Intermittent Fasting: The Definitive Guide


No doubt you’ve heard of intermittent fasting already.

One of the few health trends to achieve staying power, fasting has become popularized for the simple fact that it’s natural.

Our ancestors did it when they didn’t know when their next meal was coming; while modern technology assuages this fear, it’s still beneficial to look at the physiological effects of fasting.

Improving your health while simplifying your lifestyle is a good way to stick to healthy habits. Intermittent fasting blends bodily wellbeing and a simplistic approach to nutrition, making it an incredible tool for life.

In this ultimate guide to all things fasting, we’ll tell you everything about what intermittent fasting is, what it does to your body, and the benefits and side effects you might see.

Plus, we give you a comprehensive map to begin your own intermittent fasting habit, along with tools and tips to make your road to fasting much easier.

Last, we answer any and all remaining questions you might have when it comes to intermittent fasting.

What Is Intermittent Fasting?

what is intermittent fasting?

While it has become a catch-all word for all kinds of dieting, here is the definition of intermittent fasting:

“Also known as intermittent energy restriction, [intermittent fasting] is an umbrella term for various meal timing schedules that cycle between voluntary fasting (or reduced calories intake) and non-fasting over a given period.” (Wikipedia)

This makes sense.

Intermittent fasting involves fasting at intermittent times. It is essentially a nutrition protocol that doesn’t ask “What will you eat?” but rather “WHEN will you eat?”

The 8 Different Types of Intermittent Fasting

There seems to be an uncountable amount of intermittent fasting approaches these days. However, most of them are either ridiculous or don’t provide you with the benefits we’ll get into below.

As well, there are only a few ways to fast intermittently that have been shown to help you lose weight, improve insulin sensitivity, and more.

Here are the best, most common styles of intermittent fasting:

1. 16:8 Fasting Protocol or LeanGains

The classic fasting protocol, a man named Martin Berkhan created this LeanGains protocol, which is a barebones 16:8 fasting structure: 16 hours fasting, followed by eight hours feasting or eating.

Ideally, you skip breakfast and have an eating window between 12 or 1 PM and 8 or 9 PM. This is usually a good place to start if you’re a fasting beginner because forgoing food for 12 to 16 hours is much easier than the other protocols below.

16/8 intermittent fasting lean gains protocol

2. 5:2 Intermittent Fasting Protocol

This protocol includes five days of the week in which you eat like you normally would, breakfast and all, with two days of extremely low-calorie “fasting.”

Select two days that are spaced out by at least two days — Monday and Thursday, for example — and you only get to consume 500-600 calories on those days, preferably from whole foods like fruits and vegetables.

This one is more difficult than LeanGains due to drastically limiting calories two out of the seven days of the week. However, you put yourself in a 2000-3000 caloric deficit for your week (so long as you eat like normal on all the other days), which can translate to fat loss.

5-2 protocol intermittent fasting

3. Eat-Stop-Eat

A more extreme version of 5:2 fasting, eat-stop-eat involves two separate 24-hour fasts throughout your week.

In this protocol, you stop eating after dinner on Sunday, say; then you fast for 24 hours, a.k.a. until Monday night.

You can still eat every day, but two of the days have to have a 24-hour fasting window.

This one is difficult due to the innate desire to pig out when breaking your fast.

However, if you can eat properly with this protocol, the caloric deficit jumps up another 1000-1200 calories, plus the added benefits of two long-term fasts throughout the week. (We’ll get into benefits below.)

eat stop eat intermittent fasting

4. Alternate-Day Fasting

Adding even more fasting to the above two protocols, the name gives this protocol away: You fast every other day for 24 hours.

alternate day fasting

Yep, you heard that right.

You can do a proper 24-hour fast without any food, or you can use extremely low calories (500-600) a-la the 5:2 protocol. This approach has you fasting three or four times every week.

If you can somehow eat like normal on the opposite days, you’ll put yourself in a ridiculous 3500-4800 caloric deficit, which means you’re losing over a pound of body weight per week, mostly fat.

5. The Warrior Diet

Ori Hofmekler created this to satiate people who wanted to fast and eat the Paleo diet.

Every day of the week, you eat small amounts of raw fruits and vegetables from morning until evening, followed by a single large meal for dinner.

While not necessarily a true fast, the Warrior Diet helps you fast from processed foods and junk calories.

Sticking to tiny amounts of fruits and vegetables untampered by cooking processes allows you to get tons of micronutrients, as well as live like our primal ancestors.

Also, you can only eat so much in one sitting; this paired with little bits of fruits and veggies should most likely put you in a caloric deficit.

6. 20:4 Fasting Protocol

This is level two of LeanGains.

Pushing the fasting window to 20 hours and shrinking your eating window to four hours, you get to experience more of the fasting benefits, including enhanced autophagy (more later).

Again, four hours isn’t a ton of time; you can probably sneak in two meals and a snack in that window. This might be enough to put you into a caloric deficit as well.

20-4 intermittent fasting protocol

7. One Meal a Day (OMAD)

The most advanced version of LeanGains, level three.

It’s exactly what the name suggests: One meal, getting all of your calories within a single hour or sit-down meal.

omad intermittent fasting

OMAD is intense.

You might want to start out with 16:8, then get into a 20:4 before you swap to a full OMAD protocol.

This is where you can get in the most intense calorie deficit and the most fasting benefits from an intermittent fasting protocol.

8. Spontaneous Meal Skipping or Intuitive Fasting

Intuitive fasting is like intuitive eating in that you ask yourself at every meal, “Am I actually hungry?”

Most of the time, it might be a yes, but your body will let you know if it’s merely conditioning over an actual need to eat.

While this leads to an uneven schedule, it’ll make you feel the best satiation-wise.

You might not lose weight at all, but you will be following a natural eating pattern for your specific body, which comes with its own advantages.

Honorable Mention: 12:12 IF Protocol

One intermittent fasting protocol that I love that I didn’t mention above is the 12:12 Intermittent Fasting protocol, which is a great option for beginners because it’s not too extreme. We have a complete guide on how to implement it here.

What About Longer Fasts?

As you saw above, we said that OMAD gives you the most fasting benefits from an intermittent fasting protocol. However, there are even longer fasts that shouldn’t be intermittent but provide even more benefits for your body and overall well-being.

These one-off fasts should be done once or twice a month at max, depending on your gender, health condition, and other factors.

A long fast is anything from 24 hours onward.

Theoretically, you can fast for days while only drinking water. You can also do dry fasts — fasting without consuming anything, even water — for shorter spans of time.

The thing you have to keep in mind is to be careful.

Longer fasts are beneficial, but don’t need to be done more than once a week–once or twice a month is a terrific middle ground.

During these fasts, make sure to stay hydrated if you aren’t going with a dry fast. If you are dry fasting, make sure to have some kind of medical supervision because it can get dangerous.

Longer fasts incur autophagy, where old parts of your cells or entire old cells are broken down and recycled.

It can also help lower blood pressure, increase ketone body production (which aids in fat mobilization, usage, and eventual loss), and improve insulin/leptin sensitivity.

Expect to feel hungry and potentially lightheaded throughout a long fast.

Find zero-calorie drinks to help you get through, avoid environments with crave-able foods, and stay busy. As well, get some social support (along with that medical support we mentioned).

What Is Happening at a Cellular Level?

intermitten fasting cellular level

Fasting intermittently or even longer causes our cells to perform processes that they otherwise wouldn’t do.

These processes improve your health and biomarkers more than most other nutrition or physical fitness applications. On top of that, fasting removes toxins from your body overall.

In terms of autophagy, removing glucose from the equation — which is stored calories in your cells — allows cells the room to remove toxic and/or dead pieces of those cells.

They flush out of your system via waste, sweat, perspiration, and more.

This programmed cell death lets those cells create fresh new pieces, which in the long run helps you with enhanced longevity, along with the lower risk of multiple diseases.

This repair process can also alter gene expression, as well as other processes such as increased human growth hormone (HGH) and insulin sensitivity.

In terms of fueling your body, you deplete your body of stored glycogen, which has been in there from previous foods consumed.

Once your body is rid of glycogen — around 10-12 hours of fasting — it begins to use other sources for fuel.

Your hormone levels adjust so that stored body fat is more accessible because the main source of fuel sans-glucose is body fat ketones. Ketones are your stored fat cells getting mobilized and used for energy.

This is the reason why your body stores fat in the first place: If you get to a place of “starving,” your body can keep going on for a while off of the stored fat. (This is why people who go from eating to starvation become emaciated before passing out or dying; your body uses the fat first.)

intermittent fasting infographic

Intermittent Fasting Health Benefits

While it’s good to know what’s happening inside your body, it’s also good to know what intermittent fasting will do for you and your life:

1. It Will Help You Lose Weight

Losing weight or body fat is at the core of most people’s desire to go on a diet or try out nutritional protocols. Intermittent fasting improves both weight and body composition in two ways:

One: Boosts Metabolism

Fasting for short periods of time has been found to increase metabolic rate.

There is a thermogenic response to fasting, as well as the reduction of insulin, the increase of human growth hormone, and improved effects on glucose regulation.

What all this does is increase metabolism between 3.6 percent and 14 percent. It’s even been shown to be better than the next potential benefit as an overall weight loss strategy.

& Two: Potentially Creates A Caloric Deficit

By reducing the number of hours you eat overall, you limit your chances of consuming calories.

You simply don’t feel great eating above-maintenance calories in one sitting, even if you’re a professional athlete.

So you could unconsciously eat less in a time-restricted feeding window. This has been shown to work for fat loss purposes.

harvard study shows intermittent fasting benefits

2. Increase Insulin Resistance

Similar to metabolism, fasting works on the entire digestive system, and therefore your entire digestive health. One marker, insulin, is a big factor in determining your overall wellbeing.

Studies show that both intermittent fasting and caloric restriction have “comparable reductions in visceral fat mass, fasting insulin, and insulin resistance” while calorie restriction is slightly better for weight loss (though both are helpful).

With insulin sensitivity improving, people with Type 2 diabetes and those who are pre-diabetic might be able to reverse those diagnoses with short periods of fasting coupled with small caloric deficits.

3. Improved HGH Production

As mentioned above, intermittent fasting upregulates your body’s human growth hormone production. HGH boosts muscle growth, bone healing, weight loss, overall mood, and exercise capacity, making it an athlete’s best friend.

And having your body make it naturally due to fasting is the healthiest way to increase HGH. (Then you don’t have to take an exogenous pill or supplement, and who knows what else is in there.)

4. Reduced Inflammation

Chronic diseases are usually a result of chronic inflammation. This inflammation can come from many places, including excess unprocessed stress, poor eating and sleep habits, lack of exercise, and even where you live.

However, a lot of this stress can be mitigated via fasting; specifically, alternate day fasting, prolonged fasting, and 20:4/OMAD fasts you see during Ramadan have vastly improved inflammatory biomarkers.

5. Induced Autophagy and Cellular Repair

When you get to hour-16 or farther in a fast, scheduled cell death occurs. Our bodies recycle all of our cells within weeks, making us new people every month or so.

And MIT found that a 24-hour fast can reverse the age-related loss of stem cell function.

This essentially means you have more youthful stem cells and cellular function, which helps with reducing aging overall and keeps you healthier longer.

6. Lowered Cholesterol and Blood Pressure

Blood triglycerides (the fat in your blood), LDL cholesterol, inflammation, blood sugar levels, and insulin levels are all best when they’re at they’re absolute lowest.

Intermittent fasting helps decrease all of these key biomarkers, keeping you safe from heart disease and other cardiovascular issues.

7. Improved Brain Health

neural benefits from intermittent fasting

Your brain gets a gigantic boost from fasting. IF increases brain-derived neurotrophic factor (BDNF), as well as BDNF expression. BDNF makes neural proteins that promote neuron survival and health.

Plus, it aids in “neuronal resistance to injury,” keeping your brain safe from degenerative diseases like Alzheimer’s and dementia.

8. Anti-Aging Effects

Seeing as intermittent fasting has research to claim it “promotes multi-system regeneration, enhanced cognitive performance and healthspan,” it can safely be said that intermittent fasting is one of the best ways to prevent premature aging.

There’s no better way to prevent disease, delay aging, and minimize any chronic side effects due to dieting than intermittent fasting.

9. It’s All-Natural and Extremely Simple

You get all of these benefits AND it’s the most natural thing you can do as a human being. Historically, our species has gone through famines and droughts as well as times of abundance and plenty.

This eat-a-lot/not-eat approach is genetic. Plus, this nutritional protocol doesn’t involve sketchy supplements, unfounded scientific literature, or weird fitness gurus.

It’s simply not eating, which is as straightforward as it gets.

Anyone can do it, more or less, and it doesn’t have many adverse effects, which we get into in the next section.

Side Effects of Intermittent Fasting and a Note on Safety

As you can see, intermittent fasting is a borderline miracle drug for your health and body composition. But does the good come with any bad?

Of course, there’s no such thing as a free lunch (er… a free fast from lunch). There are some adverse properties of intermittent fasting that you need to be aware of.

The first one is hunger. (Shocker.)

Seeing as your body has been conditioned to expect three square meals a day, morning, noon, and night, it’ll be super confused when you skip that first feeding. Your stomach will grumble and you might have hunger pangs.

However, conditioning can be swapped, and after a few weeks of using an IF protocol, your body should be programmed to know you’ll be fasting in the morning or the entire day.

Another side effect you might have is lightheadedness or muscular weakness.

This is due to a lack of glucose in your system, which is what your brain primarily runs on.

Switching from glucose to ketones as the source of fuel may take a few days or weeks of fasting to get primed and set up.

Now for the note on safety: While fasting is a fantastic tool for overall health, some people should exercise extreme caution when trying to implement an intermittent fasting protocol.

Those with medical conditions, diabetes, low blood pressure, unregulated blood sugar, along with those who are underweight or have had issues with eating disorders in the past, should consult with a medical professional before fasting.

As for women, if you are trying to conceive, have a history of amenorrhea (missing menstrual periods), or are breastfeeding, definitely consult your doctor before starting intermittent fasting.

For all other women, it’s tricky.

Some science has shown that fasting doesn’t have as many benefits for women as it does for men. IF can also make women emaciated, miss periods, and even infertile.

While this may seem scary, a simple conversation with a medical professional can help you figure out if fasting is right for you and your specific situation.

Intermittent fasting is highly safe for the majority of the human population, and that might include yourself.

As for those who are at-risk for adverse effects, you may still be able to fast, but not as hard.

For instance, instead of trying OMAD or prolonged fasts, you could fast for 12-14 hours and eat for 10-12.

This will help your digestive system by taking a break from processing food all the time; plus, it can give you some of the benefits listed above (just not as pronounced as if you were fasting for a longer period of time).

How to Start Intermittent Fasting

By far, the easiest way to start intermittent fasting is with LeanGains, also known as skipping breakfast.

lean gains intermittent fasting

The reason why this is easiest is because it’s consistent (you do it every day), it involves the biggest eating window per day (eight hours), and it provides the smallest barrier to entry (it’s a single psychological and physical change, not eating one meal).

Once you have the hang of LeanGains — about two months or so — you can progress to another variation, including the alternate-day fasting protocols if you so choose.

The key is to take a look at your lifestyle and goals before choosing which fasting protocol is right for you:

  • Are you an athlete, someone who wants a simplistic lifestyle, or who wants the most out of their fasting protocol? LeanGains, the Warrior Diet, and 20:4 will be best for you.
  • Are you wanting to experience the effects of prolonged periods of fasting? Eat-stop-eat, 5:2, and OMAD will be best for you.
  • Are you wanting a healthy approach based on YOUR body and YOUR nutritional needs? The Warrior Diet or spontaneous fasting will be best for you.

As for longer fasts, sprinkle them into each month to get the enhanced autophagy effects.

Start out by trying one 24-hour fast, then keep that up monthly or add more days (max of once per week) or add to the length of the fast.

You can go up to 48 hours fairly safely; if you want to go longer, consider medical supervision and additional social help.

Once you select the fasting protocol that fits your needs and lifestyle, keep it up! This should be part of your daily life.

All it takes is a few weeks of conditioning and then maintaining that strategy for the long-term.

How to Best Break a Fast

how to best break a fast with the right foods

You don’t want to be groggy and feel like junk after breaking your fast. Therefore, you want to steer clear of most carbohydrates, especially processed junk food, highly sweetened food, and starches.

Let us suggest another approach.

Break your fast with high-quality fats and lean protein sources, paired with raw or steamed vegetables.

This does multiple things.

Instead of blasting your body with glucose from heavy amounts of carbs, leading to an inevitable insulin crash and your need for a nap, fats like olive oil or butter helps your body stay on ketones for fuel for a little bit longer.

This keeps your body and brain functioning at peak form.

Also, eating plenty of protein keeps you satiated, which will keep you from snacking throughout your feeding window, managing your hunger and calorie consumption.

As well, the veggies offer you a variety of micronutrients, giving your body the resources it needs to not only survive but thrive.

The same can be said for quality protein, giving your body all of those essential amino acids for cellular repair, muscle growth, and improved cognition.

Eating so that your body has more ketones, amino acids, and micronutrients, instead of switching it back to glucose and dragging your body down too soon, is the best way to break a fast.

After this first meal, you can save all of your yummy carbs for later in the day, for dinner.

When you do this, the carbs can work their magic and make you drowsy, just in time for a good night’s rest (which is also crucial for intermittent fasting).

Best Tools to Help with Intermittent Fasting

You don’t have to fast alone. There are tools at your disposal to make your intermittent fasting protocol even easier to track and do.

Zero is a fasting app (Apple download) where you can track your fast to the minute, which might be helpful for people who want to keep a strict schedule.

Natural green tea extract is a great supplement to take while fasting to expedite the benefits. And for some people, zero-calorie sweeteners like stevia extract or monk fruit extract can sweeten your non-caloric beverages without kicking you out of a fasted state.

Books for fasting like Eat Stop Eat by Brad Pilon and The Complete Guide to Fasting by Dr. Jason Fung, and habit books like The Power of Habit by Charles Duhigg and Atomic Habits by James Clear, are terrific resources to learn even more about fasting and how to create healthy habits in lieu of the bad ones.

The most recent guide on long-term fasting is Beyond Fasting by Dr. Daniel Pompa.

To learn more about maximizing fasts and other aspects of your fitness and health, podcasts are awesome resources. Bulletproof Radio with Dave Asprey, The Intermittent Fasting Podcast, and the FoundMyFitness podcast with Dr. Rhonda Patrick are our top choices.

And for more information on the amazing physiology of fasting, check out this viral Joe Rogan video with Dr. Peter Attia and Dr. Rhonda Patrick.

Other Hacks and Processes to Make IF Easier

  • Drink Non-Caloric Beverages: Water, black coffee, green tea, herbal tea, zero-calorie drinks like Zevia or flavored sparkling water are all great options for blunting appetite and continuing the fast. Also, lukewarm water + Himalayan pink sea salt + a squeeze of lemon gives your body a ton of minerals and other helpful micronutrients without taking you out of your fast.
  • Get Quality Sleep: Getting your seven to nine hours of sleep suppresses ghrelin, the “hunger hormone,” while ramping up leptin, which tells your body you’re fully satiated. This works wonders for your fast.
  • Have an Accountability Partner: Find a family member or friend who either wants to fast with you or can keep you on the straight and narrow. Sometimes we need support, and it never hurts to ask.
  • Ask Yourself if You’re Actually Hungry: Are you hungry or are you bored? Western civilization made it possible to eat for a hobby. You’ll need to break that hobby; rather, have fully satiating meals at times when your body truly needs food. Eating isn’t a sport (well, it is, but not in this case).
  • Count Calories: Not necessary at all, but this could help you structure your feeding window for the most nutrition without downing a giant bag of Doritos. Apps like MyFitnessPal can help you set a caloric goal each day and a place for you to track how and what you eat.

Frequently Asked Questions:

How Long Should You Do Intermittent Fasting?

If it’s truly intermittent fasting, you can do it each and every day or week, depending on your protocol.

Intermittent fasting has a ton of benefits with minimal side effects that should abate after the first few weeks of doing your specific protocol. And if you can sustain IF as a habit, your best bet is to do it for as long as you can.

As for longer fasts, the ideal range is one or two long fasts (36-72 hours) per month.

Can You Drink Beverages During a Fast?

If you’re doing a dry fast, no — nothing can be ingested.

If you’re doing a traditional intermittent fast, then it’s perfectly okay to drink water. Some people get kicked out of a fasting state (i.e. ketosis) by drinking other zero-calorie drinks that are sweetened with stevia, monk fruit extract, erythritol, and aspartame (which is bad for you anyways and should be avoided).

However, green tea and black coffee are terrific no-calorie beverages you can consume during your fasting window. These two will actually help blunt appetite, allowing you to fast even longer (as you saw in our tips and tricks above).

What Can I Eat During a Fast?

Technically, during the fasting window, you shouldn’t eat anything. During your eating window, on the other hand, it’s all up to you! This nutrition approach doesn’t have anything to do with what you eat — it just determines when.

Of course, in order to be fully healthy and getting a good nutritional profile, consider eating raw and cooked vegetables, small amounts of fruit, small amounts of animal protein, and healthy fats like extra virgin olive oil, coconut oil, butter, and ghee.

For maximum effects, avoid eating too many starchy vegetables, processed foods, unnaturally sweetened foods, and most alcohol.

How Much Weight Can You Lose with Intermittent Fasting?

This fluctuates between individual people.

If you’re starting with a ton of weight to lose, then you might see amazing fat-loss results quickly. If you have 10-15 pounds you want to shed in order to achieve your peak physical form, then it might take a while.

Nevertheless, you should be able to lose weight with intermittent fasting so long as you remain in a weekly caloric deficit.

Intermittent fasting alone doesn’t make you lose weight. You can still cram 4000-5000 calories into your eating window.

That won’t help–what will is a caloric deficit you can maintain for a week.

A great way to do this is to set a 20-percent caloric deficit — eat less than maintenance calories, which you can find out with this calculator — for six of the seven days of the week. Then, once a week, you can eat at maintenance or slightly above.

This does two things. It allows you to achieve a 2400-3600 caloric deficit over an entire week (0.75-1 pound of fat lost per week) and it keeps your metabolism from downregulating to the deficit level with that re-feed day.

By fasting intermittently, setting a caloric deficit, and using one strategic re-feed day per week, you’re in a good position to lose as much weight as your body will allow.

Put It into Practice: 3 Impactful Takeaways to Implement

  • Understand Intermittent Fasting: Learn about intermittent fasting, an eating pattern that involves cycling between periods of fasting or reduced calorie intake and non-fasting.
  • Explore Different Types of Intermittent Fasting: Research the different types of intermittent fasting, such as the 16:8 Fasting Protocol or LeanGains and the 5:2 Intermittent Fasting Protocol, to find the one that works best for you.
  • Experience the Benefits of Intermittent Fasting: Try intermittent fasting to experience its numerous health benefits, including weight loss and improved insulin sensitivity. Intermittent fasting is a natural and simple approach to nutrition that can improve your health.


There it is. Now you know everything there is to know about intermittent fasting.

Select a fasting strategy that best fits your needs and lifestyle, then use our fasting hacks to make it simple and easy. That way, it becomes a habit much faster, getting you on the right track to consistent fat loss and improved overall health.

Happy fasting

And if you loved this article, don’t forget to check out some of our most popular content on the site:1. How To Heal A Leaky Gut 2. 3 Best Probiotics For Healing Leaky Gut 3. Best Nootropic Stacks For Optimal Brainpower 4. Best Probiotics for Candida Overgrowth

Daniel Maman

Daniel is a certified personal trainer (ACE), has a Bachelors of Science degree in Sports, Exercise, and Wellness, and spends his free time keeping up to date with the latest research in health and fitness. In his free time, you can find him playing basketball, doing muay thai, camping, traveling, reading, and eating tomatoes like they're apples.

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