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24-Hour Fast: Benefits, Side Effects and How to [ULTIMATE ARTICLE]

24-hour fasting, a kind of intermittent fasting includes avoiding calories for one whole day. Although it may sound challenging to avoid food for 24 hours, it’s proved to have various potential benefits, including weight loss, improved metabolic health, and enhanced mental clarity. To help you make informed decisions about the 24-hour fast and decide if you should try it, we have gathered all you need to know in this all-in-one guide. Let’s read!

24-Hour Fast: Benefits, Side Effects and How to [ULTIMATE ARTICLE]
  • 24-hour fasting is a kind of intermittent fasting where you avoid consuming calories for one whole day.
  • Potential benefits of a 24-hour fast include weight loss, improved insulin sensitivity, cellular repair, heart health, and enhanced brain function.
  • You’re allowed to drink non-caloric fluids during fasting, such as water, black coffee, herbal tea, and plain tea.
  • 24-hour fasting can be effective for weight loss and improving health biomarkers, but results vary in individuals.
  • It’d be better to consult a healthcare professional before trying fasting.

What Is a 24-Hour Fast?

Simply put, a 24-hour fast is a kind of intermittent fasting where you need to avoid food and beverages containing calories for a continuous 24 hours.

Different versions of 24-hour fasting can have differing rules, but generally, only non-caloric fluids like water, black coffee, or herbal tea are allowed during the fasting period.

To learn more about the 24-hour fast, we need to know its principles, rules, benefits, what to eat, and what to drink. Let’s read about them in the following subsections.

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

Principles and Rules

Let’s learn some basic facts about the 24-hour fast.

  • Duration: A 24-hour fast usually takes one whole day.
  • Calorie Restriction: 24-hour fasting aims to decrease your overall calorie intake to promote fat melt and weight loss.
  • Nutrition before and after the Fasting Period: To provide enough nutrients and ensure adequate calorie intake, you must have balanced and nutritious meals before and after the fasting period to compensate calorie restriction period.
  • Adaptation: It may be challenging for some people to adapt to the rules and situations of 24-hour fasting. Challenges may include initial discomfort, hunger, or irritability. However, reports indicate that the human body adapts to fasting gradually, making it easier and easier.
  • Fasting Schedule: 24-hour fasting has different versions. Some fast from dinner to dinner, while others do it from breakfast to breakfast. Also, some try it once a week, but others do it two or three times per week. Timing can vary based on personal preference and lifestyle.
  • Safety: 24-hour fasting is generally considered safe for most healthy adults. However, not everything is suitable for everybody, and you need to consult a healthcare professional before trying any kind of fasting. So, if you’re pregnant or breastfeeding, have medical conditions, or taking specific medications, you’d better talk to a professional first.
  • Hydration: It’s necessary to drink enough water and stay hydrated during fasting.
  • Breaking the Fast: After completing a 24-hour fast, you must restart consuming food gradually to avoid discomfort or digestive issues. Start with small, easily digestible meals or snacks.
  • Listening to the Body: Needless to say, you must listen to your body's signals during the 24-hour fast. In case of feeling unwell, excessive hunger, or any discomfort, it's better to break the fast and consult a physician or nutritionist.

Learn More: What Is Fat Fasting? Carnivore Method, Meal Plan, and Food List

Dr. Jason Fung (Nephrologist and Fasting Expert):

"Fasting is not about starving yourself. It’s about voluntarily abstaining from food for a period of time to allow your body to heal and rejuvenate."

A 24-Hour Fast Benefits

Here are the main benefits of a 24-hour fast. Just note that you need to consider these benefits in line with your own status, goals, and needs. Not everything is suitable for everyone, so must try to choose the best methods that suits your situation.

Weight Loss

Different reports have indicated potential weight loss effects of intermittent fasting, including 24-hour fasting. Calorie restriction during fasting is reported to be the main cause of fat melt and weight loss [1].

Insulin Sensitivity

Fasting can improve insulin sensitivity, reducing the risk of type 2 diabetes [2]. Some studies have indicated positive effects on insulin sensitivity markers after a 24-hour fast [3].

Cellular Repair and Autophagy

Fasting triggers a cellular process called autophagy, i.e., recycling and removing dysfunctional cellular components, which helps cellular repair, maintenance, and longevity. Animal studies have suggested that 24-hour fasting can promote autophagy and cellular health.

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

Heart Health

There have been signs of improved heart health after intermittent fasting. Studies have shown reductions in blood pressure, total cholesterol, triglycerides, and LDL (bad) cholesterol levels (commonly referred to as "bad" cholesterol) after intermittent fasting [4].

Dr. Valter Longo (Biogerontologist and fasting researcher):

"Periodic fasting can be a powerful tool to promote longevity and protect against diseases. A 24-hour fast is one of the simplest ways to give your body a break and trigger cellular repair processes."


Some animal studies have shown the positive effects of intermittent fasting on increased lifespan. While more human research needs to be done to confirm such effects, fasting can have potential benefits in longevity and age-related disease prevention.


Fasting can reduce inflammation in the body. Some studies have indicated decreased inflammation markers after a 24-hour fast.

Brain Function

Intermittent fasting, including 24-hour fasting, can positively affect brain health and improve cognitive function and neuroprotection.

It’s no secret that more studies need to be conducted to ensure the long-term health benefits of 24-hour fasting. Also, remember that different bodies react differently to changes, including fasting.

Various factors, including age, sex, overall health, and lifestyle, can affect the results. So, consulting a healthcare professional is a necessary step before trying 24-hour fasting.

What Can I Eat?

Generally, you’re not allowed to consume calories during fasting, so zero-calorie foods or some drinks can be used. What can we drink then?

What Can I Drink on a 24-Hour Fast?

No food or drinks containing calories is allowed in the 24-hour fast, but you can consume non-caloric fluids as follows:

  • Water: Plain water is a necessary and probably the best fluid to stay hydrated during fasting. Drink as much water as you can during fasting; there are no limits.
  • Black Coffee: Coffee without additives like sugar or cream is allowed during a fast. However, too much caffeine may lead to increased heart rate or jitteriness, so keep it in moderation.
  • Herbal Tea: You can consume various non-caloric herbal teas, such as chamomile, peppermint, or green tea, during a 24-hour fast. Avoid teas with added sugars or sweeteners.
  • Plain Tea: You can also consume plain black or green tea without any additives during a fast.

So, now that we know the basics of the 24-hour fast let’s find out if it’s actually good or bad and if it’s good for weight loss.

Learn More: What Can You Drink on the Carnivore Diet? Is Coffee, Milk and Alcohol Safe to Consume?

Mark Sisson (Author and Founder of the Primal Blueprint):

"Intermittent fasting, including 24-hour fasts, is a natural part of our evolutionary history and can lead to significant improvements in metabolic health and well-being."

Is 24-Hour Fasting Good or Bad?

A 2013 article by Roger Collier provides the following information about the effectiveness of intermittent fasting:

  • Large bodies of research support the health benefits of fasting, but most of them have been conducted on animals.
  • Mark Mattson - senior investigator for the National Institute on Aging, part of the US National Institutes of Health - believes fasting improves disease biomarkers, decreases oxidative stress and boosts learning and memory functioning.
  • Mattson has also researched the protective benefits of fasting to neurons, and has found that avoiding food for 10–16 hours makes your body enter ketosis, i.e., burn fat for energy, and fatty acids (ketones) be released into the bloodstream. Mattson believes it can maintain memory and learning functionality and slow disease processes in the brain.
  • Researches conducted on animals indicate that in addition to potential weight loss benefits, fasting can help cardiovascular health, boost cancer-fighting properties, and support memory preservation.

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

Another article medically reviewed by Amy Richter, RD, Nutrition, in March 2023 has indicated the following findings.

  • Blood Sugar Control: Recent studies have found that intermittent fasting and alternate-day fasting can help decrease insulin resistance.
  • Reduced Inflammation: Fasting can help reduce inflammation, as chronic inflammation can cause heart disease and cancer.
  • Heart Health: Fasting, especially alternate-day fasting, can decrease cholesterol, blood pressure, triglycerides, and LDL (bad) cholesterol, which cause heart disease.
  • Brain Function: While human research is limited and mostly animals have been studied, fasting can boost brain function and structure, protecting against neurodegenerative conditions like Alzheimer's and Parkinson's.
  • Human Growth Hormone (HGH) secretion: Fasting can naturally increase HGH levels, which aid metabolism, weight loss, and muscle strength.
  • Longevity: Animal studies show that fasting can improve longevity and delay ageing, but limited human research has been conducted in this area.
  • Cancer Prevention and Treatment: Fasting may help cancer prevention and treatment, as some studies suggest it can block tumor formation and improve chemotherapy results. However, most research is limited to animal and test-tube studies, and more research is needed to confirm the results.

Learn More: 16-Hour Fast: Benefits and Results [The Best Foods to Eat]

A 24-Hour Fast Boosts Weight Loss

Studies published in the National Institutes of Health (NIH) show that intermittent fasting, including 24-hour fasts, can lead to a reduction in caloric intake and help with weight loss. Participants in various studies usually reduce their weekly caloric intake by 20-30%​.

Is 24-Hour Fast Good for Weight Loss?

  • Mattson - introduced in the above section - has been a part of various studies on intermittent fasting and caloric restriction. In one such study, overweight adults suffering from moderate asthma consumed only 20% of their normal calorie intake on alternate days (Free Radical Bio Med 2007;42:665–74) and could lose 8% of their initial body weight in two months. Also, the markers of oxidative stress and inflammation decreased in them, and their asthma-related symptoms and different quality-of-life indicators were improved.
  • In another study, Mattson et al. studied the effects of intermittent and continuous energy restriction on weight loss and various biomarkers (for conditions including breast cancer, diabetes and cardiovascular disease) among young overweight women (Int J Obesity 2011;35:714–27). The results indicated that intermittent restriction and continuous restriction were equally effective for promoting weight loss, insulin sensitivity and other health biomarkers.
  • Another article medically reviewed by Natalie Olsen, R.D., L.D., ACSM EP- in 2020, shows that about 8 hours of fasting makes the body deplete glucose, the main source of energy coming from carbohydrates, and enter a gluconeogenesis state, i.e., creating glucose from fat, and burning calories to lose weight.
  • Other studies have indicated that intermittent fasting can indeed help with weight loss. However, it is not necessarily better than other weight-loss methods, like decreasing daily calorie intake or going on a Carnivore diet. The effectiveness of fasting varies from person to person, and it can be hard to keep over time. Moreover, post-fast binging and a false sense of security on non-fasting days can impact weight loss results.
  • While fasting can generally be safe, it may be risky for certain individuals, including those with diabetes, eating disorders, or those taking special medications. So, consulting a healthcare professional before trying fasting is necessary.
  • Other studies have shown that fasting can result in weight loss by restricting calorie intake and improving metabolism. Some studies indicate intermittent fasting can be more useful for weight loss than continuous calorie limitation.

If you’re confused about how to start and complete fasting, try this 7-Day Ultimate Fasting Course For Fat Loss, which is suitable for those on Low Carb, Keto, or Carnivore diets (or those still eating donuts!). This course will teach you four fasting techniques for faster fat loss, which cover both beginners and advanced dieters.

So, if 24-hour fasting is your thing and your body composition and health status are okay with it, you may want to know how much weight you can lose in a 24-hour fast or how many calories you can burn in a day of fasting, or what happens to your body during a 24-hour fast. Scroll to find out!

How Much Weight Can You Lose in a 24-Hour Fast?

The amount of weight you can lose in 24-hour fasting is bound to various factors, including metabolism, body composition, and overall calorie intake. The early weight loss after a 24-hour fast is mainly because of water weight loss and food reduction.

Real fat loss takes a bit longer time to happen and usually results from a changed lifestyle, including a balanced diet and regular exercise, not just fasting. Consistency and perseverance are the keys to a healthy and in-shape body.

To determine the average weight loss, you need to consult a healthcare professional to examine your body status and situation.

Learn More: Does Carnivore Diet Lose Weight? How Fast? How Much?

Calories Burned Fasting for 24 Hours

The number of calories burned during a 24-hour fast varies from person to person, as it depends on various personal and environmental factors, including age, weight, sex, metabolic rate, and activity level.

However, research shows that during a 24-hour fast, the body uses stored glycogen from the liver and muscles, which is about 1,600 to 2,000 calories for an average-sized person. But the depletion of the glycogen stored, the body starts to use fat stores for energy.

Dr. Jason Fung (Nephrologist and Fasting Expert):

"A 24-hour fast can help reset your metabolism and provide numerous health benefits, including improved insulin sensitivity and reduced inflammation."

What Happens to Your Body During a 24-Hour Fast?

During a 24-hour fast, various physiological changes occur in the body as it tries to adjust to the absence of food and calories. The following are among the changes happening during a 24-hour fast.

  • Glycogen Depletion: After eating, the body breaks down carbohydrates into glucose and stores them as glycogen in the liver and muscles. During the first hours of fasting, the body uses this glycogen to provide energy, which gradually depletes, and the body switches to fat for energy.
  • Fat Burning: After the depletion of the glycogen stores, the body breaks down fat into fatty acids and releases them into the bloodstream, which are then used for energy by cells. Fasting speeds up fat burning, which results in weight loss.
  • Ketosis: Increased fat breakdown makes the liver produce ketone molecules from fatty acids, which is called the ketosis process. The body uses ketones as an alternate energy source for the brain and other tissues.
  • Insulin and Blood Sugar Regulation: No food means lower insulin levels, which enables fat mobilization and lets the body burn stored energy. Blood sugar is also regulated by releasing stored glucose from the liver (gluconeogenesis).
  • Hormonal Changes: Growth hormone increases during fasting, improving fat burning and muscle preservation. Insulin-like growth factor 1 (IGF-1) is reduced, resulting in cellular repair and longevity. Also, some changes in adrenaline levels, ghrelin and leptin, impacting hunger and satiety signals, may occur.

So, that’s what happens during a 24-hour fast. The following section will ease you into the process of 24-hour fasting and elaborates on important details you need to know before starting.

A 24-Hour Fast Improves Diabetes

According to a study published in the National Institutes of Health (NIH), a 24-hour fast can improve insulin sensitivity, reducing the risk of type 2 diabetes. One study found that intermittent fasting improved insulin sensitivity by 20-31% in participants over a short period​​.

How to Do 24-Hour Fasting?

This step-by-step guide helps you through the process of 24-hour fasting.

  • Choose a Suitable Day: Choose a normal, relaxing day for fasting that doesn’t involve strenuous physical activity or important events that might be affected by fasting.
  • Plan the Meals: Eat balanced, nutritious meals the day before the fast to provide enough nutrients for the fasting day.
  • Start the Fast: Start your fast after your last meal of the day, i.e., if you finish dinner at 8:00 PM, your fast will begin from that hour onward.
  • Stay Hydrated: Drink lots of water to prevent dehydration. You can also drink non-caloric fluids like black coffee or herbal tea.
  • Keep Busy: Don’t think about food and distract yourself with activities that take your mind off food. Do the activities you enjoy to help pass the time.
  • Listen to Your Body: Pay attention to your body’s reactions during fasting. Feeling hungry or tired is normal, but feeling lightheaded, excessively weak, or unwell needs medical care.
  • Break the Fast Gradually: Break the 24-hour fast with light, small, and easily digestible meals. Your body needs time to adjust to eating again.
  • Resume Regular Eating: After breaking your fast, resume your regular eating pattern. Consume balanced, nutrient-dense meals.

Needless to say that before making any drastic changes in your eating habits or doing the 24-hour fast, you need to consult a healthcare professional to avoid possible problems.

A 24-Hour Fast Once a Week

A 24-hour fast once a week, also known as a weekly 24-hour fast or 24-hour intermittent fasting, involves avoiding food or caloric drinks for a continuous 24-hour period once a week. The following subsections provide all you need to know.


  • Fasting Duration: The fast lasts for a full day, typically starting after the last meal of the day and continuing until the same time the following day.
  • Caloric Restriction: You should consume no calories during the 24-hour fasting period.

What to Eat?

No foods containing calories are allowed.

What to Drink?

  • Water
  • Black Coffee
  • Plain Tea
  • Herbal Tea
  • Zero-Carb Beverages


  • Weight loss
  • Insulin Sensitivity
  • Metabolic Health Benefits
  • Autophagy
  • Psychological Benefits
A 24-Hour Fast Helps Cancer Treatment

A study published in the National Institutes of Health (NIH) shows that fasting triggers autophagy, a cellular repair process where cells remove damaged components, which can contribute to increased longevity and protection against diseases like cancer and Alzheimer’s​.

24-Hour Fasting 3 Days a Week

A 24-hour fasting three days a week, aka alternate-day fasting, involves fasting every other day. Here are its rules and details.


  • Fasting Days: On the three alternate fasting days, caloric intake is limited to zero or minimal (usually less than 500 calories), i.e., you need to avoid food or caloric beverages for a full 24-hour period.
  • Non-Fasting Days: Eat regular, balanced, nutritious meals with no caloric restrictions.

What to Eat?

Regular food on non-fasting days.

What to Drink?

  • Water
  • Non-Caloric Beverages

A 24-Hour Fast Every Day

It may sound like something you may do for a short time, but longer periods of food abstaining, such as 24-, 36-, 48- and 72-hour fasting periods, may be dangerous and must b avoided unless with medical supervision.

Foods to Eat After a 24-Hour Fast

Starting to eat small and easily digestible foods gradually after the 24-hour fast is a must. You need to let your body adapt to food consumption. Also, consuming nutritious foods out of the fasting period is necessary to provide for your body’s needs. You can eat the following foods after the fasting period.

  • Fresh Fruits and Vegetables: Including berries, melons, leafy greens, cucumbers, and bell peppers for hydration, fibre, vitamins, and minerals
  • Lean Proteins: Including chicken, turkey, fish, tofu, or legumes for muscle repair and satiety
  • Whole Grains: Including brown rice, quinoa, oats, or whole wheat bread that provide complex carbohydrates and fibre for sustained energy
  • Healthy Fats: Including avocados, nuts, seeds, and olive oil to support satiety and provide essential fatty acids
  • Greek Yogurt or Cottage Cheese: for high levels of protein
  • Soups or Broths: Light, easily digestible homemade soups or broths for  hydration and nutrients
  • Hydrating Fluids: Including water, herbal tea or diluted fruit juices

Foods to Avoid After a 24-Hour Fast

The following list shows the foods you’d better avoid after a 24-hour fast.

  • Heavy, Greasy, or Hard to Digest Foods: Including deep-fried foods, fatty cuts of meat, or rich sauces
  • Highly Processed Foods: Including packaged snacks, sugary cereals, or fast food, which are low in nutritional value and high in added sugars, unhealthy fats, and artificial ingredients
  • Spicy or Highly Seasoned Foods
  • Carbonated drinks
  • Large Portions: Start with small portions to let your digestive system adapt to consuming food after a complete 24 hours of fasting.
  • Foods Trigger Sensitivities or Allergies

24-Hour Fasting Side Effects: Is It Good for You?

As mentioned above, 24-hour fasting can have various health and weight loss benefits. However, every good thing may have some drawbacks. Remember that 24-hour fast side effects differ from person to person, but knowing them helps you make informed decisions.

  • Hunger and Food Cravings: It’s normal to feel hungry after a few hours of fasting, but too much hunger needs care and attention.
  • Fatigue and Low Energy: Every normal feeling happening too much must alert you to take necessary measures. Feeling tired or low energy is fine, but not excessive fatigue.
  • Headache: Headaches are a possible side effect of fasting, often associated with dehydration or changes in blood sugar levels. It's essential to stay hydrated and monitor your body's hydration needs during the fast.
  • Dizziness or Lightheadedness: Temporary drops in blood pressure causing dizziness or lightheadedness are common during fasting, especially when transitioning from sitting or lying down to standing up. Taking it slow when changing positions can help minimise such symptoms.
  • Irritability and Mood Changes: Some individuals may experience changes in mood, irritability, or difficulty concentrating during fasting periods.
  • Disrupted Sleep Patterns

Who Should Avoid Doing a 24-H Fasting?

Not every good thing is beneficial for everyone, especially if you have a certain condition. The following group of people must consult healthcare specialists before trying fasting.

  • Individuals with Specific Health Conditions: Including diabetes, hypoglycemia, cardiovascular disease, eating disorders, or any other chronic medical condition
  • Pregnant or Breastfeeding Individuals
  • Children and Adolescents
  • Older Adults
  • Those with a History of Eating Disorders: Fasting can trigger or worsen disordered eating patterns or psychological distress in those with a history of eating disorders.
  • Underweight Individuals or Those with a History of Significant Weight Loss


A 24-hour fast can be a useful method to melt fat, limit calorie intake, and lose weight. However, it’s necessary to consult a healthcare professional before making significant changes in your eating habits.


Here you can find answers to the most frequently asked questions about 24-hour fasting.

Is Fasting for 24 Hours Good?

Yes, studies indicate that fasting for 24 hours can help weight loss, reduce insulin resistance, and improve blood lipid levels.

How Many Days Can You 24-Hour Fast?

You can do 24-hour fasting, one, two, or three days a week, depending on your health status, body composition, and goals.

How Long Is It Safe to Fast?

If it’s your first time trying fasting, you’d better start with 8- or 16-hour fasting. Then, if things are fine, you can try a 24-hour fast.

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