Time-restricted eating – a detailed intermittent fasting guide

By , medical review by

How to improve:

Overweight

Obesity

Type 2 diabetes

Metabolic syndrome

Insulin resistance

Fatty liver

High blood pressure

High cholesterol

High triglycerides

…and many more…

By Doing ABSOLUTELY NOTHING!!

A guide by Ted Naiman, MD.

Time-restricted eating

Fed vs. fasted

You can think of your body as having two different and opposing states: ‘Fed’, and ‘Fasted’.

In the fed state, insulin is elevated, and this signals your body to halt any fat burning, to store excess calories in your fat cells, and burn glucose (from your last meal) instead.

In the fasted state, insulin is low (while glucagon and growth hormone, opposing hormones to insulin, are elevated). The body first burns stored glucose from glycogen, and then starts mobilizing stored body fat from your fat cells and burning it for energy (instead of glucose).

The practical importance of all this is that you can burn more stored body fat while in the fasted state, and you store more body fat while in the fed state.

Insulin resistance

Unfortunately, we seem to be spending less time in the fasted state and more time in the fed state. As a result, our cells spend less time mobilizing and burning stored body fat, and instead we continuously use the glucose-burning pathways. This leads to a chronic elevation of insulin and a near complete reliance on glucose.

Over time, this chronic exposure to too much insulin leads to ‘insulin resistance’ where the body secretes even more insulin in response to the fed state. Chronic insulin resistance can lead to the ‘Metabolic Syndrome’: obesity, abdominal fat storage, high triglycerides, low HDL, and elevated glucose with eventual type 2 diabetes (1 in 12 humans on earth currently have full blown type 2 diabetes, while 35% of adults and 50% of older adults have Metabolic Syndrome, or pre-diabetes).

Someone with insulin resistance is burning predominately glucose on the cellular level, and they rarely get the opportunity to burn any body fat. When these people run out of glucose from their last meal, instead of easily transitioning over to the fasted state to burn fat, they become hungry for more glucose (from carbohydrates) as their bodies and cells have decreased capacity for mobilizing and burning fat for energy. This pattern can start a cycle of eating every few hours, raising glucose and insulin, and then eating more when when blood sugar drops.

Let’s put it this way. Why would an obese person frequently feel hungry? They have enough fat stores to last a very long time. The world record for fasting went to a 456 pound man who fasted for 382 days, consuming only water and vitamins and losing 276 pounds with no ill effects (although we don’t recommend this!).

But the average overweight person is used to being in the fed state, has very little practice in the fasted state, and is continually burning glucose rather than fat at the cellular level. They have insulin resistance, which is both caused by and also leads to chronically high insulin levels, which promotes fat storage and suppresses fat mobilization from the adipocytes (fat cells). They even have changes in the mitochondria, or tiny energy factories inside the cells.

The mitochondria can burn either glucose (sugar) or fat for fuel, and over time they will have a preference for one over the other; “sugar burners” have increased the pathways in the mitochondria that burn glucose and decreased, or down-regulated, the underused pathway for burning fat.

A good analogy is that of a tanker truck on the freeway filled with oil. If the tanker truck runs out of gas it stops moving, despite the fact that it has 10,000 gallons of potential fuel on board. Why? Because it prefers to run on refined gas and is incapable of burning the stored oil for fuel.

Fat adaptation

Humans have the ability to become ‘fatadapted’ and improve their ability to fuel themselves with stored body fat instead of glucose. However, this takes time and practice, and your body has to do a number of things to slowly up-regulate (or increase) your fat-burning pathways.

This includes improving insulin sensitivity to lower insulin and promote fat mobilization into free fatty acids from the adipocytes (fat cells) as well as upregulating the fat-burning pathways at the cellular level (in the mitochondria). There are several ways to improve ‘fat adaptation’ or the ability to successfully burn stored body fat for energy, and these include the following:

Low carbohydrate diets. Eating a LCHF (Low Carb High Fat) diet improves the body’s ability to utilize fat for energy, as there is more fat and less glucose available at all times.

Exercise. High-intensity exercise depletes glucose and glycogen rapidly, forcing the body to switch over and utilize more fat for fuel. Exercise also improves insulin sensitivity.

Caloric restriction. Eating fewer calories also equals less glucose available for fuel, so the body is forced to rely on stored body fat for fuel. You will naturally eat the lowest calories when you maximize nutrient density by eating whole, natural, unprocessed foods found in nature.

Intermittent fasting, and spending more time in the fasted state, which gives the body more ‘practice’ at burning fat.

Metabolic exercise

Intermittent fasting is a strategy for exercising and strengthening the body’s ability to exist in the fasted state, burning fat instead of continually burning sugar (glucose).

Just like anything else, this ability can be strengthened over time with practice. But this ability also atrophies or shrinks with lack of use, just like your muscles atrophy when you break your arm and have to wear a cast for weeks. You can consider spending time in the fasted state as a form of exercise—a METABOLIC WORKOUT.

In fact, there are many parallels between exercise and fasting. Exercise does all of the following beneficial things:

Decreases blood glucose.

Decreases insulin level.

Increases insulin sensitivity.

Increases lipolysis and free fatty acid mobilization.

Increases cellular fat oxidation.

Increases glucagon (the opposite of insulin).

Increases growth hormone (the opposite of insulin).

BUT did you know you can also accomplish all of the above by doing ABSOLUTELY NOTHING? The secret is *FASTING*.

Extending the amount of time that you spend during your day in the FASTED state (as opposed to the FED state) can accomplish all of these, very similar to exercise. That is why I say extending time in the fasted state is a form of metabolic ‘exercise.’ It can ‘train’ your body to more efficiently mobilize free fatty acids from fat stores.

Just as overweight and out of shape people struggle to jog or lift weights or participate in other forms of physical exercise, they are also generally out of practice when it comes to mobilizing stored fat for fuel. Perhaps they just need more practice!

Less feeding, more fasting

One of the best ways to achieve effortless and long-lasting fat loss in my opinion is to train yourself to eat two meals a day (and eliminate snacking). The easiest and best way to accomplish this? Leverage your natural overnight fast by skipping breakfast (drinking coffee makes this easier and more enjoyable for some people).

Typically, the fed state starts when you begin eating and for the next three to five hours your body digests and absorbs the food you just ate. Insulin rises significantly, completely shutting off fat-burning and also triggering excess calories to be stored as fat.

After the first few hours mentioned above, your body goes into what is known as the post–absorptive state, during which the components of the last meal are still in the circulation. The post–absorptive state lasts until 8 to 12 hours after your last meal, which is when you enter the fasted state. It typically takes 12 hours after your last meal to fully enter the fasted state.

Exercise helps

When you’re in the fasted state your body can burn fat that has been inaccessible during the fed state. Because we don’t enter the fasted state until 12 hours after our last meal, it’s rare that our bodies are in this fat burning state. This is one of the reasons why many people who start intermittent fasting will lose fat without changing what they eat, how much they eat, or how often they exercise. Fasting puts your body in a fat burning state that you rarely get to enter during a normal eating schedule.

Exercise can also help with fat adaptation. Your glycogen (the storage form of glucose in your muscles and liver) is depleted during sleep and fasting, and will be depleted even further during training. This can further increase insulin sensitivity.

Fasting myths

There are many myths about fasting:

“Breakfast is the most important meal of the day!”

We have all been told to eat breakfast. Unfortunately this is inaccurate advice.

When you first wake up in the morning, your insulin level is quite low and most people are just starting to enter the fasted state, 12 hours after eating the last meal of the previous day. Eating at this time raises insulin and glucose and immediately shuts off fat-burning. This is especially true for a high carb breakfast. A potentially better choice would be to push the first meal of your day out at least a few hours, during which time you can continue in the fasted state and burn stored body fat.

Interestingly, many properly fat-adapted people aren’t very hungry in the morning and have no problem skipping breakfast.

This is appropriate, as throughout our evolution humans were hunter-gatherers. Rather than eating a large breakfast first thing in the morning we would hunt and gather throughout the day, having a larger meal later in the day. I highly recommend mimicking this pattern by skipping breakfast.

“Eat small frequent meals.”

There has been plenty of troubling advice here. We have been told to eat frequently to “keep your metabolism going” and “don’t let your body enter starvation mode.” This is all likely the exact opposite of the truth: in order to burn fat, you want to spend as much time in the fasted state as possible and get very efficient at living on stored body fat rather than caloric intake from constantly eating.

“Fasting leads to burning muscle instead of fat.”

Many people are concerned that if they start fasting they will either stop making muscle or maybe even burn muscle. If you are under weight and fast for a prolonged period of time, this may be true. Otherwise, this is simply not true.

In fact, growth hormone is increased during fasted states (both during sleep and after a period of fasting).

“Your metabolism slows down when you are fasting.”

Growth hormone is highly anabolic (builds muscle), and is used in combination with testosterone by bodybuilders who want to simultaneously build as much muscle and burn as much fat as possible. Growth hormone elevates in fasting, which can help preserve muscle, and this makes sense. In our hunter-gatherer ancestors, if fasting and going without food made you weaker and slower you would never catch or find any food and you would die and humans would become extinct.

For shorter fasts, this is completely false. A number of studies have proven that in fasting up to 72 hours, metabolism does not slow down at all and in fact might speed up slightly thanks to the release of catecholamines (epinephrine or adrenaline, norepinephrine, and dopamine) and activation of the sympathetic nervous system (sympathetic nervous system is often considered the “fight or flight” system, while the opposite is the parasympathetic nervous system or the “rest and digest” system).

It makes sense that this fight or flight sympathetic nervous system would be activated during the daytime, when hunter-gatherer humans are most active and in the fasted state (looking for food), followed by parasympathetic “rest and digest” mode in the evening after eating a large meal.

“If I don’t eat I will get low blood sugar [hypoglycemia].”

Studies have shown that healthy persons who have no underlying medical conditions, who are not taking any diabetes medications, can fast for long periods of time without suffering from hypoglycemia. In fact, many people find sensations of hypoglycemia or low blood sugar (in non-diabetics) result from eating a very high glycemic index food a few hours prior (blood sugar spikes, then insulin spikes, then blood sugar drops rapidly).

However if you are a diabetic, especially if you are on any diabetes medications, you definitely need to check with your doctor before starting a fasting protocol. Some diabetes medications can lead to severe hypoglycemia when fasting (mostly insulin and sulfonylurea drugs like glipizide, glimepiride, and glyburide). [Be sure to check with your doctor prior to starting a fasting protocol if you have any medical problems, diabetes or otherwise. You and your doctor can use our guide for low carb diets and diabetes medications as the same concepts apply to fasting.]

How to fast intermittently

There are a number of ways to actually perform intermittent fasting, but the easiest and most popular varieties involve taking advantage of your natural overnight fast by skipping breakfast and pushing the first meal of the day forward a number of hours. Once you have passed the 12 hour mark from dinner the night before, you are truly in a fasted state and you begin to rely on stored body fat for fuel.

It is likely that the longer you stay in the fasted state, the deeper your fat adaptation will get. In fact, if you can maintain this intermittent fast for 20 to 24 hours you may achieve an even higher rate of lipolysis (breakdown of stored body fat into free fatty acids, available for burning in the cells) and fat oxidation (burning of fat in the mitochondria).

When you first start out with intermittent fasting, you may feel hungry and low energy. In this case I recommend starting out with “baby steps”, by just pushing breakfast out an hour or two at first, then slowly increasing the fasting interval. As time goes by and you become more “fat adapted”, it is easier to fast. This is identical to exercise in those who are sedentary: it is painful and extremely difficult at first, and then once you are adapted it gets easy and even enjoyable.

LCHF diet for fat adaptation

Many individuals and clinicians find it easier to fast if you are already on a LCHF (low carb high fat) diet, as these diets naturally lead to fat adaptation and are naturally lower insulin secretion and glucose utilization. In fact, I HIGHLY recommend the combination of a very low carb diet with intermittent fasting.

The closer you get to a ketogenic diet (extremely low in carbohydrates, moderate in protein, and high in fat) the easier it is to go for hours and hours without eating.

For those who do incorporate carbohydrates in the diet, I would recommend that these mostly consist of FIBER, which is indigestible and does not contribute to the elevation of glucose and insulin and can also add to satiety. If you do decide to eat digestible carbohydrates, I recommend not eating these first thing in the morning, as this can contribute to fat storage and sabotage fat burning, as well as setting you up for a blood sugar and hunger roller coaster for the rest of the day.

Popular forms of intermittent fasting

There are several popular ways to accomplish intermittent fasting and I will discuss the three most popular varieties next. All of these involve lengthening the overnight fast by skipping breakfast and postponing the first meal of the day. All of these also involve eating no calories at the beginning of the day and the majority of your calories later in the day, a concept called a caloric ‘reverse taper’.

Leangains – also called 16:8

Keep in mind that for the purpose of these discussions we will consider the baseline standard diet to involve 12 hours of fasting (overnight) plus 12 hours of an eating window during the day consisting of three meals, breakfast, lunch, and dinner. The sad reality however is that most people are actually eating first thing in the morning until late at night, with lots of snacking, so the eating window for the average American is probably even larger than 12 hours.

Leangains, as popularized by bodybuilder Martin Berkhan, is by far the most popular method of fasting intermittently. This form of fasting consists of skipping breakfast every morning and pushing the first meal of the day to lunch, eating within an eight hour window. The idea is to fast for 16 hours (overnight plus the first ~6 hours of your day), then eat all your calories in an 8 hour window.

For example, let’s say you get up at 6:00 a.m. You would skip breakfast and eat nothing for six hours, then lunch at noon and dinner at 8:00 p.m. Snacking inside your eating window is allowed (although I will say that generally speaking you want to try to consolidate calories into larger meals rather than snacking). This 16:8 split (16 hours fasting and 8 hours eating) is recommended every single day.

If you had one day off from this protocol and followed this the other six days of the week, that would amount to an additional 4 hours of fasting per day compared to the standard 12:12 split that we are assuming to be baseline (12 hours fasting and 12 hours eating). Four hours per day times six days per week equals about 24 hours of total additional fasting per week. [4 hours fasting per day times 6 days per week = 24 hours]

Warrior diet

The Warrior Diet, as popularized by Ori Hofmekler, consists of fasting during the majority of the day, then eating all of your calories in the evening. The goal is to skip breakfast and lunch, then eat a large dinner in a four hour window at the end of the day. This is a 20:4 hour split (20 hours of fasting and then a 4 hour eating window at the end of the day).

This method of fasting does allow you to eat very large very satisfying meals at the end of the day, and might be perfect for someone who was going out to dinner to eat in a social setting, where a ton of calories and food might be involved (like one of those all-you-can-eat Brazilian steak houses!).

Fasting this long during the day can be more difficult for some, but it may also lead to a deeper level of fat adaptation and lower insulin. If one followed this protocol roughly every other day (let’s say three days a week), that would equate to eight extra hours of fasting compared to the 12:12 baseline standard diet, times three days per week would also equal about 24 hours of total additional fasting per week. [8 hours fasting per day times 3 days per week = 24 hours]

Eat stop eat

Eat Stop Eat, as popularized by bodybuilder Brad Pilon, involves fasting for an entire 24 hours, two days per week. Let’s say you eat your last meal of the day at 8:00 p.m. the day before. You fast overnight and then all the following day, skipping breakfast and lunch, and then pushing dinner out to 8:00 p.m. (for a full 24 hours with no calories).

I only recommend this two days per week (nonconsecutive). Many people think that the following day they will binge on so much food that the benefits of the fasting on the previous day will be negated, but this is usually not true, especially if you are consciously aware of the risk.

Studies have repeatedly shown that persons may overeat by a couple hundred calories the next day, but still not come anywhere close to eating as much as they would have by eating normally both days (in other words, you are still left with a net caloric deficit even after eating more food the day after your fast).

Each day that you fast in this fashion adds 12 hours of fasting compared to the standard 12:12 split we are calling baseline, and two days per week of this again equals about 24 hours of total additional fasting per week. [12 hours fasting per day times 2 days per week = 24 hours]

It’s all good

With all of these fasting methods, the goal is to skip a meal or two, avoid snacking, and consolidate calories. All of these methods are effective, and you can mix and match these as much as you would like. I would highly recommend keeping it flexible. Fast for as long as is convenient on any given day, and break your fast whenever you need to or want to. Anything beyond a 12 hour window is going to be at least somewhat beneficial towards anyone’s goals.

If you planned on fasting 16 hours but only make it 14, that’s ok and you are still much better off than if you had eaten all day long. I think a good goal would be 24 hours per week of additional fasting (additional to the standard 12:12 baseline). This could be 2 days of 24 hour fasting (Eat Stop Eat), 3 days of 8 hour fasting (Warrior Diet), or 6 days of 4 hour fasting (Leangains).

Coffee = awesome

During the fasts feel free to drink ANY noncaloric beverage you want, including but not limited to: water, coffee, tea (hot or iced), or any other beverage with no calories. However I would NOT recommend any calories AT ALL, as it takes frightfully few calories to increase insulin and sabotage your fast. Fat is the macronutrient that raises insulin the very least, which is why so many people are adding fat (butter, coconut oil, etc) to coffee in the morning.

However, I do not routinely recommend this or any other source of calories while fasting, as this may limit some of the benefits of fasting. If you will absolutely “die” without at least a tiny splash of cream or fat in your coffee, well then do it. You will likely be better off with it than if this prohibition against fat in your coffee keeps you from trying to fast intermittently at all (95% fasting much better than 0% fasting)!

However I would try to keep the cream in your coffee to an absolute MINIMUM quantity, and you should also use this opportunity to learn to drink coffee black (this is something anyone can learn over time, believe it or not).

Enjoy your newfound freedom from food

Once you are properly fat adapted, intermittent fasting can be easy, fun, enjoyable, and liberating—while making you leaner and healthier in the process! Let’s say you are following the Leangains protocol. Breakfast every day during the work week is now just black coffee, tea or water, how easy is that? No more worrying about what you are going to grab for breakfast as you rush around in the morning and struggle to get to work on time or get the kids ready for school.

This saves you time, work and effort and is literally a form of metabolic exercise! In the meantime, you are improving your insulin sensitivity and strengthening your fat adaptation. This is a win in many ways. It frees you to eat large and satisfying meals later without feeling the deprivation, without watching calories, and without restricting yourself.

And on days where you skip breakfast and lunch, you will be amazed at how much extra time you have not worrying about what to eat, where to get it, and when to find time to eat it. Your productivity will be higher and you will have more free time. (please read our guide on OMAD for more potential benefits and concerns about eating one meal per day).

Some pointers

Check with your doctor before initiating intermittent fasting, ESPECIALLY if you are diabetic and on diabetes medications!

You can generally take any vitamins or supplements you want while fasting as long as they don’t have calories, but you don’t need any supplements as you will be eating plenty of nutrient-dense foods every day.

You don’t have to worry about losing muscle from lack of protein during your fast, as long as you eat adequate protein at the meals before and after fasting.

You will not lose muscle while fasting as long as you are exercising regularly, and I specifically recommend resistance training such as lifting weights.

Following a LCHF (low carb high fat) diet pairs nicely with intermittent fasting, as both improve fat adaptation.

It is perfectly fine to exercise while fasting, either cardio or lifting weights (lifting weights is better for body composition and I highly recommend it for everyone, as this will further your goals considerably) See our guide on exercise to decide what type of exercise best suits your goals.

Drink plenty of water and non-caloric beverages while fasting; coffee and tea in the morning make fasting considerably more enjoyable.

Don’t use intermittent fasting as an excuse to eat tons of junk food when you are eating—continue to eat responsibly, sticking with whole natural foods with high nutrient density and avoiding highly processed foods!

More

Intermittent fasting for beginners

Intermittent fasting video course

Fasting Q&A

