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[Expert Guide] 20-Hour Fast Results, Side Effects, and How to

A 20-hour fast is one of the kinds of intermittent fasting numerous individuals try to lose weight and body fat or address certain health concerns. This article is to find its real results, side effects, and step-by-step how-to guide. Also, it’ll find out what you need to eat, drink and avoid and for whom this 20/4 fasting may not be suitable. So keep scrolling to learn it all.

[Expert Guide] 20-Hour Fast Results, Side Effects, and How to
  • A 20-hour fast includes avoiding calorie consumption for 20 consecutive hours and eating in a 4-hour window.
  •  20-hour fasting can have various benefits for the body, including better cardiometabolic health, regulated blood glucose, cell repair and rejuvenation, and weight loss.
  • Despite its benefits, fasting for 20 hours can have some side effects in some individuals, including hunger and cravings, malnutrition, fatigue and low energy, dehydration, headaches and lightheadedness, irritability and mood swings, digestive problems, bad breath, and sleep disorders.
  • The body goes through various physiological changes during a 20-hour fast.
  • 20-hour fasting is generally safe, but some groups of people need to do it with caution.

What Are The Fasting Ratios?

Intermittent fasting can be done in different ratios, and the numbers represent the length of time you can fast each day.

  • 24: 24 hours of fasting includes avoiding calorie intake for a complete 24 hours. Learn more about the 24-hour fast.
  • 20: 20 hours of fasting means avoiding calorie intake for 20 hours and eating for only 4 hours.
  • 18: 18 hours of fasting gives you six hours to avoid and 18 of fasting for a day.
  • 16: 16 hours of fasting allows you to eat in a span of eight hours and avoid calorie intake for 16 hours. The 16/8 fasting schedule is a popular method of intermittent fasting.
  • 22: 22 hours of fasting includes a fasting period of 22 hours with a 2-hour eating window.

Among all these fasting ratios, we’ve focused on a 20-hour fast. Let’s get into details and learn more about it.

What Is a 20-Hour Fast?

The 20-hour fasting, aka 20/4 or Warrior Diet, includes fasting for 20 hours and eating for 4 hours. Let’s review its principles, rules, and benefits and see what we can eat and drink in this kind of intermittent fasting.

20-Hour Fast Rules

Principles and Rules

During the 20-hour fasting period, you can consume non-caloric beverages like water, black coffee, or plain tea, but no solid food, calories, added sugars, or sweeteners are allowed.

After the 20-hour fasting period, you have 4 hours to consume your daily caloric intake. Make sure to eat healthy foods that provide you with the necessary nutrients.

If you regularly consume high amounts of calories and eat slowly, an OMAD diet may also be suitable for you, as it includes consuming only one meal a day.


Recent studies have indicated that fasting for 20 hours a day can have the following benefits:

  • Improving Cardiometabolic Health
  • Regulating Blood Glucose (Blood Sugar)
  • Boosting Cell Repair and Rejuvenation
  • Improving Weight Loss

What Can I Eat on a 20-Hour Fast?

During fasting, no calories are allowed. But out of the fasting period, i.e., in the 4-hour window, you must consume enough nutrients and healthy fats as your body needs them for ketosis.

The best foods you can consume can be chosen from among the following whole foods:

  • Fatty Meats
  • Bone Broth 
  • Organ Meats
  • Full-fat Yoghurt
  • Full-Fat Cheese
  • Cottage Cheese
  • Low-Carb Vegetables

Learn More: What Is Fat Fasting?

Ori Hofmekler (Author of "The Warrior Diet"):

"The Warrior Diet is based on intermittent fasting and controlled overeating. By fasting for 20 hours and eating one large meal at night, you can trigger a unique metabolic state that enhances fat burning, muscle building, and overall health."

What Can I Drink on a 20-Hour Fast?

Zero-calorie drinks, including water, plain tea, black coffee, and herbal tea, are some of the best beverages you can drink during the 20-hour fast.

Learn More: The 18-Hour Fast: Does It Work for Weight Loss?

Is Fasting for 20 Hours Safe?

Intermittent fasting, specifically the 20-hour fast, is generally safe for people without special health concerns. However, its biggest risk is nutrient deficiencies. That’s because your eating time is too short, and it can be challenging to eat a variety of foods in only four hours.

Also, it may not be a good choice for everyone. Although all people who’d like to try fasting need to consult a healthcare professional first, children, the elderly, pregnant or breastfeeding women, people with eating disorders, and those with certain health problems need more caution.

The 20-hour fasting may also involve some side effects, which we’ll cover in the final sections of this article. Now, one of the main questions is whether 20-hour fasting is good for weight loss.

Learn More: The 60-Hour Fast Benefits and Results: Is It Safe?

Is a 20-Hour Fast Good for Weight Loss?

Different clinical studies have found that alternate-day intermittent fasting (IMF) can result in significant weight loss in obese individuals.

Also, an animal study conducted by Juliet Guuthardt et al. in 2015 indicated that intermittent fasting could positively affect fat loss and help maintain lean mass,  increase hypothalamic norepinephrine content, and increase neuropeptide Y gene expression in diet-induced obese male mice.

Based on Guuthardt’s study, weight loss resulting from 20-hour fasting is actually caused by increased beta-oxidation or fat-burning. After some hours of fasting, our body starts to use its fat reserves to produce energy.

In other words, in this situation of low insulin and high norephedrine levels, the body begins to identify and select excess fat stores for fuel.

A 2005 study conducted by Heilbronn et al. indicated that a 20-hour fast has helped some individuals become leaner and more muscular at the same time.

Another study by Mansell, Fellows, and Macdonald in 1990 indicated that fasting can increase metabolic rate by about 14%.

Hatori et al. conducted a study in 2012 and found that intermittent fasting could boost metabolism and fight the obesity epidemic in the US.

Additionally, individual experiences show high levels of satisfaction in intermittent 20-hour fasting. You can start one under the supervision of experts using this 7-Day Ultimate Fasting Course For Fat Loss.

Dr. Joseph Mercola (Natural Health Advocate):

"Intermittent fasting, including the 20-hour fasting window, helps to regulate your hormones, improve insulin sensitivity, and support mitochondrial health."

How Much Weight Can You Lose on a 20/4 Fast?

It’s no secret that the amount of weight lost after a 20/4 fast depends on various factors, from health status and body composition to the diet, overall calorie intake, and many more.

However, some individuals may lose 500 to 1000 calories per day can and lose about 1 to 2 pounds per week. Remember that it’s a rough number, and each individual’s results may vary.

So, to reach this amount of weight loss or more, what happens in our body?

Learn More: 40-Hour Fast: Does It Work? [Benefits and Side Effects]

What Happens to Your Body During a 20-Hour Fast?

During a 20-hour fast, various physiological changes occur so your body can adapt to the absence of calories. Although the effects of fasting for 20 hours differ from person to person, and factors including overall health, activity level, and previous dietary habits affect the results, here are the common changes in the body.

Glycogen Depletion

When you eat a regular diet, your body stores carbohydrates in the liver and muscles as glycogen for energy. During fasting, glycogen is used, the stores get depleted gradually, and the body starts to use other sources (i.e., fat) to provide energy.


By decreasing glycogen, the body starts to use fat for energy, i.e., fatty acids are broken down in the liver to produce ketones as the new energy source and create a metabolic state called ketosis.

Insulin Sensitivity

During the fasting period, insulin is reduced, which increases insulin sensitivity. More insulin sensitivity can positively affect blood sugar regulation and metabolism.


After some hours of fasting, autophagy is triggered. It is a process of removing damaged cells and cellular components, which can boost longevity and prevent diseases.

Hormonal Changes

Fasting can influence different hormones (e.g., growth hormone may increase or the hunger hormone - Ghrelin - may fluctuate), promote fat breakdown, and maintain muscles.

A 20-Hour Fast Restricts Calories

Studies published in the National Institutes of Health (NIH) suggests that intermittent fasting, including the 20-hour fast, can lead to a significant reduction in daily caloric intake, promoting weight loss. Also, studies indicate that fasting can reduce caloric intake by 20-30% without deliberate calorie counting​.

Increased Fat Burning

With no glycogen to use, the body heavily uses fat to provide energy, which means increased fat burning.

Energy Conservation

During fasting, the body slightly decreases energy consumption to conserve energy.

Mental Clarity and Focus

Some individuals report fasting has improved their mental clarity and focus, which can be caused by the production of ketones, which can be used as an alternative fuel for the brain.

Appetite Suppression

Fasting can decrease appetite in some people, which may be because of hormonal changes and the body's adaptation to the fasting state.

Just note that fasting affects each individual differently, and feeling temporary discomfort, hunger, or fatigue during the fasting period is also common. To alleviate such feelings, you can drink enough water and consume electrolytes.

Now that we know the process and the changes that may occur in our body, it’s time to learn the steps of actually doing a 20-hour fast.

Ori Hofmekler (Author of "The Warrior Diet"):

"Our ancestors ate this way for thousands of years. Fasting during the day and feasting at night is the most natural and effective way to achieve optimal health and fitness."

How to Do 20-Hour Fasting?

  • Step 1: Choose Your Fasting Window: Choose your 4-hour eating period. For example, you can eat between 2:00 PM and 6:00 PM each day.
  • Step 2: Start Gradually: If it’s your first time trying fasting, start with a 12- or 16-hour fasting and gradually increase the fasting period in the coming days to reach 20 hours.
  • Step 3: Stay Hydrated: It’s necessary to drink enough water. You can also drink plain tea or black coffee (without sugar or cream) to stay hydrated, curb hunger, and even provide some energy.
  • Step 4: Plan Your Meals: Before starting fasting, plan your meals to avoid wasting time in your 4-hour eating period. Balance the meals with whole foods and ensure they contain various nutrients.
  • Step 5: Break Your Fast Mindfully: Avoid large meals after the fasting period and start with a small, light meal to avoid pressuring your digestive system.
  • Step 6: Listen to Your Body: Pay attention to your body's signals, especially about hunger and fullness. Eat enough and avoid overeating or undereating.
  • Step 7: Maintain a Consistent Schedule: Fix the 4-hour eating period and start the fasting period at a specific time. Sticking to a fixed schedule helps your body adapt sooner.
  • Step 8: Monitor Your Progress: Stay alert to your body’s reactions and changes during the fasting period to see if you need to improve anything.
  • Step 9: Be Patient and Flexible: Your body needs some time to adapt to the fasting situation. Be patient and modify your plan and schedule to meet your needs.
  • Step 10: Seek Professional Guidance: It’s necessary to consult a healthcare professional if you have any medical conditions or if you're unsure if fasting is suitable for you.

To reach fast results, some consider 20-hour fasting for a whole week or month. Let’s take a look at each one and see how they work.

Learn More: Carnivore Diet Fasting: Types, Food List & Meal Plan

A 20-Hour Fast Improves Diabetes

A study published in the journal Cell Metabolism found that intermittent fasting improved insulin sensitivity, reducing the risk of type 2 diabetes by enhancing the body's response to insulin​.

20-hour Fast One Week

Some people choose to try 20-hour fasting every day for a week. They do so for various health and fitness goals. Let’s learn more about this kind of intermittent fasting.


  • Fast for 20 hours each day.
  • Eat in the 4-hour eating period each day.

What to Eat?

  • During the 20 hours of fasting period, only non-caloric beverages like water, black coffee, or plain tea are allowed.
  • During the 4-hour eating period, you’d better eat nutrient-dense, whole foods, including lean proteins (chicken, turkey, fish, tofu, legumes, or lentils), healthy fats (avocados, olive oil, nuts, and seeds), complex carbohydrates (sweet potatoes, brown rice, quinoa, whole-grain bread), fruits and vegetables.

What to Drink?

Out of the fasting period, you can drink as you normally do. But inside the fasting period, you can stay hydrated only with non-caloric drinks, including:

  • Water
  • Black Coffee (No Sweetener or Additives)
  • Plain Tea


  • Weight Management
  • Fat Burning
  • Improved Insulin Sensitivity
  • Better Mental Clarity
  • Cellular Repair
  • Simplicity

CAUTION: Consider consulting a healthcare professional before making drastic changes to your eating habits to avoid possible health problems.

A 20-Hour Fast Improves Health

Studies published in NHLBI shows that fasting can help regulate hormones such as insulin, leptin, and ghrelin, which play crucial roles in hunger, metabolism, and overall energy balance​​.

20-Hour Fast One Month

Muslims who do fasting for religious reasons almost do this kind of fasting. Others try this kind of intermittent fasting for health and fitness reasons. This kind of fasting involves fasting for 20 hours a whole month.


  • Fast for 20 hours every day consecutively for one month.
  • Eat only during the 4-hour eating period.

What to Eat?

During the 4-hour eating period, you can eat the following whole and nutritious foods:

  • Lean Proteins: chicken, turkey, fish, tofu, legumes, or lentils
  • Healthy Fats: avocados, olive oil, nuts, and seeds
  • Complex Carbohydrates: sweet potatoes, brown rice, quinoa, whole-grain bread
  • Fruits and Vegetables

What to Drink?

In the 4-hour eating period, you’re allowed to drink as you normally do. It’s better to watch your calorie intake, though. During the 20-hour fasting period, you can only consume zero-calorie beverages, including:

  • Water,
  • Black Coffee, and
  • Plain Tea.


  • Fat Melt,
  • Weight Management,
  • Better Insulin Sensitivity,
  • Improved Mental Clarity,
  • Cellular Repair,
  • Simplicity and Discipline

It’s worth noting that extended fasting may not be suitable for everyone, especially those involved with certain medical conditions. Pregnant or breastfeeding women or individuals with eating disorders must consult a healthcare specialist first.

A 20-Hour Fast Is Flexible

A 20-hour fast involves eating one meal within a 4-hour window, allowing flexibility in meal timing to fit various lifestyles and preferences​ (National Institutes of Health (NIH))​.

Foods to Eat After a 20-Hour Fast

After a 20-hour fast, break your fast gradually with small and easy-to-digest meals to let your digestive system adapt. You can consume the following foods.

  • Bone Broth: It contains minerals, collagen, and amino acids, helping gut health.
  • Fruits: Watermelon, cantaloupe, or berries help with proper hydration and gently raise blood glucose levels.
  • Vegetables: Various cooked or steamed vegetables, such as spinach, broccoli, or carrots, are good options as they contain vitamins and minerals and are easy to digest.
  • Proteins: Lean proteins like grilled chicken, turkey, or fish provide essential amino acids for muscle repair and growth.
  • Healthy Fats: Avocados, nuts, olive oil, and animal fats provide healthy fats needed for satiety and nutrient absorption.
  • Yoghurt: Plain, unsweetened yogurt with probiotics is good for gut health and digestion.
  • Complex Carbohydrates: Whole grains like quinoa, brown rice, or whole-grain bread are good sources of sustained energy.
  • Eggs
  • Smoothies
  • Water

Foods to Avoid After a 20-Hour Fast

The main thing you need to avoid after a 20-hour fast is a heavy, sugary meal. It’s better for your digestive health to avoid the following foods after a 20-hour fast.

  • Highly Processed Foods: Fast food, chips, cookies, and sugary snacks rapidly increase blood sugar and may lead to digestive discomfort.
  • Fried Foods: Fried foods are usually greasy, heavy, and not so easy to digest. That’s why they’re not ideal to break a fast.
  • High-Sugar Foods and Beverages: Foods and drinks like soda, candies, and pastries also increase blood sugar levels too fast.
  • Alcohol: Consuming alcohol right after a 20-hour fast and empty stomach can cause dehydration and digestive problems.
  • Spicy Foods: Spicy foods can irritate the digestive system, especially after fasting for 20 hours.
  • Too Much Dairy: Too much dairy to break a fast may lead to digestive discomfort.
  • Caffeine in Excess: Too much caffeine can irritate the stomach and negatively impact digestion.
  • Artificial Sweeteners
  • High-Fat Meals: Very high-fat meals that are heavy on the stomach are not good options after 20-hour fasting.

20-Hour Fast Weight Loss Results

The findings of a 2020 study conducted by Cienfuegos et al. - researchers of the University of Illinois, Chicago - indicated that 18-hour and 20-hour fasting were both effective for weight loss by decreasing the participants’ eating portions without counting calories.

This study revealed that after 10 weeks into the research, both groups who fasted for 18 and 20 hours lost about 3% of their starting weight.

Findings of recent research show that no matter which fasting period you choose, lower calorie intake caused by fasting simply helps weight loss.

Also, the participants of this study witnessed improvements in insulin resistance and oxidative stress by the end of the research, implying that 20-hour fasting can positively affect overall metabolic health and even resistance to chronic disease.

In addition, we know that various factors affect the weight loss results of a 20-hour fast in each individual. Factors such as metabolism, activity level, starting weight, and overall dietary habits are among the most important ones.

Additionally, the rate of weight loss may decrease over time as the body adapts to the fasting schedule.

Having said that, the general results of fasting for 20 hours for one week and one month can be as follows.

One Week Results

Individual factors aside, weight loss after one week of 20-hour fasting may be around 1 to 3 pounds (0.5 to 1.4 kilograms) for most individuals.

Some may experience more significant initial weight loss because of a reduction in water weight and stored glycogen in the muscles and liver.

One Month Results

After one month of 20-hour fasting, you may witness more significant results than one week. On average, you may lose about 4 to 10 pounds (1.8 to 4.5 kilograms) in a month.

Also, if you need help starting fasting, you can simply use this 7-Day Ultimate Fasting Course For Fat Loss.

20-Hour Fast Side Effects

Despite its significant positive effects, the 20-hour fast may contain some side effects for some people. Talking about side effects doesn’t mean they’ll necessarily happen to you, but you need to know them to take necessary measures and avoid problems.

Here are the most common side effects of 20-hour fasting.

Hunger and Cravings

A 2018 study conducted by Sundfør, Svendsen, and Tonstad on 112 participants divided into two groups based on the amount of calorie intake revealed that the group with 400 or 600 calories consumed on 2 nonconsecutive days each week for 1 year showed higher hunger scores than the group consuming a low-calorie diet with continuous calorie restriction.

Another study conducted by de Toledo et al. in 2019 on 1422 participants trying fasting for 4–21 days showed that hunger symptoms only appeared in the early days of fasting, and the body gets adapted to the new situation after some days.


If you follow the above-mentioned steps to do a 20-hour fast, you may not experience any malnutrition. However, if you do fasting for long periods without paying attention to providing enough nutrients in the eating windows, you may face malnutrition.

A 2019 study conducted by Grajower and Horne confirmed this point and found that poorly constructed energy restriction diets can lead to the same negative results.

Fatigue and Low Levels of Energy

Different studies conducted by Harvie and Howell (2017) and Finnell et al. (2018) indicated that fatigue and feeling low energy are among the common side effects of intermittent fasting.

However, other studies by Nugraha et al. (2020) showed that intermittent fasting can lower fatigue, especially when the body gets used to the fasting situation.

Headaches and Lightheadedness

A review by Allaf et al. published in 2021 revealed that in 18 different studies on people doing intermittent fasting, mild headache was among the side effects.

According to a 2010 research by Torelli and Manzoni, fasting headaches cause pain in the frontal area of the brain and are mainly caused by low blood sugar and caffeine withdrawal.

Irritability and Mood Swings

Based on a study conducted by Mongraw-Chaffin in 2019, mood swings and irritability can happen during intermittent fasting due to decreased levels of blood sugar [1].

Another study by Tinsley et al. (2019) supports these findings, indicating that hypoglycemia during fasting can create feelings of irritability, anxiety, and poor concentration [2].

A 2016 study by Watkins and Serpell conducted on 52 women showed that the participants were more irritable after an 18-hour fasting period than a non-fasting period.

Despite irritability, the participants of this study experienced a higher sense of achievement, pride, and self-control after the fasting period compared to the start of the study [3].

Digestive Problems

Based on different studies by de Toledo et al. (2019) and Cui et al. (2020), some individuals may experience constipation, diarrhea, nausea, and bloating while doing intermittent fasting [4].

Tinsley et al. (2019) have found that decreasing food consumption, changes in eating habits, and the body’s effort to adapt to the new situation may cause these digestive issues [5].

Bad Breath

It’s no secret that avoiding food for long hours, like fasting for 20 hours, can cause bad breath. This point has also been confirmed by different studies conducted by Gonçalves et al. (2019), Mogilnicka et al. (2020), and Bovey et al. (2018).

According to a 2015 study by Anderson, bad breath caused by a 20-hour fast happens because our body starts to burn fat and produce Acetone, which is a by-product of fat metabolism.

Another study by Kapoor et al. (2016) shows that dehydration, which can happen during a 20-hour fast, can also cause bad breath.

Sleep Disorders

Phillips (2019) has conducted research on the effect of intermittent fasting on sleep and found that some individuals may experience sleep disorders like insomnia or over-sleepiness after fasting.

Another 2019 study by de Toledo et al. indicated that 15% of 1422 participants experienced sleep disorders because of intermittent fasting.

However, other studies conducted by Kalam et al. (2021) and Cienfuegos et al. (2021) reported there was no significant relation between intermittent fasting and sleep disorders. 

Who Should Avoid Doing 20-H Fasting?

Generally, everyone can try fasting, but some groups seriously need to consult a healthcare professional before doing a 20-hour fast because they’re in special situations like the following groups.

  • Pregnant and Breastfeeding Women
  • Children and Adolescents
  • Those with Eating Disorders
  • Those with Underlying Health Conditions
  • Those with a History of Nutrient Deficiencies
  • Underweight Individuals
  • Those on Specific Medications
  • Highly Active Individuals


A 20-hour fast can be a good option for some individuals looking for fast fat melt, weight loss and those with specific health concerns in need of quick changes.

However, to decide if intermittent fasting, especially a 20-hour one, is suitable for you or to determine the weekly and monthly schedule, it’s wise to consult a healthcare professional or dietitian first to avoid possible health problems.


In the following subsections, you can find answers to the most frequently asked questions about 20-hour fasting.

Is a 20-Hour Fast Better Than 16 Hours?

Some say fasting longer is better as insulin gets lower as time passes, yet some studies show that 16-, 18-, and 20-hour fasting periods are equally effective. Anyhow, your body may answer better to one of these methods. Consulting a healthcare professional will help you decide the best one.

Is a 20-Hour Fast Better Than an 18-Hour Fast?

According to recent studies, if you consider losing weight, an 18-hour fast can be as effective as a 20-hour fast.

Can I Take Vitamins When Fasting?

Taking vitamins when fasting depends on your body's needs and is generally accepted. But taking some vitamins like gummy ones or those containing calories can break your fast. How to Get Vitamin C on Carnivore Diet? [Best Meat to Avoid Scurvy]

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