The term intermittent fasting is quite popular these days, but what exactly is an intermittent fast and how do you do it? As opposed to other types of dietary strategies, with intermittent fasting, it's not about what you eat, but when you eat.
I'm here to help you learn more about numerous aspects of this simple health technique so that you can answer the following questions, and much more:
I've been practicing intermittent fasting for over a decade for its health benefits because intermittent fasting works incredibly well with the Keto Diet. Together, they reduce inflammation and improve physiological function in the body and cognitive performance in the brain.
First, let's start with the basics and answer the question, what is fasting?
According to the National Institute of Aging, one of 27 Institutes and Centers that comprise the National Institute of Health “in a fasting diet, a person does not eat at all or severely limits intake during certain times of the day, week, or month. A practical effect of a fasting diet may be fewer calories because there is less time for regular eating.”1
Fasting simply entails refraining from consuming food for a period of time. This practice was a key component of our ancestor's lifestyle and I believe that our bodies are built to fast as well.
Intermittent fasting is a type of fasting, also known as time-restricted feeding and intermittent energy restriction. The term intermittent fasting refers to various meal timing schedules that cycle between fasting and feeding over a given period of time.2
Robb Wolf states that an intermittent fast is abstaining from food for 36 hours or less, though he also makes the point that this determination is somewhat arbitrary. For our purposes, this is the definition of intermittent fasting that we will use here.3
It is likely that our hunter-gatherer forebears fasted regularly. This was not by design, but because of a limited caloric supply and the unfortunate reality of potential starvation. Food was scarce and not in the abundant supply that it is for many of us today in an industrialized world where grocery stores dot the modern landscape.
Much of ancestral life revolved around procuring food via gathering, a labor-intensive practice, or hunting which required moderate to great exertion. It is therefore believed that after a big kill, the group gorged on food and then rested, digested and fasted, while preparing to hunt again.
Fasting was a given component of the hunter-gatherer lifestyle. Because we have not evolved extensively since ancient times, it is logical and very likely that we are not built for three square meals a day, but rather to eat far less, and only when we're hungry. That's where intermittent fasting comes into play, especially where weight loss and inflammation are concerned.
Now let’s address the question, why is intermittent fasting good for you? The answer is simple. When we fast our insulin levels go down and our fat burning goes up. This gives us the opportunity to go from burning glucose and glycogen stores, to burning fat, or ketone bodies, and fat stores.
Fat is a much cleaner burning fuel than sugar, and complex carbohydrates, which quickly turn into sugar when digested. If you're looking for more information on using fat for fuel, check out my article The History of Low-Carb Keto Diets. And please note, the Keto Diet is not a fad diet, it is a medical therapy that was created over a century ago to treat pediatric epilepsy by adjusting brain metabolism through the elimination of dietary carbohydrates. For more information, you'll want to read my article, Interview With A Top Keto Diet Expert.
But why does fasting work and do so many amazing things? First and foremost, fasting frees up the energy in the body that would typically be used for digesting food. Although it is not often recognized, digestion taxes our systems quite heavily on a physiologic level.
Breaking food down has a metabolic cost as it diverts energy from processes like cellular destruction, regeneration, and repair. Fasting frees up biological resources for the aforementioned functions. This leads us to the second reason that makes fasting fantastic, and that is healing.
With intermittent fasting we are not just skipping food and meals and reducing our caloric intake, we are doing something more sophisticated. We are training the body to go from carb-burning to fat-burning which allows us to deplete glycogen stores, and then tap into fat stores.
This process leads to weight loss, but additionally, fat is a much cleaner burning fuel than sugar. Fat burning improves cellular function overall, as well as brain energy metabolism which promotes the healing of brain diseases like MS.
The energy gained by fasting leads to cellular repair which heals inflammation. When we fast our body releases endogenous substances that are somewhat similar to steroids, a common anti-inflammatory drug. This also happens with calorie restriction, which is when we restrict calories to very low levels.4
According to the National Institute of Aging, “calorie restriction means reducing average daily calorie intake below what is typical or habitual, without malnutrition or deprivation of essential nutrients.”5
Some people prefer to eat a small amount of food during a fast which makes the practice more calorie restriction than intermittent fasting. Both practices give the body a chance to rest and heal by reducing stress in the gut. And both can provide great health benefits.
How do intermittent fasting and the Keto Diet work together? It’s not as complex as it may seem. Intermittent fasting is a health strategy that focuses on when you eat, as opposed to what you eat.
But most people I know who practice intermittent fasting are also very focused on what to eat and follow an incredibly healthy diet. I use intermittent fasting while following a Keto Diet. These potent tools help me battle MS by reducing inflammation. But fasting does more than heal your body and brain, it can lead to weight loss too.
According to Robb Wolf:
Body fat is stored as triglycerides—bundles of three fatty acids bound by glycerol (sugar) molecules. Fasting causes triglycerides to split apart into fatty acids, which are then beta-oxidized (burned) predominantly in the liver to produce energy (ATP) and ketones.
By restricting calories and becoming a fat burner, many people that are overweight find that the pounds melt right off.
Wolf goes on to say:
Fasting is like an internal keto diet. Instead of running on dietary fat, your cells run on body fat. The results are similar: increased fat burning, ketosis, and all the benefits that come with keto.6
Many practice intermittent fasting to lose weight, but the other benefits are equally, if not more important, as I mention above. If you suffer from a neurodegenerative disease7 intermittent fasting can be a powerful tool in your disease battling arsenal.
Anecdotally speaking, intermittent fasting has given my very well-worn digestive tract a chance to heal. Since I was not diagnosed with Celiac Disease until age 30, my gut has endured decades of abuse.
As we know, intermittent fasting heals the body by diverting energy from digestion to metabolic healing, thereby releasing endogenous anti-inflammatory substances. The benefits of intermittent fasting are numerous. Here are just a few:
Starting intermittent fasting is easy for some and terribly hard for others. I know that it can be delightfully intuitive from personal experience, but don’t be fooled, it took me a very long time, in fact, well over a decade, to get there.
My low-carb journey began in the late 1990s when I went on a Candida Diet. Little did I realize then where it would lead me decades later.
The next step was going on a Grain-Free Diet. Grains, including but not limited to, rice, millet, and wheat, are full of energy in the form of carbohydrates. This highly caloric food turns into sugar in your body, which means that grains are the arch-enemy of ketosis.
I adhered to a strict grain-free diet for many years before I started fasting, and still follow the grain-free way of eating to this day.
That's why every recipe on this website is not just gluten-free, but grain-free too. And my New York Times best-selling book, Paleo Cooking from Elana’s Pantry, along with my other two books, are also 100% grain-free.
I also have drastically reduced my intake of fruit to little or none. I do eat berries sometimes because they are low in carbohydrates and full of fiber and antioxidants, making them a keto compatible choice.
Once I completely eliminated all grains, including starchy foods like corn and potatoes, from the menu, and reduced fruit intake, I was on the path to low-carb living, which assists with blood sugar regulation, making the transition to fasting far less painful.
The next step in this evolution was the Keto Diet, a high-fat diet, that’s moderate in protein and low in carbohydrates. By switching to very low-carb, I created the context for my body to easily switch from burning carbohydrates to burning fat and fat stores, which is the goal of the Keto Diet, as well as the objective of intermittent fasting.
At that point, I began increasing my fat intake with treats like homemade Keto Fat Bombs.
Ok, this all sounds great but what is ketosis, you ask. Ketosis is a metabolic state that occurs when you consume a very low-carb, moderate protein, high-fat diet that causes your body to switch from using glucose as its primary source of fuel to running on ketones.9
Ketones are produced when the body burns fat, and are primarily used as an “alternative” fuel source when glucose isn't available. According to Jimmy Moore, author of Keto Clarity, ketones are the preferred fuel source for the muscles, heart, liver, and brain.
With my metabolic flexibility well established, my body was incredibly accustomed to tapping into fat for fuel and burning ketones by the time I made intermittent fasting part of my healthy lifestyle.
All in all, undertaking the practice of intermittent fasting was a slow development that happened organically and felt very good.
For more information on my journey and support with yours, check out :
So, given my own intermittent fasting experience, I can tell you that it may be good to first reduce your carbohydrate intake in order to become better adapted to burning fat before you begin a regular intermittent fasting practice. Here are some of my favorite tools for cutting carbs to get you into ketosis:
Another possible alternative is to start with very short fasting windows. Which leads us to the next question –what is the appropriate length of an intermittent fast?
I’m often asked, how long should you do intermittent fasting, or how long is an intermittent fast? Intermittent fasting is typically anywhere from eight hours to a day and a half.
The practice of intermittent fasting features cycles of fasting and eating often referred to as a fasting schedule.
Figuring out how to do an intermittent fast is pretty simple. You will have specific periods for eating, and then periods of abstaining from food. The feeding window can be anywhere from 8-12 hours, resulting in a fasting window of 16-12 hours, giving the body a long break to rest and heal.
Intermittent fasting can take a variety of forms. Some people practice it by skipping a meal, other by eating every other day.
I bet you didn't realize it, but if you're sleeping, you're practicing intermittent fasting. When you sleep you heal, part of that is due to the mechanism of sleep itself, but it's also because you're not eating. If you have 6-8 hours of sleep each night and you don’t eat for an hour before you go bed then you are doing intermittent fasting 8/16 –that means 8 hours for fasting and a 16-hour window for feeding. Interestingly, lack of sleep can lead to a number of medical issues including weight gain.
You can extend your fast by simply skipping breakfast, there are a number of other ways to do an intermittent fast. Here are some of the most popular.
The 12/12 intermittent fast is the perfect beginner’s fast. This entails 12 hours of fasting and 12 hours of feeding. This is a relatively short fast that gives the body a 12-hour break to fast and heal in between dinner and breakfast. Sleeping during this fast makes it the easiest listed here.
The 16/8 intermittent fast is similar to the fast above but extends the fasting period to 16 hours, with an 8-hour feeding window.
This is my favorite fast and the one that I have practiced daily for years now. What does it look like for me? I fast after dinner, sleep for 9 hours, then skip breakfast, and eat lunch around noon.
The 16/8 fast works really well for me because I have no interest in food when I wake up in the morning. I'm thirsty as can be, crave water, and enjoy rehydrating after a long night of sleep.
If I’m fully hydrated and a bit hungry in the late morning, I will have a cup of tea with coconut milk (high in fat), and if I’m hungry from my morning workout, I eat a square or two of high-fat 85% dark chocolate. I’m not rigid around the fast since I’m not trying to lose weight.
Here are more tips on how to go low-carb and break your fast:
Some people practice intermittent fasting in a one day on, one day off fashion. This is called ADF and means eating one day and fasting the next. According to Robb Wolf, this is the most effective method of intermittent fasting for weight loss, which makes sense since you’re cutting your caloric intake in half.
The easy intermittent fasting rules above are a great guideline for weight loss and healing everything from autoimmune conditions to neurological disease, cancer,10 and more.
As previously discussed here, fasting although very popular now, is not at all a modern phenomenon. Periodic fasting was espoused and practiced by the ancients.
Foregoing or abstaining from food for an extended period of time for spiritual purposes was part of the fabric of ancient cultures11 and among other things is a statement that one believes the spiritual is greater than the material.12
Growing up in a Jewish community, I was exposed to religious fasting for penitence and atonement from an early age.
What is a religious fast? Fasting for Judaism includes abstaining not only from food but from water as well. From personal experience, I can tell you this is terribly challenging and very uncomfortable. It is the lucky person who emerges from this completely abstemious fast without a pounding headache. This type of religious fasting is not what I recommend if you are doing so for health reasons. Water, along with proper hydration are critical when fasting for health.
Other religions that use fasting as a ritual include Christianity and Islam.
Moving right along through the ages, the Greeks have used therapeutic fasting since 500 BCE. Hippocrates, known as the father of medicine, recommended fasting to patients who experienced various symptoms of illness, stating,
To eat when you are sick is to feed your sickness.13
Just a few decades later, the ancient Greek philosophers made their position on fasting clear as well.
Plato, author of The Republic stated, “I fast for greater physical and mental efficiency.” Incidentally, a more modern philosopher by the name of Benjamin Franklin, was also a proponent of fasting, stating, “The best of all medicines are resting and fasting.”14
My favorite thing about Franklin's quote is that fasting is resting, it gives the digestive system a rest so that the body can expend this extra energy on healing at a cellular level.
Intermittent fasting is an amazing strategy for most people, but we're all biochemical individuals. If you have specific questions about how to fast, will intermittent fasting work, will I lose weight and the like, this section is for you!
I like to drink water while I fast, sometimes I will squeeze half a lemon or lime into a quart of water.
Coffee is a liquid and you can drink it during a fast.
However, according to Jimmy Moore, author of Keto Clarity, caffeine spikes blood sugar which can blunt the production of ketones, hence impairing the burning of fat.
Exercise may not be recommended for all who fast, best to check with your medical provider. Personally, I work out, very gently, every day before I eat anything. Great things can happen on a metabolic level if you are healthy when you fast and work out simultaneously.
The only times I’ve had a headache when fasting are from drinking too little, or too much water. Drinking little water can result in dehydration, a common cause of headaches, whereas drinking too much water dilutes electrolytes, which can also cause headaches. If you have blood sugar issues you may get a headache when you begin intermittent fasting. Everyone is different and it is best to check with your medical provider about this.
I'm living with three autoimmune diseases –Celiac Disease, Multiple Sclerosis, and Hashimoto's Thyroiditis. Constipation is something I've battled on and off during adulthood.
I've found that fasting greatly reduces my digestive distress and constipation, and improves my gut motility. If you are dealing with constipation from intermittent fasting or a Keto Diet, you may want to try electrolytes. You can also check out my post, How I Healed My Gut.
I do not practice intermittent fasting to lose weight, but to maintain a healthy weight. As I’ve aged, I’ve found that my caloric demand has decreased and that when I do eat it is best to consume nutrient-dense food like bone broth, healthy fats, vegetables, and high-quality meat. People who are overweight often find that intermittent fasting, along with a low-carb high-fat (LCHF) diet helps with weight loss.
Intermittent fasting itself should not make you gain weight. There are a couple of related behaviors that might cause you to put on pounds rather than taking them off.
First, coffee. Yes, drinking too much coffee during your fast can make you gain weight because excess caffeine can increase blood glucose levels, instead of ketone bodies which burn fat.
Second, be sure to stick to low-carb food during your feeding windows if you want to continue burning fat and losing weight. Fasting doesn't give you a free pass to drink alcohol, eat chips, or scarf down a pizza. Though if you do want to drink, I have a fabulous Sugar-Free Keto Margarita for you that shouldn't kick you out of ketosis.
Finally, sleeping. Make sure you get enough sleep as sleep deprivation can wreak havoc on your blood sugar levels and prevent you from losing weight.
I've found that intermittent fasting helps me with brain function and emotional regulation. I definitely feel less anxious and more calm with my IF and Keto Diet. For more information check out this interesting article in Psychology Today that discusses mood disorders and ketosis.
Common side effects of intermittent fasting may include hunger, cravings, headaches, low-energy, and flu-like symptoms. Personally, since I transitioned gently from grain-free to low-carb, to keto prior to intermittent fasting, I did not experience any of these miserable side effects.
Since everyone is different, there is no way to know how you’ll feel during an intermittent fast without giving it a try. I find that I feel good when I fast. I have more mental focus and improved physical performance on a number of levels
I’m often asked is intermittent fasting safe. That’s a tricky question to answer because we’re all biochemical individuals. I would not recommend fasting or intermittent fasting without speaking to your medical provider first.
Intermittent fasting can be a great tool for some people to improve health and reduce inflammation, but there are some people who need to be especially careful because of the risks, this includes:
Now I’d love to know if you’ve practiced intermittent fasting. If so, did it work for you?
Hey all you IFers, what type of intermittent fasting do you do? How do you feel when you do it? Come on EPers, I need your anecdotes! Leave a comment to help our community learn more about fasting.