Intermittent fasting has exploded in popularity in the last few years. It has been known for decades that severely restricting calories (by roughly 40% from normal caloric intake) improves health and increases lifespan in animals ranging from fruit flies to monkeys. However, most people lack the discipline to maintain such a lifestyle (and for good reason – it isn’t a pleasant one). That’s why there was so much excitement about the discovery that completely cutting out calories for shorter periods of time, known as intermittent fasting or metabolic switching, could have an even greater benefit.

When you go without food for even just a few hours, your body switches into a fat-burning mode called ketosis. Ketosis is interpreted as a warning of lean times to come, so cells throughout your body begin recycling old components in a process called autophagy.

But how long is long enough in a fast to start seeing these healthy processes happen? Researchers and practitioners have experimented with a wide variety of intermittent fasting schedules and durations, ranging from a simple overnight fast of roughly twelve hours to more complex schemes such as eating normally for five days and then essentially starving for two days. Many of these intermittent fasting schedules and durations appear to have health benefits ranging from weight loss, to improved blood sugar control, to improved blood lipid markers and reduced inflammation.

Which intermittent fasting schedule is right for you? This article will explain the basis of intermittent fasting and help you figure out the best style for you as an individual.

What you will find in this article:

What happens to your body during a fast | What fasting style is right for you? | Intermittent fasting schedules and their benefits | How do you get started with intermittent fasting? | Is intermittent fasting right for you? 

How does intermittent fasting work and what happens when you fast? Learn more below.

What Happens During a Fast?

To understand the advantages and disadvantages of different intermittent fasting schedules, it’s important to first appreciate what goes on in your body during a fast. Of course, we can only talk in terms of averages, but let’s assume that you finish dinner at 6pm and then start a fast. Here’s the basic progression:

1. When you finish dinner, your body starts breaking down the food for energy. It uses some of the energy to fill your liver with glycogen, which is just a string of glucose (sugar) molecules. This glycogen is an easily tapped energy supply, as we’ll see later. Any excess calories are converted to triglycerides and stored in fat cells. We can think of triglycerides as a simple storage form of fat. Each triglyceride consists of three fatty acids and one glycerol molecule.

2. For the next eight hours – until roughly 2am, your body is running off of the food from your dinner. The fat, carbohydrate and protein from that meal are broken down and used to meet your body’s energy requirements. But by about 2am, you’ve used it all up.

3. For the next four hours – until roughly 6am, your body draws down the glycogen that it tucked away in your liver. The liver breaks off one glucose molecule at a time from strings of glycogen and releases them into your bloodstream. This helps maintain your blood sugar at its normal level, somewhere around 100 mg/dl.

4. At 6am, you’re 12 hours into your fast. When you wake up, you probably feel some hunger pangs. Your stomach secretes a hormone called ghrelin that screams to your brain “Hey, we’re hungry down here. Get us some breakfast!” But your body doesn’t wait around. It starts breaking down the triglycerides stored in your fat cells – turning them back into fatty acids and glycerol that are secreted into your bloodstream. Your heart, muscles and many other tissues happily use the fatty acids directly, absorbing them from your blood and burning them in their inner power plants called mitochondria.

5. If you don’t eat, by noon you’re 18 hours into your fast. The rate of fat breakdown increases. Although your muscles and other tissues are fine with using the fatty acids flowing into your blood, your brain isn’t. The brain is walled off from the rest of the body by a structure called the blood-brain barrier (BBB). Fatty acids are big enough that they don’t flow well through this barrier. For this reason, your liver starts to absorb some of the fatty acids, chops them up into smaller pieces called ketones, and secretes them back into the bloodstream. These ketones easily pass through the blood-brain barrier and the brain happily burns them for energy.

So we can summarize this progression as follows:

  • 0-8 hours after your last meal – energy comes from food in your stomach
  • 8-12 hours – energy comes from glycogen in your liver
  • 12-18 hours – energy comes from triglycerides in fat with a small amount of ketone production (“light nutritional ketosis”)
  • 18+ hours – energy comes from the triglycerides in fat at a higher rate with lots of ketone production (“heavy nutritional ketosis”)
How many hours of fasting per day and per week is right for you? It depends on your weight loss and health goals.

What Fasting Style is Right For You?

Let’s consider some of the most popular intermittent fasting schedules that you could try! They fall into three main categories:

1. Time-restricted eating (TRE)

This essentially involves packing all of your daily calories into a window of anywhere between 12 and 2 hours per day. Another way of looking at it is as a fasting period between 12 and 22 hours per day.

2. Alternate day fasting (ADF)

This approach allows you to eat normally one day and then fast (or at least greatly reduce your calories, to less than 500-800 calories) the next day. We’ll assume that this works out to four days of normal eating and three days of fasting every week.

3. 5:2 “Diet”

This plan allow you to eat normally for five days and then fast (or greatly reduce your calories) for two consecutive days.

In considering different intermittent fasting schedules you might ask yourself the following questions:

  • How many hours per week does this schedule result in any level of fat-burning ketosis?
  • How many hours per week does this schedule result in heavy ketosis?
  • How many times per week does this schedule force my body to switch into ketosis? This is referred to as the “metabolic switch” – from mostly burning sugar to mostly burning fat.

Let’s compare various intermittent fasting styles for whether and for how many hours per week they will get you into ketosis, versus whether and for how many hours per week you might feel hungry if you follow them! The numbers in the columns below rely on the assumption that the daily intermittent fasting or time-restricted eating (TRE) plans are followed every day of the week. Hunger pangs are usually the worst for the first two hours after the start of light ketosis and for two hours before bedtime for long fasts.

Plan Ketosis (hrs) Heavy Ketosis (hrs) Metabolic switches Longest Fast (hrs) Hunger (hrs)
12/12 0 0 7 12 0
14/10 14 0 7 14 14
16/8 28 0 7 16 14
18/6 42 14 7 18 14
22/2 70 42 7 22 14
ADF 72 54 3 36 12
5:2 48 44 1 60 8

The first number in each time-restricted eating plan (e.g. 12/12, 22/2) is the number of fasting hours in the day, the second is the number of “fed” hours in the day.

There are many different intermittent fasting schedules.

 Check out this intermittent fasting calculator to help you figure out when and how much to eat depending on your fasting schedule and activity levels.

Intermittent fasting schedules

Time-restricted eating, 12/12

Remember that this plan involves eating as you like for 12 hours and then fasting for the next 12 each day. This is usually done as overnight fasting. For example, finish dinner by 7pm and have breakfast at 7am the next morning. Don’t take in any calories after dinner until breakfast.

This is a great way to ease into intermittent fasting. It really doesn’t involve much in the way of dealing with hunger, but it doesn’t burn a lot of fat either. Bottom line – best for people new to fasting. But as simple as it is, it still provides benefits. It still prompts your body to at least start a daily metabolic switch from burning sugar to burning fat. Finishing eating a few hours before bedtime is especially important as it gives your gut a chance to rest. Even this plan may help improve insulin sensitivity if practiced for long enough.

Time-restricted eating, 14/10

All that’s required in this plan is to delay breakfast a bit. For example, finish dinner at 7pm and have breakfast at 9am. You’ll have to endure a couple of hours of hunger pangs every morning until breakfast, but it’s pretty easy to do and you will get used to it over time. You’ll have a couple of hours of ketosis (fat burning) every day and get your body used to switching between burning sugar and burning fat. This sort of “metabolic switching” helps maintain you in a good overall metabolic shape. This schedule is the logical next step after mastering 12/12 fasting.

Time-restricted eating, 16/8

If you can get your fasting up to this level, you’re doing great! This is a fantastic long-term destination for most people. It’s the approach used by many researchers in the field. It’s done most easily by having an early dinner (e.g. finish by 6pm) and a late breakfast (e.g. 10am). Yes, you’ll have to learn to ignore your stomach growling a bit in the mornings, but even that fades over time. If you’re not overweight or perhaps mildly so, this plan will help you gradually shed a few extra pounds (or kilograms) as you spend roughly 28 hours a week burning fat. A study in obese adults showed that fasting between 18:00 and 10:00 for 12 weeks leads to mild weight loss and improved blood pressure.

Time-restricted eating, 18/6

This plan is popular among serious fasters. Many of them skip breakfast altogether and eat two meals a day. An example schedule is finishing dinner by 6pm and not eating again until lunch the next day around noon. Followers of this schedule spend 42 hours a week in fat-burning mode and even get up to the brink of heavy ketosis every day. If you get to the point of following this schedule most days of the week, you’ll know that you’ve become a fast master.

Time-restricted eating, 22/2

This approach is also known as “OMAD” or “one meal a day”. As the name implies, it generally involves skipping breakfast and lunch while packing all calorie consumption into a 2-hour window or single meal at dinnertime. As with all intermittent fasting plans, it’s best if you can finish your eating a few hours before going to bed. This plan is great for people looking to lose serious weight because it puts you in fat-burning mode 70 hours every week with 42 of those hours representing heavy ketosis.

Although it sounds hard, many people settle into this schedule for months or even years once they get used to it. A small study conducted by Jason Fung’s research group showed that fasting for 24 hours 3 times a week and eating only dinner on fast days, eliminated the need for insulin in type 2 diabetic patients. This intermittent fasting approach also resulted in improved HbA1C, lower body mass index, and reduced waist circumference.

To do OMAD long-term, it’s best to work with a physician and a dietitian or nutritionist to make sure that you are getting all the nutrients your body needs in your one meal or 2-hour daily eating window.

Alternate day fasting (ADF)

ADF involves eating normally one day and then fasting the next. Sometimes instead of a complete fast, practitioners allow themselves up to 500 or so calories on “fast” days. If you take that approach, it’s important to get most of those calories from good fats like olive oil and to avoid carbohydrates. Carbs will kick you out of ketosis (fat-burning) and diminish the benefits of ADF.

Another way of looking at ADF is as three 36-hour fasts per week. Here’s an example schedule – eat normally on Monday until 6pm. Don’t eat at all (or at least very little) until breakfast on Wednesday morning. Finish dinner by around 6pm Wednesday night. Now don’t eat again until breakfast on Friday. Eat normally the rest of the day and finish dinner by 6pm or so Friday night. Now don’t eat again until breakfast on Sunday.

As you can see from the table, ADF gives you the most time in ketosis and heavy ketosis of all the plans we’re presenting here. It also involves only three mornings and three evenings per week when you’re likely to get a bit hungry. The downside is that you’re only forcing your body to make the metabolic switch between sugar and fat-burning three times a week. This schedule is great for the first several months of a serious weight loss plan. Some people are able to maintain it long-term, but many fall back to one of the other schedules after they’ve lost most of the weight.

ADF is effective for weight loss especially when combined with a low-carb diet. ADF seems to cause similar weight loss compared to daily caloric restriction, although ADF may to lead to greater reductions in fasting insulin and insulin resistance in insulin-resistant patients.

The 5:2 “diet”

Finally, the 5:2 plan involves eating normally for five days each week and then fasting for the last two. For example, you might decide to fast Monday and Tuesday and then eat normally the rest of the week. You can also look at it as one 60-hour fast per week. Finish dinner at 6pm on Sunday night and don’t eat again until breakfast on Wednesday morning, for example. This schedule provides the second greatest amount of heavy ketosis per week. However, it does the least to promote metabolic flexibility since you’re only making the switch from burning sugar to burning fat once per week. If you do follow this plan, it will be important to break your fast with a healthy meal and to monitor your blood sugar levels before and afterwards, to make sure that you are improving your blood sugar control and not causing a large spike in blood sugar.

The 5:2 diet also be of interest to people suffering from auto-immune disorders since a weekly 60-hour fasting period is long enough to kill off and allow replenishing of large numbers of old white (immune) cells.

Of course, there are countless variations on these basic themes. Some people practice 16:8 every day but relax a bit over the weekend. Others supplement a TRE plan like 14:10 with one 24-hour fast each week. No matter what plan you end up following, it’s good to track the amount of time you spend in ketosis and heavy ketosis each week. It’s the time spent burning fat – especially the heavy ketosis periods – that’s important in losing weight.

Now we have the knowledge to put together this final summary table:

Plan Benefits Best For
12/12 No extra hunger, easy to adopt. People new to intermittent fasting.
14/10 Some fat-burning but pretty easy for most people to handle. Healthy people looking for an easy way to fast without a great deal of suffering.
16/8 Nice daily fat burn that will help encourage a gradual move toward a healthy weight. Normal weight or slightly overweight people looking for an intermittent fasting style they can adopt for life.
18/6 Gets the body to the brink of heavy ketosis every day. Body builders and serious athletes as well as overweight people who have mastered easier intermittent fasting schedules.
22/2 Lots of good fat burning every day. Even puts the body into regular autophagy for recycling of old proteins. People looking to lose serious amounts of weight, reverse type 2 diabetes, lower their risk for Alzheimer’s and Parkinson’s.
ADF The greatest period of heavy ketosis of any of the popular intermittent fasting plans. Lots of autophagy too. People looking to jumpstart serious weight loss or reduce risk of disease.
5:2 A long fasting period of 60 hours each week. People looking to lose significant weight.

How do you get started with intermittent fasting?

Human nature is to jump into a new practice with both feet. Unfortunately, most of us tend to flame out quickly, despite our good intentions. We highly recommend that you start your introduction to intermittent fasting with the 12/12 plan. It’s a great way just to consistently cut out those late-night snacks that really pack on the pounds. The 12/12 plan is really the essence of intermittent fasting – learning that you’re not going to die if you don’t eat for a few hours.

Once you master this fasting schedule, you can move on to anything you’d like. The most logical and perhaps easiest progression is to start to move dinner up and breakfast back as you adopt a 14/10 schedule. However, some people will move straight from 12/12 to one of the more ambitious plans like ADF or 5:2.

Use the following flowchart to decide what intermittent fasting schedule is right for you, and keep the recommendations below in mind:

  • Don’t adopt intermittent fasting if you have type 1 diabetes, an eating disorder, or are already underweight. Pregnant women and nursing mothers should avoid fasting as well. Intermittent fasting should not be practiced at all by children under 18 unless under the supervision of a physician.
  • Stay hydrated while you fast. Drink lots of water and unsweetened tea or coffee. Try to avoid beverages with artificial sweeteners, but it’s not the end of the world if you slip up.
  • Consider monitoring your blood glucose (sugar) as well as your ketones with a device like KetoMojo or BIOSENSE.
  • If you start to feel bad, eat! Intermittent fasting is a wonderful health intervention, but don’t take it too far. You can even try to stay in ketosis by avoiding carbs and getting the bulk of your calories from good fat like olive oil.

A mobile app such as the LIFE fasting tracker from LifeOmic can help you integrate intermittent fasting into your lifestyle in a safe and effective manner. Remember that intermittent fasting isn’t a diet. It’s best to think of it as a healthy life-long habit like exercise and eating lots of good fruits and vegetables. Don’t give up if you have a bad week. Start off slowly and build up over time. Slow and steady wins the race!

Doing a fast? Share your experiences with us on Instagram and Facebook  with the hashtags #SeizeLIFEFast #LIFEfastingapp.

Download the LIFE Fasting Tracker app here.

Don Brown

Dr. Brown is LifeOmic’s Chief Executive Officer and one of the most successful serial software entrepreneurs in the Midwest. He received a bachelor’s in physics from Indiana University in 1978, a master’s in computer science from IU in 1982, an MD from the Indiana University School of Medicine in 1985, and a master’s in biotechnology from Johns Hopkins University in 2017.

Don is an avid outdoorsman who loves hiking, rock climbing, skiing, and snowshoeing with his eight children – especially in and around Park City, Utah.

LifeOmic® is the software company that leverages the cloud, machine learning and mobile devices to improve healthspans – from prevention and wellness to disease management and treatment.



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