An 18-hour fast can be a useful dietary approach to weight loss and fat melt due to restricting calorie intake. But the question is what its side effects can be, what about its short-term and long-term effects, whether it is good for everybody, what we can eat and drink, and what possible effects it can have on our general health, hormones, and blood sugar. Keep scrolling to learn it all!
Table of Contents
An 18-hour fast is a type of intermittent fasting which can positively affect weight loss and health.
An 18-hour fast can promote ketosis, insulin sensitivity, and fat burning.
Various studies indicate an 18-hour fast can boost weight management, cardiovascular health, brain health, and reduced inflammation.
There are many similarities and some differences between an 18-hour fast and other types of intermittent fasting methods like 16-hour, 20-hour, and 24-hour fasts regarding ketosis duration and adaptability.
There are tips you need to follow to break the fast in a healthy way and to eat and drink foods and beverages providing the necessary nutrients.
What Is an 18-Hour (18:6) Fast?
An 18-hour fast is a dietary approach to restrict daily calorie intake. It includes avoiding calorie sources for 18 hours and consuming food, calories, nutrients, vitamins, and minerals during a 6-hour eating period.
Let’s review the rules and principles you need to follow in an 18-hour (18:6) fast.
An 18-hour fast is a popular kind of intermittent fasting with simple principles to follow. Here are the main rules of an 18-hour fast.
Time-Restricted Eating: An 18-hour fast includes eating during a 6-hour eating period when you must consume daily calories and nutrients. Choosing the eating period based on your goals and lifestyle is up to you, but eating between 12:00 PM and 6:00 PM is common.
Fasting Period: The 18 hours fasting period includes avoiding food and calorie intake for 18 hours, but drinking non-caloric beverages like water, black coffee, or plain tea is allowed.
Choose the Fasting Window: Set specific hours for the fasting period. You can spend a part of it at sleep (overnight) and fast from 6:00 PM until 12:00 PM the following day.
Stay Hydrated: Drink water and zero-calorie drinks like black coffee or plain tea to stay hydrated.
Avoid Caloric Intake: Consume no calories during fasting hours for the best results.
Nutrient-Dense Meals: Avoid too many calories or sweet foods and consume nutrient-dense meals to provide essential vitamins, minerals, and macronutrients to support your overall health and meet your body’s needs.
Listen to Your Body: Monitor your body’s changes and reactions and make modifications to make the most of fasting.
Consult a Healthcare Professional: Talking to a physician or dietitian before starting the diet helps you design a plan that best suits your needs and status and avoid health issues.
Consistency: Be consistent with your eating and fasting windows. Perseverance and consistency help you achieve your goals.
Now that we know the rules let’s see what physiological changes our body goes through during an 18-hour fast.
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What Happens During an 18-Hour Fast?
Calorie restriction makes various changes in our body. Here is the process that takes place during an 18-hour fast.
Glycogen Depletion: After the last meal, your body begins to burn the glucose stored in your liver and muscles, which is glycogen and is the main energy source during the early fasting hours.
Insulin Levels Decrease: Avoiding food decreases insulin levels, and lower insulin makes your body use stored fat for energy.
Lipolysis: After some hours of fasting, your body starts breaking down stored fat through lipolysis. Diapose tissue releases fatty acids, which move to the liver and turn into ketones to be released into the bloodstream.
Ketosis: As the number of ketones increases in the blood, ketosis starts, i.e., the ketones are used to provide energy.
Increased Growth Hormone: Growth hormone levels can increase due to fasting. Growth hormone release can boost fat metabolism and help maintain muscles.
Autophagy: Long hours of fasting make the body enter a state of autophagy to break out damaged or dysfunctional cells and recycle them to help cellular rejuvenation and repair.
Enhanced Insulin Sensitivity: Regular fasting helps your body get more efficient at regulating blood sugar using insulin.
Increased Fat Burning: Fasting restricts calories and carbohydrates, which makes the body use fat for energy, resulting in more fat burning and weight loss.
Mental Clarity: There have been individual reports of mental focus and clarity during fasting periods, probably caused by increased ketone production in the brain.
Break of Fast: When the fasting period is over, and you break the fast, your body starts to use glucose for energy, so the insulin levels increase.
So, what kinds of benefits does an 18-hour fast offer? Scroll to find out!
18:6 Fast Diet Benefits
An 18-hour fast or 18:6 fast can have various benefits. Here are the findings of some research studies about the advantages of this fasting diet. Its long-term and short-term benefits will follow.
A 2015 study by Wegman et al. shows that 18:6 intermittent fasting can slow down ageing.
Another study by Faris et al. 2012 found that 18-hour intermittent fasting can decrease inflammation in the body.
Phillips conducted a study in 2019 about the effects of intermittent fasting on human health and discovered that it can promote brain health.
In a 2020 study by Li et al., the effects of intermittent fasting on digestive health were studied, and the results indicated that it could positively impact gut health.
The findings of a 2018 study by Anton et al. revealed that intermittent fasting, including an 18-hour fast, can facilitate weight loss.
In another medical study examining the relationship between intermittent fasting and heart health in 2020, Dong et al. found that fasting positively affects cardiovascular health.
Another study by Hutchinson et al. conducted in 2019 showed that intermittent fasting can help prevent diabetes.
As mentioned above, an 18-hour fast is one of the popular fasting methods, but it’s not the only one. Another prevalent fasting method is a 16-hour fast. Let’s compare the two to see which one can be a better option.
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The main difference between an 18-hour fast and a 16-hour fast lies in the duration of the fasting window. Both are forms of intermittent fasting, but the fasting periods are slightly different. Let's compare the two:
The Differences Between a 16- and 18-Hour Fast
An 18-hour fast involves avoiding calorie intake for 18 hours, while a 16-hour fast includes avoiding calories for 16 hours.
Duration of Ketosis
Both fasting periods can result in ketosis. However, fasting longer (18 hours) gives you more time in ketosis than the 16-hour fast.
A 16-hour fast can be easier to adapt for beginners due to shorter fasting hours.
Deciding if a 16- or 18-hour fast is better is up to you as it differs from individual to individual. You need to consider your goals, preferences, health status and lifestyle to choose the best fasting method.
However, if you’re a beginner or can’t fast for long hours for medical reasons, a 16-hour fast can be a better option for you. But if your lifestyle lets you fast for longer periods and needs faster results, try an 18-hour fast.
Another popular fasting method used especially by athletes or those seeking faster fat-melt results is a 20-hour fast. The following section will compare it with the 18-hour fast to let you make informed decisions.
An 18-Hour Fast vs a 20-Hour Fast
An 18-hour fast and a 20-hour fast are two types of intermittent fasting, with 18 hours and 20 hours of avoiding calorie intake, respectively.
Both methods help fat metabolism, boost weight loss, enhance autophagy, and support insulin sensitivity.
The Differences Between a 20- and 18-Hour Fast
There are not a lot of differences between these two fasting methods, and the main difference concerns the fasting period.
A 20-hour fast includes fasting for 20 hours and eating in only 4 hours.
An 18-hour fast involves fasting for 18 hours and eating in only 6 hours.
Is an 18- or 20-Hour Fast Better?
It’s no secret that your goals, needs, preferences, health status, and general health determine the best fasting method, and no method is good or bad by itself.
However, an 18-hour fast can be a good option for those looking for less restrictive or challenging methods that still contain benefits.
A 20-hour fast can be better for those experienced in fasting and prefer a more condensed eating window.
Let’s compare the 18-hour fast with another form of intermittent fasting, which can be challenging for beginners, but is preferred by athletes and veteran dieters!
Both an 18-hour fast and a 24-hour fast are types of intermittent fasting, but their eating periods are different.
The 18-hour fast’s main benefits are improved fat burning and insulin sensitivity, among others. It is relatively easier to implement compared to longer fasting durations.
The 24-hour fast includes avoiding calories for a complete 24 hours, making it a challenging and strict fasting method. Despite not being suitable for beginners, this fasting method provides enhanced autophagy, deeper ketosis, and potentially greater metabolic benefits.
The Differences Between a 24 and an 18-Hour Fast
The main difference between a 24-hour and 18-hour fast is related to the fasting and eating periods. During a 24-hour fast, you need to avoid consuming calories for a consecutive 24 hours.
Also, the 24-hour fast cannot be practised regularly as it takes a whole day, and your body needs to recover, so you can’t do it oftentimes.
An 18-hour fast may be more suitable for experienced dieters, but a 24-hour fast needs veteran dieters whose bodies are adapted to the strict fasting situation.
Is an 18-Hour Fast Better Than a 24-Hour Fast?
Individual goals, status, and considerations asides, a 24-hour fast can provide faster results, but it’s better for experts. An 18-hour fast is good for experienced dieters who can fast for long periods but also need a suitable eating window.
Now that we know all about different types of intermittent fasting let’s focus on the 18-hour fast and see how we can do it using a step-by-step guide.
Choose Your Fasting Schedule: The fasting plan must match your lifestyle and goals.
Start with an Early Dinner: It can give you a lot of time to adapt to the fasting situation without feeling the side effects.
Stay Hydrated During the Fast: Keep drinking water throughout the fasting period to stay hydrated, which helps decrease headaches and dizziness. You can also drink other non-caloric beverages.
Avoid Caloric Intake: Consume no calories during the fasting period, i.e., avoid solid foods, juices, or anything increasing your insulin levels.
Plan Your Eating Window: Plan ahead your eating window so you can consume all your daily calories and nutrients within this time frame.
Avoid Overeating: Eat in moderation to let your digestive system adapt to the new eating habit.
Listen to Your Body: Check the changes in your body and plan accordingly.
Be Consistent: Stick to your schedule to make the most of fasting.
Eat Nutrient-Dense Meals: You need to provide the necessary vitamins, nutrients, and minerals during the six hours of the eating period to get ready for the fasting period.
We have mentioned nutrient-dense foods here and in the above sections, but what does it really mean? What exactly can we eat and drink in an 18-hour fast?
What Can I Eat?
During an 18-hour fast, you cannot consume any calories. But in the eating period, you can eat various food groups, including vegetables and high-quality protein found in meat, poultry, and seafood.
Also, consume Greek yoghurt and other dairy products if tolerated. Generally, you can eat everything you usually do, but it’s better to opt for high-quality, low-calorie, and high-protein options.
What Can I Drink?
During the 18-hour fast, you can only drink non-caloric drinks like water, herbal tea, coffee, and plain tea. But, during the eating period, you can drink whatever you like. Again, it’s better to choose from low-calorie options.
You may wonder what exactly an 18-hour fast schedule may look like. Well, scroll to the next section to find out.
A Sample 18-Hour Fast Schedule
6:00 AM: Wake up and start the day with a glass of water for hydration.
6:30 AM: Drink black coffee or tea without additives to suppress appetite and provide a gentle energy boost.
7:00 AM: Continue fasting by drinking water or other non-caloric beverages during the morning.
12:00 PM: Break the fast and enter the 6-hour eating period. Have a balanced and nutrient-dense meal, including greens, protein (chicken, tofu, or beans), healthy fats (avocado, nuts), and a light dressing.
3:00 PM: Have healthy snacks if you feel hungry after the 12 o’clock meal, and consume Greek yoghurt with berries or a handful of nuts.
6:00 PM: Eat your last meal before starting the fasting period again, and include grilled fish or chicken with roasted vegetables and quinoa in this meal to provide the necessary nutrients.
6:30 PM: Start fasting and consume only non-caloric beverages until the end of the fasting period.
12:00 AM (Midnight): Continue fasting while sleeping until 6 AM.
So, if it is the fasting schedule, can we do it every day?
It is possible to do an 18-hour fast every day. It is also known as daily or daily time-restricted fasting.
This type of fasting includes avoiding calories for 18 hours each day and eating during a 6-hour eating window.
Just note that this kind of fasting can be challenging for beginners and even experienced dieters. So, it’s wise to consult a healthcare professional and do the necessary checkups to avoid health problems.
What to Eat?
No Processed Foods
Water and Non-Caloric Beverages
An 18-Hour Fast Once a Week
Practising an 18-hour fast once a week is a method practised by beginners or those who seek specific health and fitness goals.
Doing an 18-hour fast once a week involves fasting for 18 hours one day a week and having regular eating patterns for the rest of the days.
What to Eat?
The eating rules during this type of intermittent fasting are similar to other kinds, i.e., avoiding calories during fasting and continuing regular eating during eating periods.
How to Break an 18:6 Fast?
Start Slowly with Small, Balanced Meals.
Have a Nutrient-Dense Meal.
Avoid Heavy or Greasy Foods.
Listen to Your Body.
Avoid Overeating and Undereating.
Eat Healthy Snacks.
Avoid Sugary and High-Calorie Beverages.
The Best Foods to Eat After an 18-Hour Fast
Foods to Avoid After an 18-Hour Fast
Heavy and Greasy Foods
Sugary Foods and Beverages
Dairy Products (for some individuals)
Alcohol immediately after breaking the fast
An 18-Hour Fast Side Effects: Is It Good for You?
So far, we have elaborated on the positive effects of the 18-hour fast, but every good thing has some drawbacks. Here are the potential side effects of an 18-hour fast.
Note that not all of them will happen to everyone, but it’s necessary to know about them and take necessary measures to avoid health problems.
Hunger and Cravings
Fatigue and Dizziness
Low Energy Levels
Potential Disruption of Menstrual Cycles
Improved Metabolic Health
Enhanced Cellular Repair and Longevity
Potential Cognitive Benefits
Possible Longevity Effects
Hunger and Cravings
Fatigue and Dizziness
Temporary Weight Loss
Improved Insulin Sensitivity
Increased Focus and Alertness
Potential Digestive Changes
An 18-Hour Fast Weight Loss Results
Let’s go through some studies examining the weight loss results of an 18-hour fast.
Intermittent fasting with calorie restriction positively affects weight loss and cardio-protection in obese women.
Source: Klempel MC, Kroeger CM, Bhutani S, Trepanowski JF, Varady KA. Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women. Nutr J. 2012 Nov 21;11:98. doi: 10.1186/1475-2891-11-98.
Intermittent fasting can have more significant results on fat loss than traditional calorie-restricted diets.
Source: Tinsley GM, La Bounty PM. Effects of intermittent fasting on body composition and clinical health markers in humans. Nutr Rev. 2015 Oct;73(10):661-74. doi: 10.1093/nutrit/nuv041.
Intermittent fasting impacts insulin sensitivity and helps regulate blood sugar.
Source: Barnosky AR, Hoddy KK, Unterman TG, Varady KA. Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Transl Res. 2014 Oct;164(4):302-11. doi: 10.1016/j.trsl.2014.05.013.
Intermittent fasting can decrease some risk factors related to cardiovascular disease.
Source: Harvie M, Wright C, Pegington M, McMullan D, Mitchell E, Martin B, Cutler RG, Evans G, Whiteside S, Maudsley S, Camandola S, Wang R, Carlson OD, Egan JM, Mattson MP, Howell A. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. Br J Nutr. 2013 Oct;110(8):1534-47. doi: 10.1017/S0007114513000792.
Alternate-day fasting boosts weight loss in normal-weight and overweight participants.
Source: Varady KA, Bhutani S, Klempel MC, Kroeger CM, Trepanowski JF, Haus JM, Hoddy KK, Calvo Y. Alternate day fasting for weight loss in normal and overweight subjects: a randomised controlled trial. Nutr J. 2013 Nov 12;12:146. doi: 10.1186/1475-2891-12-146.
Your starting weight, body composition, calorie intake, activity level, and overall metabolism are among the main factors affecting the amount of weight you can lose.
Weight loss has no fixed or specific amount during an 18-hour fast. However, 0.5 to 2 pounds a week is considered a safe and sustainable weight loss rate.
Calories Burned Fasting for 18 Hours
Various individual factors, including age, weight, gender, metabolism, and activity level, determine the number of calories you can burn in an 18-hour fast.
Generally, a rough estimate of the calories burned during fasting can be calculated using the Harris-Benedict equation (BMR), which considers gender, age, weight, and height. For example:
For women: BMR = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years)
For men: BMR = 66 + (6.23 x weight in pounds) + (12.7 x height in inches) - (6.8 x age in years)
Once you get the BMR, multiply it by the number of hours of fasting (18) to get an approximate number of calories burned during the fasting period.
An 18-Hour Fast Ketosis
An 18-hour fast can create the state of ketosis, where the body burns fat instead of carbohydrates to provide energy. During an 18-hour fast, glycogen stores get depleted, and insulin levels decrease, resulting in the breakdown of fat into fatty acids.
Once the fat is turned into fatty acids, the liver turns them into ketones to be used to provide energy. This state of ketosis usually happens after a long time of fasting, i.e., after 12 to 12 hours of avoiding calories.
Remember that ketosis occurs at different times in different individuals, so don’t expect to see similar results as others because our bodies respond differently to changes and triggers.
Will I Lose Weight If I Fast 18 Hours a Day?
Ketosis promotes weight loss and fat melting, so fasting for 18 hours can help lose weight by restricting calorie intake.
One of the earliest reasons for losing weight is consuming fewer calories, which can be attained by an 18-hour fast. Also, you need to consume healthy and low-calorie foods in the 6-hour eating period for optimum results.
Note that while intermittent fasting can be a useful way of losing extra body weight, it may not be suitable for everyone. To ensure it works for you, consult a health professional first.
Is Fasting for 18 Hours Good for You?
Generally, fasting for 18 hours, or other kinds of intermittent fasting, is good for the body as it provides a situation for the digestive system to rest. They can also calm the mind and create peace for the individual.
Also, intermittent fasting can have various health benefits if done properly and under appropriate circumstances. However, deciding if it is "good for you" needs professional consultation and depends on your overall health, medical history, lifestyle, and individual needs.
Even some individuals need to avoid fasting. Let’s see who they are and if you are in one of these groups.
Who Should Avoid Doing 18-H Fasting?
The following groups of individuals must consult a healthcare professional to ensure it is suitable and harmless for them.
Pregnant or Breastfeeding Women
People with Eating Disorders
Children and Adolescents
Individuals with Underlying Health Conditions
Athletes or Active Individuals
Individuals with Low Body Weight or Nutritional Deficiencies
Individuals with Hormonal Imbalances
An 18-Hour Fast Diet and Autophagy
Autophagy is a cellular process happening during long hours of fasting, like an 18-hour fast. Autophagy is a beneficial mechanism for cells as it removes damaged or dysfunctional components and recycles them to support cellular health and regeneration. It is also found to have anti-ageing effects.
Another positive effect of an 18-hour fast can be on blood sugar levels. The following section talks about this effect.
Blood Sugar After an 18-Hour Fast
Physical activity, metabolism, and overall health status are among the main factors affecting individual blood sugar after an 18-hour fast.
Generally, blood sugar levels decrease during fasting, especially when carbohydrates are limited or not consumed. An 18-hour fast can reduce blood sugar levels and increase insulin sensitivity.
So, if an 18-hour fast can decrease blood sugar, is it suitable for bodybuilders and athletes? Scroll to find out!
An 18-Hour Fast and Bodybuilding
Despite what many may think, an 18-hour fast can be compatible with fitness and bodybuilding exercises. The main consideration for bodybuilders is to preserve muscles and the following points.
Protein Intake: Consuming enough protein during the 6-hour eating window helps promote muscle protein synthesis.
Timing of Workouts: Try to do the workouts during the 6-hour eating period or close to the end of it so you can consume enough nutrients after the training.
Nutrient-Dense Meals: Eat nutrient-dense during the eating period to provide essential vitamins and minerals for muscle repair and overall health.
Hydration: Drink plenty of water during both fasting and eating periods, as proper hydration helps muscle function and performance.
Fasted Training: Some bodybuilders train during fasting, which can benefit some individuals. However, remember to consult a healthcare and workout specialist to ensure you can have sufficient energy and performance during workouts.
Gradual Adaptation: Avoid heavy workouts if you're new to intermittent fasting to let your body adapt to the fasting state.
Monitoring Progress: Closely monitor your progress, including muscle mass, strength, and overall well-being, to make sure an 18-hour fast aligns with your bodybuilding goals.
One of the main concerns of bodybuilders is muscle loss. Can an 18-hour fast cause muscle loss in bodybuilders?
Will an 18-Hour Intermittent Fast Affect Muscle Loss?
Although it differs from person to person, and individual factors, including diet, physical activity, and overall health, affect the performance of an 18-hour fast regarding muscle loss, an 18-hour fast is less likely to cause significant muscle loss, especially if done correctly with a balanced diet and appropriate exercise.
To avoid muscle loss during an 18-hour fast, consider the following tips.
Consume enough protein.
Try regular strength/ resistance training.
Keep a nutrient-dense diet.
Drink enough water.
Avoid extreme calorie deficits and consume enough calories during the 6-hour eating period.
An 18-Hour Fast and Pregnancy
You may be able to do fasting only if your doctor approves it. But intermittent fasting, including an 18-hour fast, is generally not recommended during pregnancy; here is why:
The possibility of nutrient deficiency during fasting can disrupt the baby's growth and development.
Fasting can result in low energy levels, which may not be enough for both the mother and the growing baby.
Fasting can cause blood sugar fluctuations, which can be risky during pregnancy.
Long hours of fasting can cause hormonal imbalances, which can be harmful during pregnancy.
The 18-hour fast is a type of intermittent fasting that helps weight loss and provides different health benefits. This article compares the 18-hour fast with other fasting methods, like 16-hour, 20-hour, and 24-hour fasts, regarding adaptability and goals.
Also, it provides guidelines for doing an 18-hour fast healthily, including meal suggestions and breaking the fast.
It also covers the potential side effects, the individuals who should avoid this fasting method, its impact on autophagy, blood sugar, and its compatibility with bodybuilding and pregnancy.
Remember to consult a physician or dietitian to make the most of the fasting period and stay healthy.
The following Q&As answer the most frequently asked questions about an 18-hour fast.
Will a Glass of Wine Ruin an 18-Hour Fast?
Yes, alcohol contains calories, so drinking a glass of wine or any amount of alcohol during an 18-hour fast will break your fast.
How to Start Eating After an 18-Hour Fast?
Start with small, nutritious portions containing protein and vegetables, and gradually return to your regular eating habits.
What Is The Best Thing to Eat Last Before an 18-Hour Fast?
You can eat various foods, but everything containing fruits, veggies, brown rice, and a protein source like baked chicken can make you feel full longer and provide enough vitamins and minerals.
How Long Does It Take to See Results from 18-Hour Intermittent Fasting?
Although it differs from person to person, it can take 2-4 weeks for your body to adapt to intermittent fasting. After that, the results may gradually appear, with an average weight loss range of 3-8% over 8-12 weeks.
How Often Should You Do an 18-Hour Fast?
You can even do an 18-hour fast daily, but it all depends on your body, health status, needs, and goals.