips a metabolic switch, which moves you from sugar-burning (or fat-storing) mode to fat-burning mode. 

In fact, fat is stored in the body for this very reason: to provide energy when food is scarce. Believe it or not, with enough body fat, you can fuel yourself for a shockingly long time. Case in point: in the 1970s, researchers fasted a morbidly obese man for 382 days. He entered the fast at 456 pounds and exited at 180 pounds with no serious side effects because body fat fueled his extended fast. 

Losing excess fat is just one reason extended fasting is good for you. Let’s explore the top four reasons to fast here:

Benefits of Extended Fasting

Why practice extended fasting? Consider the following benefits:

  1. Weight loss. As you might imagine, long periods of fasting stimulate weight loss. One 2019 study from PLOS One, for example, followed 1,422 volunteers on an extended-fasting program over the course of a year. When the study was over, the participants had lost significant weight.

It is worth noting that most weight loss early in the fast will be water weight, especially if you’re not fat-adapted. This means that you will most likely regain some weight when normal eating is resumed. But as your metabolism adapts to using fat for energy, more of this weight loss will be fat loss. 

  1. Lower blood sugar and insulin levels. High blood sugar and insulin levels are the defining features of type 2 diabetes, a serious metabolic disorder affecting about one in 10 Americans. All types of fasting lower blood-sugar levels, but longer fasts move the needle further and faster. Dr. Jason Fung, bestselling author of The Complete Guide to Fasting, has successfully used both intermittent and extended fasting protocols to reverse type 2 diabetes in his Toronto clinic.
  2. Ketosis. As mentioned above, fasting lowers insulin levels. This, in turn, signals your liver to start burning fat and producing ketones. Ketones have many functions, but they primarily serve as brain fuel. In one study, higher ketone levels were linked to better mental performance in older adults. This means that fasting will incite ketosis, which is likely to improve cognitive function.
  3. Autophagy. In the absence of nutrients (i.e. during a fast), your cells activate a recycling program called autophagy. Think of autophagy as an anti-aging cleanup mechanism: the old, damaged cell parts go in, and refreshed parts come out. All things equal, longer fasts activate more autophagy than shorter ones. Though we can’t effectively measure autophagy in humans (and you won’t feel it), autophagy unquestionably helps your cells stay vital and healthy. So, do an extended fast and increase your cell recycling and refreshing!

Now that you know the benefits of fasting, let’s explore the process. 

Building Up To Extended Fasting

If you’re new to fasting, it’s best to start slow. Don’t start with a two-day fast. Instead, baby-step your way towards fasts of two or more days by starting with overnight fasts. 

An overnight fast of 14 to 16 hours has been shown to lower blood sugar, improve insulin function, and enhance your wake/sleep cycle. This style of fasting typically includes two meals per day—one at 12 PM and one before 8 PM or one at 9 AM and 5 PM, for example. After a couple of weeks of overnight fasting, you can graduate to intermittent fasts of 18 to 24 hours. 

These shorter fasts help your body fat-adapt, making extended fasts—which generally range from two days to about a month—much easier. 

Another tip to help you access body fat for fuel? Eat a ketogenic diet before and after your fast. Keto, like fasting, helps lower insulin levels, priming your cells to burn fat instead of sugar. 

Breaking Your Extended Fast

When you fast for days on end, your stomach shrinks. To prevent indigestion, it’s important to not gorge yourself the first meal back. Instead, you should think small. 

Make your first meal a small one, such as a protein shake or a few hundred calories of lean meat. Protein is not only easier to digest than fat, but also it switches off muscle catabolism (muscle loss). After your mini-meal, wait about an hour before eating a regular-size meal that’s rich in healthy fats like olive oil, avocado oil, or coconut oil. These fats will help you stay in ketosis. 

A final tip is to eat iodine-rich foods (like seaweed, shrimp, tuna, and eggs) following a fast to support the production of your thyroid hormones T3 and T4. This tip is especially important for women, who generally need more thyroid support while fasting. 

For more info on safely breaking a fast, watch our video featuring celebrity health coach Thomas DeLauer. 

Tips For Extended Fasting Success

If you’re planning on fasting for over 36 hours, these tips will help you succeed:

  • Take electrolytes. Insulin levels drop during a fast, causing increased sodium loss through urine. Taking potassium, magnesium, and phosphorus while fasting can also help prevent refeeding syndrome, in which crucial minerals are depleted to rebuild glycogen, protein, and fat in your body following a fast. If you don’t want to supplement electrolytes, drink bone broth. It’s an electrolyte-rich superfood. Enjoy a cup of bone broth twice a day during longer fasts. Though it has a bit of protein, it shouldn’t meaningfully interfere with your fasts.
  • Hydrate. Fasting has a diuretic effect, meaning you lose fluids quickly. Replace those fluids with noncaloric, unsweetened, non-dairy beverages like coffee, tea, bone broth, and—yes—water.
  • Ride out the hunger. Hunger doesn’t increase the entire time you fast, but rather it ebbs and flows. Most people get hungriest on the second day of an extended fast. After that, it gets easier.
  • Track your fast. Consider an app like the LIFE Fasting Tracker to make a fasting plan, track key metrics, and keep yourself accountable.
  • Fast with others. This helps you stay motivated. Plus, it’s fun to share metrics like hours fasted, ketone levels, and glucose ketone index (GKI) measurements.
  • Monitor ketone and glucose levels. As you get deeper into a fast, your ketone levels should rise and your glucose levels should fall. This is an indication that your metabolism is adapting properly. Use an accurate at-home device like the Keto-Mojo meter to track your ketones, your glucose, and your GKI. See this article on glucose levels and this article on ketone levels for more info on these metrics. 
  • Plan your first meal carefully. If you don’t break your fast wisely, your gut won’t be happy. See this video for tips.

Is Extended Fasting For Everyone?

Most people can likely handle extended fasting without significant problems. Out of 1,422 non-obese participants in the aforementioned fasting study, less than one percent had adverse effects. That said, these fasts were conducted in supervised clinical settings. You should always consult with your primary care provider before trying dramatic dietary changes.

In some cases, supervision is recommended, especially for those with type 2 diabetes. Many diabetics are on drugs—like insulin and metformin—which can push blood-sugar levels dangerously low during a fast if these medications aren’t properly adjusted. 

The following groups should avoid extended fasting altogether:

    • Underweight people
    • Children
    • Anyone with an eating disorder

Finally, hunger is normal while fasting. But if you’re feeling shaky, dizzy, or weak, you may be experiencing hypoglycemia (low blood sugar). That’s a signal to break the fast. 

If you’re considering extended fasting, you’ll want to do your homework to get things right. Your first assignment after reading this article? Read The Complete Guide to Fasting by Dr. Jason Fung.

The Final Word

When you fast for 36 hours or more, you enter into the realm of extended fasting. The benefits of this practice include weight loss, lower blood sugar, ketosis, and enhanced autophagy.

To help your body adapt to using body fat for energy, start with shorter fasts and work your way up to longer fasts. When breaking your fast, eat a small protein-rich meal, then wait 60 minutes and have a regular meal with plenty of healthy fats.

Taking electrolytes, drinking bone broth, and tracking your fast will help you succeed during longer fasts. Finally, extended fasting is safe for most people, but certain groups (like type 2 diabetics) should be careful and may need supervision. Regardless, you should always check with your primary care provider before making dramatic changes to your diet. 

Keto-Mojo is a participant in some affiliate programs and some of the links above will generate a small commission if you make a purchase through a product link on our site. This is at no cost to you and all proceeds go directly to the nonprofit Ketogenic Foundation [501(c)3] to assist with their mission funding education and research into the ketogenic diet and lifestyle. Keto-Mojo in no way profits from these links.



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