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The 20 Hour Fast: Benefits and How To

By Liam McAuliffe Updated on

In recent years, health-conscious people have turned to intermittent fasting in droves. While the  16/8 intermittent fasting is most popular, there are more intensive, and possibly more rewarding approaches.  The 20 hour intermittent fasting regimen is considered the next level of intensity. 

In this article we’ll explore the benefits, drawbacks, and practical aspects of the 20 hour fast. 

20 Hour Fast History

The 20 hour fast came into modern popularity in the early 2000s as a key part of  “The Warrior Diet.”

This diet was created by Ori Hofmekler based on his personal interpretation of living like an ancient warrior. The fast was part of a regimen that included intensive training sessions and a limited ‘overfeeding’ window. [1] Hofmekler’s original protocol suggested exercise, small snacks during the day, and a four hour feast.  

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How to 20 Hour Fast

Since then, 20 hour intermittent fasting has evolved. In recent years, Hofmekler’s Warrior Diet has been reimagined as having a stricter 20-hour fasting period.

The 20 hour fast is also referred to as the 20:4 diet. It’s simple: just fast for 20 hours, then eat within 4. 

Coffee and tea are allowed during the fasting period, but no other calorific drinks or snacks are permitted. 

If you’re a slow eater with high-calorie requirements, then this diet will look very similar to the OMAD diet

For most people on the 20 hour fast, an ideal eating window will take place within daylight hours. The benefits of eating in alignment with your circadian rhythm are so powerful that they’ve given rise to the circadian rhythm fasting approach to IF. 

20 Hour Fast Benefits

Now that you’re familiar with the basics of 20 hour intermittent fasting, let’s take a closer look at its various health benefits. These include:

  • Improved body composition
  • Improved insulin sensitivity
  • Improved cellular health 
  • Increased HGH production
  • Increased BDNF production
  • Reduced inflammation
  • Improved productivity

20 hour fast for improved body composition

Many people who begin a 20 hour intermittent fasting protocol find that excess fat “melt” away. More often than not, this usually happens without the hunger pangs or lethargy associated with more ‘conventional’ weight loss methods. 

What makes such painless weight loss possible?

It all has to do with something called increased beta-oxidation — a.k.a. fat-burning. After several hours in the fasted state, your body begins metabolizing its own fat reserves for energy. In this state insulin is low and norephedrine is high, setting the stage for your body to identify and select any excess fat stores for fuel. [2

20 hours of fasting is also sufficient to trigger your liver to produce energy molecules called ketones. When levels of ketones in your blood become elevated, you’re in full fat-burning mode called ketosis

Cycling in and out of a ketogenic state each day is an ideal way to maximize your body’s insulin sensitivity. With greater insulin sensitivity, when you do eat, the calories you consume will be directed towards muscle cells. 

Fasting 20 hours a Day For Weight Loss

Some people find themselves getting leaner and more muscular at the same time during 20 hour intermittent fasting. [3]

This has to do with an increase in your metabolic rate–the amount of calories your body burns at rest. One study showed that fasting can boost your metabolic rate by as much as 14%. [4]

Another study published in the journal Cell, found that IF was so effective at increasing metabolism that it could be the antidote for America’s obesity epidemic. [5]

Blending the 20 hour fast with Circadian Rhythm Fasting may further increase your metabolism by synchronizing your food intake with the faster metabolic rate that occurs during the daytime. [6]

20 Hour Fast for Improved Insulin Sensitivity

Improving insulin sensitivity is tied to other benefits that go above and beyond weight loss and muscle gain. 

Most people on the Standard American Diet suffer from chronically high insulin levels and associated risks of  heart disease, type 2 diabetes, and  PCOS.  Insulin resistance can even lead to problems with cognitive function and memory. [7

Improving insulin resistance with 20 hour intermittent fasting can result in:

  • Clearer, smoother skin
  • Reduced water retention
  • Reduced diabetic symptoms
  • Reduced risk of heart disease
  • Reduced PCOS/more regular periods
  • Clearer thinking and improved mood

20 Hour Fast For Cellular Health

Death and rebirth are a part of life — even on the cellular level. 

20-hour fasting can catalyze the death/rebirth cycle, which, when it comes to cells, is known as autophagy. Just like a car needs the occasional spare part or oil change to run at its best, your body needs to undergo certain maintenance processes to maintain a high level of functionality. [8

Fasting for 20 hours a day helps your body heal, repair, and recycle needed components. It also gives cells in the digestive tract time to rest and restore their tight junctions, reducing the likelihood of leaky gut

During the fasting period your body’s production of stem cells skyrockets. These special cells are associated with increased energy, reduced cellular wear and tear, and other known markers of youthfulness. [9

Lead researcher Dr. David Sabatini concluded that his MIT-based study “provided evidence that fasting induces a metabolic switch in the intestinal stem cells, from utilizing carbohydrates to burning fat.” Further, this switch “enhanced [cell] function significantly.” [10]

diagram of autophagy process eliminating disease

Image: Nature

20 Hour Fast for Increased HGH

Higher serum HGH (human growth hormone) levels are associated with improved muscle mass, better memory, reduced water retention, and more. HGH levels tend to inversely correlate with cortisol levels, meaning they can rescue the body from a high-stress state. [11

One study involving 200 people found that fasting for 24 hours increased HGH by a factor of 20 in men and 13 in women. [12

This exponential change explains why IF is so renowned for triggering sudden changes in mood and appearance. 

20 Hour Fast for Increased BDNF

BDNF-1, or brain-derived neurotrophic factor, is another important endogenous hormone. While HGH is correlated with increased muscle cell production, BDNF-1 is associated with increased brain cell creation. Neuropsychiatrist John J. Ratey aptly calls it “Miracle-Gro for the brain.”

Higher levels of BDNF are correlated with faster learning and neural pathway development. Conversely, lower levels of BDNF are correlated with degenerative diseases like Parkinson’s and Alzheimer’s. [13]

It’s not yet known why fasting boosts BDNF-1, but the connection makes logical sense from an evolutionary perspective: In ancient times we would’ve needed to think faster (be better, more creative hunters and gatherers) in times of caloric scarcity, and it appears BDNF is the neurological ingredient that helps us to rise to the occasion. 

20 hour Fast for Reduced Inflammation 

Chronic inflammation has recently been implicated as the driving force behind virtually all chronic diseases — and fasting can  be an antidote.  Across the globe 3 out of 5 people die from chronic inflammatory diseases including heart disease, stroke, respiratory diseases, cancer, obesity, and diabetes. [5] [6] [7]

In America alone, 60% of the population has at least one chronic inflammatory condition. 2

Thankfully, 20 hour intermittent fasting can reduce inflammation in a couple of ways: It can help  ‘seal up’ the digestive tract, reducing gut inflammation as well as stopping inflammatory pathogens from entering the bloodstream. And it can trigger your body to recycle damaged cells that would otherwise trigger inflammatory responses. 

20 hour intermittent fasting may be a holistic preventative measure for people with rheumatoid arthritis, Sjogren’s, MS, and other inflammatory disorders. [14][15

20 Hour Fast for Improved Productivity

Fasting for 20 hours a day often leads to marked increases in productivity. 

For one, meal prep and cleanup time is dramatically reduced. You can skip your lunch break at school or work and instead spend that time listening to a podcast, going for a walk, or reading an informative book. There’s no longer any need to resort to unhealthy fast-food meals, either. 

Instead of worrying about meals, 20 hour intermittent fasting frees you up to enjoy your work and cherish your play.

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20 Hour Fast: Potential Drawbacks

While 20 hour intermittent fasting can provide numerous benefits, there are possible drawbacks for women and athletes. 

Women on 20 Hour Fast

The calorie restriction seen with 20:4 fasting affects a women’s hypothalamus differently men’s. 

The hypothalamus is in charge of releasing luteinizing hormone (LH) and the follicle-stimulating hormone (FSH). Disrupting these two hormones can damage a woman’s fertility rates, ovary size, and bone health. [17

Check out our dedicated intermittent fasting for women article for more details. In the meantime, women who are interested in IF may want to opt for 16/8 or even 12/12 IF instead. 


Athletes who train at a high level often have correspondingly high calorie requirements. These nutrition demands make 20:4 fasting virtually impossible. There’s no way Michael Phelps could’ve eaten his reported 12,000 calorie daily diet within a four hour window.

People with Eating disorders

For people with a history of binge eating, the ‘feasting window’ of 20:4 fasting can feel eerily similar to an out-of-control binge. 

At-risk individuals may become obsessive about their eating window, or feel guilty after eating large amounts of food all at once. 

Other Groups

Children, pregnant women, and the elderly may also want to shy away from 20:4 intermittent fasting. 

If you have a history of hormonal or endocrine problems, you may also want to consider 16/8 intermittent fasting or gentler approaches. 

Foods to Eat On a 20 Hour Fast

Since a 20 hour fasting only allots a four-hour eating window, the foods you eat during this intermittent fasting window are crucial to refueling your cells and getting the most out of IF. 

Remember that your body will be converting fat into ketones, so it’s important to fuel your body with plenty of healthy fats and fatty whole foods. 

The best choices include nourishing whole foods like: 

It’s important to note that intermittent fasting is not necessarily about restricting calories. It’s about optimizing your metabolic health by restricting the time in which you consume them.

With this in mind, a well-formulated intermittent fasting meal plan will provide 1300-2000 calories per day for most people.

What does this look like?

Here’s a 3-day sample menu to keep you nutritionally-replete while 20-hour fasting:

Ribeye steak and Eggs scrambled with pancetta

new york strip steak on slate

12 oz Ribeye Steak (untrimmed)1200
2 Eggs (scrambled)200
1 avocado (top eggs)322
½ cup blackberries30
2 oz heavy cream (to top berries)202

Short Ribs and Broccoli Rabe

12 oz beef short ribs1036
2 deviled eggs400
6 oz broccoli rabe12
2 tbsp olive oil239
½ cup strawberry35
1 cup cottage cheese213

Lamb Chops and Asparagus

Rack of lamb1320
6 oz asparagus (sauted in tallow)120
2 tablespoons tallow230
1.5 oz. dark chocolate240

Bone broth 

Bone broth may seem like a new trend, but it’s been around for thousands and thousands of years. Hippocrates and other early physicians recommended bone broth for digestive and overall health. 

Recent research reveals that bone marrow (a primary ingredient in bone broth) is packed with nutrients including trace minerals, fat-soluble vitamins, calcium, gelatin, and more. 

Other research shows that bone broth’s gelatin content allows it to protect the digestive tract from irritation and inflammation. [19][20

Organ meats

Organ meats are another brand of natural superfoods. They’ve been a central part of the human diet for the entirety of our existence — with the sole exception of the last 100 years. 

From an evolutionary perspective, organ meats are among the very best foods to eat during a restricted eating window. 

Organ meats are also uniquely helpful in the sense that they provide targeted nutrition. In other words, eating the liver of a healthy animal can nourish your own liver. Eating the heart of a healthy animal supplies heart-healthy antioxidants to your own heart. This “like-supports-like” quality can enhance the benefits of 20 hour fasting. [21


For those who don’t have access to quality organ meats, supplements can be a powerful alternative. Beef organ supplements feature the same vitamins, minerals, and rare peptides found in their fresh counterparts. 

Full Fat Dairy 

Full-fat dairy products are extremely high in calories, vitamins, minerals, and more. And they’re usually very high in calories, which is perfect for a 20 hour fast.  

You can view a list of the best full-fat cheeses here

20 Hour Fast: The Takeaway

If you want to lose weight, focus better, and reduce chronic inflammation, a 20 hour fast may be right for you.

20 hour intermittent fasting is likely closer to our evolutionary diet than most of the so-called ‘healthy’ diets that are trendy today. After a successful hunt, our ancestors feasted. Then they fasted until the next successful hunt. From this perspective, our metabolism is optimized for a fasting and feasting cycle. 

20 hour intermittent fasting is considered a more intensive IF plan. As with any fasting protocol, it’s important to listen to your body and make adjustments when necessary. There are numerous gentler approaches to try if the 20 hour fast feels too extreme.

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