What is The Best Fasting Schedule?

What is Fasting

Fasting is a dieting method where you fast from food for a set duration of hours and only eat within a specific time frame. There are many schedules for fasting, all of which have a variety of benefits and risks.

How Fasting Affects The Body

Fasting redirects where our body’s fuel source comes from. When following a regular diet and eating consistently throughout the day, our body’s primary energy source is glucose [1]. When fasting, the body begins to utilize fatty acids for fuel [1]. The cycle that the body goes through when fasting is called the fed-fast cycle.

There are four stages of the fed-fast cycle [1]. The fed state, the early fasting state, the fasting state, and the starvation state [1].

The Fed State

The fed state begins right after finishing a meal and continues till about 3 hours after eating [1]. During this time, glucose rises, which leads to insulin secretion [1]. Glucose fuels the body, and the body uses it for energy [1]. Once the body has enough glucose to use for fuel, the body stores the extra caloric intake in adipose tissue (body fat) [1].

The Early Fasting State

The early fasting state occurs from 3 hours to 18 hours after eating [1]. Your insulin levels fall, and your body begins to convert glycogen from the liver into glucose for energy [1, 2]. Then, your body beings lipolysis, which means fat cells and fatty acids begin to be broken down for fuel [1].

The Fasting State

The fasting state begins about 18 hours after eating and continues till 36-48 hours after eating [1]. At approximately 24 hours, the body starts transitioning into ketosis [1, 3].

Being in a state of ketosis means that your glycogen stores are depleted, and the body begins breaking down fat for energy [1]. When fat is used as the body’s fuel source, fat loss and body composition changes result [3].

The Starvation State

The starvation state begins approximately 48 hours after eating food [1]. In this stage, glycogen stores are completely depleted, and insulin continues to decline [1]. The kidneys then begin to generate sugar through gluconeogenesis to fuel the brain [1]. The breakdown of amino acids is halted to preserve muscle, and fat cells remain the only fuel source for the body to use for energy [1].

General Guidelines and Recommendations For Fasting

While the time schedules for fasting are different for each fasting diet, some guidelines and recommendations are consistent for all fasting diets.

Maintaining a healthy and balanced diet before and after fasting is important. While there are no guidelines for what to eat when doing a fasting diet, there are consequences of eating unhealthy. Binge eating before and after fasting can negatively affect the metabolic response to fasting [2]. You should eat your recommended daily caloric intake before fasting [2]. The USDA guidelines suggest that men should consume between 2,400 to 3,000 calories per day, and females in the same age group should eat between 1,800 to 2,400 calories per day [4]. Nutritious foods should be consumed, such as whole grains, lean protein, fruits, vegetables, and nuts [4]. Added sugars, alcohol, and saturated fats should be avoided [2, 4].

Drinking water is extremely important when fasting to avoid dehydration [2]. The amount of water you should drink depends on many factors, such as the climate or your activity level [5, 6]. But, the general guideline is for adult males to drink between 3-3.7 liters of water per day and adult females to drink 2.2 – 2.7 liters of water per day [5, 6].

During the fasting period, especially if it is a long time, you should consider avoiding strenuous physical activity because fasting can the risk of fatigue and dizziness [2].

Overall, fasting diets are safe for healthy individuals as long as they are not underweight [2, 7]. Healthy individuals typically only experience mild side effects that may occur in healthy individuals, such as dizziness, weakness, irritability, headaches, and hunger [7, 9].

Those who are pregnant or breastfeeding and adolescents should avoid fasting diets because they require regular meals to maintain essential nutrients [2, 8].

Individuals who have been diagnosed with eating disorders, or are at risk of developing an eating disorder, should avoid fasting diets [2, 8]. Fasting diets can increase a fixation with food, increase self-restrictive habits, and promote binge eating behaviors [8, 11].

Lastly, fasting diets are not recommended for individuals with type 2 diabetes [2, 7, 8, 10]. Fasting greatly increases the risk of hypoglycemia, especially for those who take glucose-lowering medications [2, 7, 8, 10].

Comparison of Fasting Schedules

Clinical trials have not yet determined the best fasting diet schedule for weight loss and health benefits [7]. Since there is no conclusive evidence, let’s compare the methods, health benefits, health risks, and pros and cons to determine which may be best for you individually.

16:8 Intermittent Fasting

Method: 16:8 intermittent fasting means that you fast from food for 16 hours of a 24-hour day and then eat within an 8-hour window. It is recommended that you consume healthy and nutrient-rich foods and eat your daily recommended calories for maintaining weight as it is not a calorie-restrictive diet.

Sample Schedule:

Day 1: Begin fast at 8 PM
Day 2: Continue fasting until 12 PM. Eat as normal from 12 PM till 8 PM. Begin Fasting again at 8 PM.
Following days: Follow the same schedule as the day before.

Pros:

Easy to follow.
You can easily eat three meals and snacks within the eating window.
You can easily meet your daily calorie recommendations.
It is flexible and can fit into most people’s daily schedules.
Many research studies show that 16:8 intermittent fasting delivers weight loss benefits [13, 14, 15].
It may help you maintain musical mass even if you lose weight [12].
It may reduce your cravings and decrease your desire to snack [14, 15]
It could improve fasting glucose [15].
It could decrease biomarkers of chronic diseases [7, 8, 15, 16, 17,]
It may improve your sleep [16].

Cons:

You may experience hunger and fatigue while your body adjusts to fasting.

Who Is It Best For: 16:8 intermittent fasting is best for healthy individuals who are looking to try intermittent fasting for the first time or for those who want to practice intermittent fasting for a long duration of time.

20:4 Intermittent Fasting

Method: When you do 20:4 intermittent fasting, you fast food for 20 hours of a 24-hour day, and then you eat within a 4-hour window. 20:4 intermittent fasting is not meant to be calorie-restrictive, so it is recommended that you consume your daily recommended caloric intake in your eating window.

Sample Schedule:

Day 1: Begin fasting at 7 PM.
Day 2: Continue fasting until 3 PM. Eat as usual from 3 PM till 7 PM. Begin fasting again at 7 PM.
Following days: Follow the same schedule as the day before.

Pros:

There are no calorie restrictions.
Studies show that you may lose weight as a result of 20:4 fasting. Weight loss may be due to ketosis or a caloric deficit, as it may be hard to consume recommended calories for maintenance [13, 14, 15, 17].
It may result in lean muscle retention [12].
You may have an improved glycemic response and improved fasting glucose [10, 15].
It may offer increased protection against chronic disease [7, 16].
It may improve your cognitive functioning [17].

Cons:

Fasting for 20 hours may take some time to adjust to.
It may be very difficult to consume your daily recommended calories in 4 hours.
The 4-hour eating window may be too strict.
It may be too difficult to continue long-term.
There is an increased risk for hypoglycemia for those with type 2 diabetes [7, 8, 10].
Increased risk for eating disorders [7, 8].

Who Is It Best For: 20:4 intermittent fasting is best for those who have practiced other forms of fasting, such as 16:8 intermittent fasting, before attempting. Those who have practiced fasting prior may have an easier time adhering to the diet. It is also best for those who do not mind eating a large quantity of food within a short time frame.

Alternate-Day Fasting

Method: Alternate-day fasting, as the name states, is when you fast on alternate days of the week. It is different from other fasting diets because you are allowed to eat 25% of your daily caloric intake on fasting days. On non-fasting days, you are allowed to eat ad libitum, meaning you can eat as you please.

Sample Schedule:

Day 1: ad libitum (eat without restrictions)
Day 2: 25% kcal (of normal daily intake)
Day 3: ad libitum
Day 4: 25% kcal
Day 5: ad libitum
Day 6: 25% kcal
Day 7: ad libitum

Pros:

You can eat some food on fasting days.
There is no restriction on what foods you can eat on fasting days.
There is strong evidence that alternate-day fasting results in weight loss [16, 18, 19, 20, 21, 22].
Studies show that you may experience changes in body composition [18].
It may improve your cardiovascular health, reduce your blood pressure, and reduce your heart rate [ 16, 18, 19, 22, 24].
It may improve symptoms of asthma because fasting reduces inflammation [21].

Cons:

You may be hungry on fasting days.
The total fasting duration could extend for over 30 hours, depending on when you begin eating on fasting days.
Only being able to consume 25% of daily calories may be difficult.
There is an increased risk of hypoglycemia for those with type 2 diabetes [10].

Who Is It Best For: Alternate-day fasting is best for those who want to try a fasting diet that allows you to consume some food on fasting days.

5:2 Fasting Diet

Method: 5:2 fasting is similar to alternate-day fasting. You eat like normal for five days of the week and fast for two days of the week. On fasting days, you are allowed to consume some food, typically 400 – 500 calories.

Sample Schedule:

Day 1: ad libitum (eat without restrictions)
Day 2: 400-500 kcal
Day 3: ad libitum
Day 4:ad libitum
Day 5: 400-500 kcal
Day 6: ad libitum
Day 7: ad libitum

Pros:

You get to eat some food on fasting days
Five days of the week are unrestricted
Several studies show that their are weight loss benefits for 5:2 fasting [13, 14, 16, 17, 18, 19, 20, 21, 22, 25]
It can decrease biomarkers of chronic diseases [7, 8, 15, 16, 17]
It may improve inflammatory markers [7, 16]
It may lead to a significant reduction in insulin concentration [10, 15, 16, 25]
It may improve your sleep [16]
It may improve your cognitive abilities [17].

Cons:

You may still be hungry on fasting days.
There is an increased risk for hypoglycemia for those with type 2 diabetes on fasting days [7, 8].
There is an increased risk for eating disorders due to restrictions on fasting days[7, 8].

Who Is It Best For: 5:2 fasting is best for those who want to try fasting yet still are allowed to consume some calories while fasting. This could also be a great transitional fasting diet that can prepare you to do alternate-day fasting.

Periodic Fasting (24-hour fast once a week)

Method: Periodic fasting is where you fast for a full 24-hour period once per week. On the fasting day, no calories are allowed, only water or zero-carb beverages. All other days of the week are unrestricted.

Sample Schedule:

Day 1: ad libitum (eat without restrictions)
Day 2: ad libitum till 1 PM
Day 3: Fast until 1 PM
Day 4: ad libitum
Day 5: ad libitum
Day 6: ad libitum
Day 7: ad libitum

Pros:

It is more sustainable than other diets because you only fast once per week.
The day and time window for fasting is flexible. You can choose any 24 time period during the week.
There are no food restrictions throughout the week.
There is a possibility of weight loss if caloric deficit for the week [18, 28].
It may lead to fat loss if you enter into a state of ketosis [3, 17].
It may decrease the risk of chronic disease [3, 7, 27].
It may improve blood sugar and decrease insulin in the following morning [3, 10, 15, 26].
May improve memory and cognitive performance [17, 27].

Cons:

It may be difficult to fast for 24 hours without practice.
It may increase the risk of binge eating [11].
Doing a 24-hour fast may not be effective for weight loss if you overconsume on other days of the week [18, 28].
You may only experience water weight loss during fasting.
It could increase the risk of hyperglycemia for those with type 2 diabetes [7, 8, 10].

Who Is It Best For: Periodic fasting is best for those who find it difficult to fast every day or on alternate days. Fasting once a week can be just as beneficial for those with busy schedules for a social life that makes it difficult to fast more often.

One Meal A Day Diet

Method: The one meal a day fasting method consists of fasting for 23 hours of a 24-hour day and eating your entire amount of daily calories in a 1-hour window.

Sample Schedule:

Daily: Fast until 5 PM and eat one giant meal that includes all of your daily calories. Stop eating a 6 PM.

Pros:

There are no calorie restrictions.
The 1-hour window for eating can occur at any time of the day (at the same time every day)
Weight loss and body composition reductions can occur even when consuming daily caloric intake in 1 hour [29, 30, 31, 32]

Cons:

It may be difficult to eat your daily recommended caloric intake in 1 hour.
There is a possibility that eating so much food in one sitting can increase total cholesterol, LDL cholesterol, and HDL cholesterol [29, 30, 31, 32].
It may cause elevated blood pressure [29, 30].
It may cause an impaired or delayed insulin response [30, 31].
It can increase the risk of eating disorders, such as binge eating [11].
You may experience extreme hunger while fasting until mealtime [29]

Who Is It Best For: The one meal a day fasting schedule is best for people with busy schedules who do not mind eating all their daily calories in one huge meal.

Which Fasting Schedule is the best?

Overall, 16:8 intermittent fasting is the best overall. Most fasting diets offer the same benefits, but 16:8 is by far the least restrictive to follow. But, research has not yet discovered which fasting schedule is the best. Whichever fast you choose, it is best to listen to your body and find the fasting schedule that works best for you.

Learn everything about fasting

FAQ

What is the best fasting schedule to lose weight?

Every fasting schedule results in weight loss if done correctly. For weight loss to occur, the fasting window needs to be followed consistently. You must eat a healthy and nutritious diet and not consume more than your daily recommended caloric intake when you are allowed to eat.

Does sleeping count as fasting?

Yes! The hours that you spend sleeping count as fasting hours.

What is the healthiest way to fast?

The healthiest way to fast is to ease yourself into fasting. Start with an easier schedule, such as 16:8 intermittent fasting, before you try to fast for 24 hours or more. Remember to eat healthy and nutritious foods before and after fasting to make the metabolic shift easier on your body.

What can you have while fasting?

What you can have while fasting depends on the diet you are following. If you are required to fast from food, you may drink water and other zero carbohydrate beverages while you are fasting. If you are allowed to consume some calories while fasting, you should stick to nutrient-dense foods.

How long is it best to fast for?

Research does not say how long it is best to fast for. Some fasting schedules may be easier to follow for a longer duration of time than others. It would be best to listen to your body and stop fasting if you experience any side effects.

Wrapping Up

When comparing fasting schedules, it is clear that they all have similar risks and benefits. There is no clear winning fasting schedule that will work for everyone. Find what one works best for you. Before beginning a new fasting schedule, or if you experience any side effects, make sure to speak with your doctor.

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