Intermittent Fasting on a Keto Diet

Updated Nov 22nd, 2022 – Written by Craig Clarke

Medical review by Dr. Barton Jennings

Intermittent Fasting, or “IF,” is a relatively new craze that can easily be incorporated into any diet. It revolves around timing your food intake in a way that can help improve overall health and boost long-term weight loss results.

Keto dieters often add intermittent fasting regimens to their keto meal plan for the unique benefits of fasting. It can also be used as a weight loss plateau busting strategy or a quick hack for entering ketosis.

That said, questions still remain regarding how long, how much, and how frequently we should use intermittent fasting on keto.

To learn everything you need to know about this incredibly simple tool, let’s take a closer look at the following fasting topics:

What is Intermittent Fasting?

Intermittent fasting is a way of eating where you cycle between a feeding state and a fasting state.

Your body is in a feeding state when you are eating your food, and you are in a fasting state when you are not consuming calories (i.e., when you are between meals without any snacks or calorie-containing beverages).

Each state stimulates vastly different biological processes in the body. Extending our time in the fasted state, for example, helps boost autophagy, reduce insulin levels, increase fat burning, and promote ketone production.

We’ll learn more about these mechanisms and how intermittent fasting benefits the body below. But first, let’s develop a better understanding of what “IF” looks like in practice.

Overview of Intermittent Fasting Methods & Common Keto Fasting Regimens

How to start intermittent fasting

There are a seemingly endless variety of intermittent fasting plans. Here are the most common methods:

  • Skipped meals. This is when you skip a meal to induce extra time in the fasted state. Usually, people choose breakfast, but others prefer to skip lunch or dinner.
  • One meal a day (OMAD). It is as simple as it sounds: Restrict yourself to one meal a day and fast until tomorrow’s meal.
  • Eating windows. The most common eating window regimen is the 16/8 schedule. Simply put, this means that you’ll condense your entire macronutrient intake within an 8-hour eating window. For the remaining 16 hours, you’ll be in a fasted state (i.e., consuming no calories whatsoever). As the body adapts, many keto dieters will extend their fasting window and reduce their eating window.
  • Keto coffee fast. An excellent IF method for keto beginners. Simply drink Bulletproof or Ketoproof coffee as a breakfast replacement and see if it helps you extend your fasting window to dinner. Though it’s not a true fast (because the beverage contains fat calories), you’ll still be fasting from carbs and protein, which provides most of the same benefits as intermittent fasting.
  • 24-48 hour cleanse. This is a prolonged fast that may also be referred to as alternate day fasting. With this fasting regimen, you’ll have a fasting day or two fasting days followed by a day of eating as much food as you’d like.

We don’t recommend starting with a 1-2 day prolonged fast. Instead, try restricting yourself to a specific eating window (and use ketoproof coffee as needed).

Typically, people restrict themselves to an 8-hour eating window between 1 and 9 pm or 12 and 8 pm. Once you have the hang of eating on a schedule, you can try more prolonged periods of 18-24 hour fasting.

That said, if you’re struggling with skipping breakfast, give the keto coffee fast a try. This can help boost your ketone levels and reduce your appetite naturally.

Whether you decide to do it every day, once a week, or twice a week is up to you. There is no such thing as the best intermittent fasting plan, so do what makes you feel best and listen to your body.

If it doesn’t work for you, do not force yourself to fast. Intermittent fasting isn’t required to lose weight and improve health on a keto diet. Restricting yourself unrealistically is pointless – it’s not worth it if it makes you unhappy and unhealthy.

How Does Intermittent Fasting Work?

How IF works
The whole point of intermittent fasting is to allow ourselves to increase the amount of food we can intake at one time. Our bodies naturally can only take in a certain amount of food at once, so we are creating a sort of limit on our calorie intake.
This is also a great method for people that overeat. I tend to see people that forget to count the snacks that they have throughout the day, and wonder why they are putting weight on.

Your body will adjust itself to fasting, and you will find yourself not as hungry as you used to be. This allows you to properly record and maintain the nutrient values of what you intake.

In this fasting state, our bodies can break down extra fat that’s stored for the energy it needs. When we’re in ketosis, our body already mimics a fasting state, being that we have little to no glucose in our bloodstream, so we use the fats in our bodies as energy.

Intermittent fasting is using the same reasoning – instead of using the fats we are eating to gain energy, we are using our stored fat. That being said, you might think it’s great – you can just fast and lose more weight. You have to take into account that later on, you will need to eat extra fat in order to hit your daily macros (the most important thing). If you’re overeating on fats here, you will store the excess.

While there are some weight loss advantages to fasting, it’s more for the convenience of timing. Do not fast solely for the weight loss if you do not enjoy doing it. There are other benefits, though, and we’ll discuss these too.

Intermittent Fasting – Why Meal Timing Matters

Meal timing matters

Simply put, intermittent fasting is the dietary strategy of restricting your food consumption to a specific window of time. For example, one of the most common intermittent fasting approaches is fasting during an 18-hour window of time and eating during the 6-hour window of time that is left in the day.

Let’s say your last meal was at 6 pm last night and you ate nothing else after that. To implement an intermittent fast, simply restrict eating until 12 pm the next afternoon (yes, sleeping time counts as fasting time). To do this every day, only eat between 12 pm and 6 pm and fast for the remainder of the day.

There are many different variations of intermittent fasting as well. Dr. Dom D’Agostino, a well-known ketogenic diet researcher, suggests doing a longer intermittent fast for 3 days, 3 times a year. This means not eating for 3 days, and eating normally until the next fast.

Another way to try fasting for yourself is by incorporating intermittent fasts of 16 or more hours per day into your lifestyle. This is a much more accessible strategy that allows you to experience many of the benefits of taking a short break from calories.

Despite the simplicity of this concept, it may not be readily apparent how beneficial intermittent fasting can be for you, especially if you are already benefiting from the ketogenic diet.

Interestingly enough, the keto diet and intermittent fasting work better when used together, creating a positive feedback loop of better results. To get a better idea of what I mean, let’s take a closer look at how this strategy works and the benefits it can yield.

How Intermittent Fasting Works: Insulin Reduction, Ketone Production, and Autophagy Induction

From a weight-loss perspective, intermittent fasting works by making it harder to overeat throughout the day. A simple rule like “skip breakfast” or “only eat between 5 pm and 8 pm,” can help prevent you from reaching for snacks or drinking calorie-dense beverages throughout the day that contribute to weight gain.

Even if you do build up a ferocious appetite while fasting, you’ll still find it difficult to overeat. In fact, intermittent fasting tends to decrease daily energy consumption and promote fat loss.

This means that you may be able to eat as much as you want and reach your goals as long as you stick to a shortened eating window or a limited number of meals.

When you first try intermittent fasting, your body will need to adjust itself to this new eating schedule. You may be hit hard by hunger pangs and potent cravings at first, but they will soon dissipate as your cells feast on stored fat and ketones.

The key mechanisms behind your ability to fast, lose fat, and improve your health throughout the process are insulin reduction, ketone production, and autophagy. As we accumulate time in a fasted state, our insulin levels decrease incrementally. This promotes the release of fat from our fat cells and stimulates the ketone-producing process known as ketogenesis.

As you continue your fast, you’ll enter a deeper state of ketosis, become more efficient at burning fat,  and ramp up the self-cleaning process known as autophagy.

Autophagy: A Nobel Prize Winning Lifestyle “Hack”

In 2016, the Nobel Prize in Physiology or Medicine was awarded to Yoshinori Ohsumi for discovering some of the mechanisms of autophagy — the process by which the cell devours itself. Superficially, this sounds like a terrible thing for our cells to do — until you consider what is really taking place.

When our cells undergo the process of autophagy, non-essential parts like damaged proteins are recycled and invading microorganisms and toxic compounds are removed. This means that autophagy plays an important role in stopping the aging process, reversing disease, and preventing cancer, but it doesn’t happen all the time.

Fasting, protein restriction, and carbohydrate restriction are the three main ways that can initiate different autophagic processes — all of which are not the same. This is part of the reason why a ketogenic diet has so many positive effects, and it also shows you why intermittent fasting is a way to improve your diet even more.

Though this is reason enough to fast more, autophagy isn’t the only upside of intermittent fasting. In fact, the combination of keto and fasting can provide us with a wider array of benefits.

The Science-Backed Benefits of Intermittent Fasting

From improving blood sugar control and body composition to helping with the treatment of cancer and diabetes, the case for restricting when we eat extends far beyond weight loss. Below, you’ll find several reasons why intermittent fasting on a ketogenic diet may be a good thing for you:

1. Improved Blood Sugar Control and Insulin Sensitivity

By giving your body an occasional break from calorie consumption, you can help improve blood glucose levels and enhance insulin sensitivity.

This makes it a helpful tool for type 2 diabetes prevention. In fact, one clinical trial found that intermittent fasting may even be a better strategy than maintaining the same calorie deficit with six meals per day.

When combined with the keto diet — which also has been found to help with insulin resistance and type 2 diabetes — the two dietary strategies can work synergistically for improving blood sugar control and reducing insulin levels. However, more research is needed on the effects of using them in tandem.

2. Decreased Body Fat Percentage and Better Body Composition

According to a 2014 review of intermittent fasting research, intermittent fasting alone was found to promote weight loss at a rate of roughly 0.55 to 1.65 lbs (0.25–0.75 kg) per week. This was accompanied by a 4-7% decrease in waist circumference, indicating that they lost belly fat.

Another IF research review discovered that an alternate day fasting protocol decreased body fat by up to 16% over 3-12 weeks. When this finding was compared with daily calorie restriction, the researchers concluded that “these diets are equally as effective in decreasing body weight and fat mass, although [alternate day fasting] may be more effective for the retention of lean mass.”

In other words, intermittent fasting may help us reduce our body fat percentage more efficiently than constant calorie restriction.

3. Enhanced Mental Clarity

Fat and ketones are the most energy-efficient fuels for your body to run on, and your brain is a huge consumer of energy. Once your body is keto-adapted, your brain can effectively run on ketones, which are derived from fat breakdown in the liver.

When our brain uses ketones,  our energy levels tend to even out with less mental fatigue and more focus. This is why keto dieters tend to notice sustained energy levels throughout the day rather than the peaks and troughs of energy that we tend to experience on a high-carb diet.

Intermittent fasting is one of the easiest ways to boost your ketone production and experience the mental clarity that ketone burning brings.

As a result, you may naturally find yourself being more productive before you consume your first calories of the day.

Mental Clarity and Keto

4. May Boost Overall Fitness Results

People tend to think that if you don’t utilize pre- and post-workout meals, you’ll lose muscle when you work out.

This isn’t necessarily true, and it is even less so when you are adapted to ketosis.

Fasting while training can lead to a number of benefits in the long run, including:

  • Higher metabolic adaptations – Studies show that your training performance will increase in the long run when you are exercising in a fasted state.
  • Improved muscle synthesis – Studies show that muscle gains are heightened when you train in a fasted state and utilize proper nutrient intake.
  • Improved response to post-workout meals – Studies show that the speedy absorption of nutrients after a fasted workout can lead to better results.

IF and Muscle

There have been numerous studies on fasting while training, including one on Muslim athletes during Ramadan. It concluded that there is no effect on performance in training while fasting – so you have no need to worry.

5. Simplifies Your Keto Meal Plan and Make Weight Loss Easier to Maintain

Maintaining weight loss results is the most difficult part of any diet. We often rely on willpower to restrict ourselves, which never works in the long run. Rather than relying solely on restriction, it is better to implement simple weight loss strategies that feel relatively effortless for you.

For some, intermittent fasting is the perfect example of this. They just follow a simple fasting regimen that works best for their schedule and find it more challenging to gain weight than to keep it off.

By incorporating intermittent fasting with your keto or low carb diet, you may find that it takes the weight of meal prep and planning off your shoulders while it takes the excess weight from your belly.

Keep in mind, however, this is just one example of a weight loss tool. There are dozens of other strategies and hacks you can try that may work better for you.

Intermittent Fasting and Muscle

Muscle loss reduced when fasting
Two ground-breaking studies have recently been published on the effects of intermittent fasting on males. One group of researchers studied the effects that 16 hours of intermittent fasting had on males that lift weights. They found that muscle mass stayed the same, fat mass decreased significantly, and the males who fasted for 16 hours a day burned more fat for fuel compared to the control group that only fasted for 12 hours.

Another study showed that combining 20 hours of fasting with resistance training resulted in an increase in muscle mass, strength, and endurance, and this was achieved by eating ~650 calories per day less than normal.

The benefits of intermittent fasting translate to untrained overweight and obese individuals as well. One study published in Obesity Reviews found that eating fewer calories is effective for fat loss, but it does come with some muscle loss. However, if the subjects fasted for 24 hours and ate as much as they wanted on the next day for a period of 12 weeks, they lost significantly less muscle mass.

Yes — you read that correctly — 24 hours of intermittent fasting without any resistance training and these subjects were able to preserve more muscle mass than the subjects that ate fewer calories every day without fasting at all. This finding contradicts our common sense, but when we dig deeper into autophagy we can find the mechanism behind this result.

Muscle Loss Prevention and Autophagy

Before the Nobel Prize was awarded to Yoshinori Ohsumi, other researchers were making groundbreaking discoveries about autophagy. In 2009, an article was published in Cell Metabolism entitled Autophagy Is Required to Maintain Muscle Mass. In this article, researchers described how deactivating an important autophagy gene resulted in a profound loss in muscle mass and strength.

This happened because autophagy is necessary to clean up damaged proteins and mitochondria in muscle cells. If autophagy is never activated then the proteins and mitochondria will remain damaged and the muscle cells will begin to die, which causes a loss in muscle and strength.

Autophagy and Muscle Loss

This seems counter-intuitive because we assume that the nutrients we eat will repair the damage, but it makes sense when you think about it another way.

If you want to refurbish a room, it is best to clean the room and remove the old furniture before you put the new furniture in. The same thought process applies to your cells. We must use intermittent fasting to let autophagy clean the room of the cell before we put in new furniture. If we don’t, our cells can become cancerous.

Intermittent Fasting and Cancer

Although there is little to no literature on the effects of 2 or 3 day fasts on muscle loss in humans, many clinical trials are currently being conducted on the effects of 2 or 3 day fasts on cancer patients.

In initial case studies, people who were undergoing chemotherapy voluntarily fasted for anywhere between 48 to 140 hours. Each person reported fewer side effects and improved quality of life regardless of how long they fasted.

This may suggest that fasting for 2 days to a week can have a protective effect on the cells in the body while they are undergoing intense bouts of toxicity.

Fasting and Cancer

Other studies in animals have found that fasting was possibly as effective as chemotherapeutic agents in delaying the progression of different tumors and increased the effectiveness of chemotherapeutic drugs against melanoma, glioma, and breast cancer cells. Although this research may not apply to your life, it does suggest that intermittent fasting can help support your body in times of toxic stress.

Simplifying the Mechanisms Behind the Benefits: Autophagy, Ketones, and Fat Loss

Intermittent fasting is so powerful because you can use it to restrict calories, induce ketosis, and activate the processes of autophagy that are brought about by protein restriction and starvation.

If this scientific jargon is throwing you off, think about what it takes for you to clean your room. You may clean it in your spare time or have a set time on the weekend to do it, but what happens when the weekend comes?

Tasks or chores come up and you prioritize something else. After a week without cleaning, your room is just a bit dirtier than usual, but after a month of being too busy to clean, your room is filthy.

Cleaning out your cells through fasting

This is what happens to our cells when we eat three or more meals a day that completely fulfills our calorie needs every day. Even by eating the healthiest of foods, your cells still can get backed up with non-essential proteins and toxic compounds — so what can you do?

To make sure that you clean your actual bedroom, you fast — not from food — but from being consumed by other obligations. To make sure that your cells can clean themselves, you must fast from food.

This fasting process will not only activate this clean process for your cells, but it will increase your ketone production and promote fat burning as well. Simply put, by adding intermittent fasting to your keto lifestyle, you can experience the benefits of keto more quickly along with the effects of autophagy.

Furthermore, If you start implementing intermittent fasting and exercise (such as walking, cycling, or lifting weights) together, you can raise ketone levels, burn more fat, and increase autophagy more than you would with intermittent fasting alone.

Overall, whether you add exercise or not, the evidence for intermittent fasting suggests it would be a great addition to the keto lifestyle for most people. However, before you start, it is important to be familiar with the negative symptoms that may arise.

How Long Can We Last Without Food?

Once you start fasting, you may feel ravenously hungry as your mind is flooded with images of your muscles disintegrating into thin air. This is simply how your mind reacts to the threat of starvation — just because it feels like you will lose all your muscle and starve doesn’t mean you will.

In fact, Mahatma Gandhi survived for 21 days without food while only taking in sips of water. During times when people had no food or water at all, they were able to survive for 10 to 14 days. However, these are just anecdotes — what does the science say?

Many studies have been done during hunger strikes and religious fasts confirming that humans have the capacity to survive even longer than Gandhi did during his fast.

How long can we survive with no food?

One monk, for example, set out to do a 40 day fast with medical supervision while maintaining his daily activities in the monastery. After 36 days, the medical professionals had to step in due to “profound weakness” and low blood pressure when standing. Although the monk fasted for 15 days longer than Ghandi, the medical professionals were able to stop the fast in time so that he could recover.

Another study tracked 33 political prisoners who were on hunger strike. The prisoners fasted for 6-24 days before being hospitalized for dehydration due an inadequate intake of fluids and electrolytes (not because of starvation). Breaking their fast was described as being “uncomplicated”.

Keep these studies in mind as your body tries to play tricks on you during your first day of fasting.  Even after three days of fasting, health complications are highly unlikely. However, it is important to know about the possible issues that can be caused by fasting. If you choose to incorporate fasting into your daily diet, you typically want to eat every day as well. Occasionally going on a longer period of fasting.

Should You Be Worried About Refeeding Syndrome?

Some legitimate health complications can arise when you fast or are malnourished for longer than five days. One of these complications is called refeeding syndrome, which is caused by potentially fatal shifts in fluid and electrolyte balance that can happen when we eat after a period of undernourishment.

This happens because the concentration of fluids and minerals in our bodies relies heavily on what we eat. Low carbohydrate diets, like the keto diet, increase the excretion of vital minerals like sodium and potassium.

If you add a prolonged fast to the ketogenic diet, you can lose an unhealthy amount of these essential minerals.

Mineral rich ketogenic meal

Fasts that are shorter than 5 days, however, aren’t likely to cause issues — especially if you break your fast with a low carbohydrate meal that is filled with mineral-rich foods. As an example, a meal with dark leafy greens, avocado, and salmon with some unrefined salt would be an ideal way to break a longer fast.

During a shorter fast that lasts less than 24 hours, however, there is no need to worry about refeeding syndrome at all.

That said, it is vital to ensure proper hydration and mineral balance while fasting (with the help of our article on keto supplement recommendations). By doing so, you can reduce the risk of experiencing the keto flu as well.

A Potential Downside: Keto Flu, Carb Restriction, and Intermittent Fasting

As we restrict carb intake (via fasting or keto eating), our bodies excrete more fluid and minerals than usual. If you aren’t fastidious with your electrolyte and water intake, you may run the risk of experiencing fatigue, headaches, digestive issues, and other flu-like symptoms.

In most cases, this is just a sign of mild dehydration and can be remedied by increasing your water, sodium, magnesium, and potassium intake. If you’d like to learn more about the keto flu and how to remedy it with specific intake guidelines, click here.

Keto and Intermittent Fasting FAQs

What is the best keto intermittent fasting plan?

As with any diet plan, there is no such thing as the best keto fasting regimen or protocol for everyone. Though there are a seemingly infinite number of methods you could try, what ultimately matters is formulating the approach that works best for you.

This will require some self-experimentation and the help of a healthcare professional. As you are experimenting with different options, make sure you are monitoring your health, well-being, and progress to figure out if a particular keto fasting plan is healthy and sustainable for you.

How should I break my fast?

A micronutrient-rich low carb meal filled with healthy fats is one of the best ways to break a fast.

An example of this would be a keto-friendly salad (like our Quick Keto Avocado & Salmon Salad). This will help replenish your vitamin and mineral needs, keep you in ketosis, and prevent the bloating that can occur after breaking your fast with high-carb foods.

Can I take exogenous ketones while fasting?

Since exogenous ketones provide us with calories, consuming them will technically break your fast. Furthermore, they will cause an artificial increase in ketone levels that decreases your body’s natural ketone production.

If you are looking for a way to boost your ketone levels while fasting, black coffee is the way to go. Its caffeine content will promote natural ketone production without adding any calories to your diet.

For anyone who doesn’t mind adding some fat calories to their coffee, try blending in a high-quality MCT oil for an additional natural ketosis boost.

Putting It All Together: Formulating Your Keto Intermittent Fasting Diet Plan

Now you know that there is nothing to fear when it comes to intermittent fasting.

Although you will feel hungry at first, your body will adjust by activating autophagy and burning more fat and ketones for fuel.

Whether you are fasting for 16 hours or three days, it is vital that you replenish your fluid and mineral levels to avoid keto flu symptoms.

Supplementing with sodium from unrefined salt and potassium & magnesium from mineral-rich keto foods/supplements may be necessary for you to prevent excess mineral loss caused by ketogenic diets and fasting.

Supplement with additional micronutrients

As with any significant diet change, make sure you’re monitoring your health with a healthcare professional while you experiment with different intermittent fasting regimens.

The best keto intermittent fasting plan for you will be one that improves your objective health measures, subjective well-being, and body composition in a sustainable way.

To learn more about personalizing your keto diet for optimal results, we’ve included several resources below:

Plus, if you are ready to experiment with IF, we’ve included a 3-day keto fasting plan as well.

Practical Application: Tim Ferris’s 3 Day Ketosis Boost

Keto Fasting Protocol
If you want to raise your ketone levels or kickstart your ketogenic journey, try a 3-day “fasting” protocol like the one below. If intermittent fasting doesn’t sound like it’s something that can work for you, you can also kickstart your keto diet by fat fasting.

Thursday Evening

  • Eat a ketogenic dinner and make that the last meal of the day. Go to bed as normal.

Friday Morning

  • Get out the door and walk within 30 minutes of waking. Drink coffee or tea if needed, but it is best to limit your caffeine intake because it will cause you to excrete more minerals and fluid than usual.
  • Bring at least 1 liter of water, with some added unrefined salt, and sip as you walk to avoid cramping.
  • Walk for 3 to 4 hours, sipping water as needed.
  • Arrange phone calls or something similar for your walk to make the time productive.
  • The idea behind the walk is that you use up your glycogen stores, forcing your body to move more quickly into deep ketosis. The quicker you get into ketosis, the less time you spend feeling drained.
  • If you prefer to shorten the time frame, you can do a 45-60 minute bout of HIIT exercise.

Friday Day (post walk/workout)

  • Consume MCT oil 2-3 times throughout the day.
  • An affordable, good quality MCT Oil we recommend is NOW Foods MCT Oil.
  • This provides you with energy until your ketone levels elevate naturally.

Saturday Morning

  • Upon waking, test your blood ketones with a ketone blood testing kit like the Precision Xtra. Your ketones should be at 0.7mmol or greater.
  • If you’re at 0.7mmol, proceed with your fast.
  • If you’re under 0.7mmol, consider going for another extended walk, and then re-test.

Saturday & Sunday Day

  • Add further MCT oil or coconut oil if you need a boost. They can be omitted once you are in deep ketosis.
  • Incorporate some salts in your water throughout the day. This can either be in the form of table salts, or via a specially formulated solution such as SaltStick electrolyte replacement pills.

Sunday Evening

  • Break your fast with your favorite ketogenic meal. Take a look at our Keto Recipes to find your new favorite!

This process can be used as a way to get you into ketosis more quickly, so you can transition gracefully into a ketogenic lifestyle or as a way to stimulate autophagy and fat loss. If you can’t go without fat for the full 3 day fast — it’s okay — you will still elicit many of the benefits of fasting by limiting your protein and carbohydrate intake.