Intermittent fasting
for beginners

Intermittent fasting is the ancient secret of health. It is ancient because it has been practiced throughout all of human history. It’s a secret because this powerful habit has been virtually forgotten.

But now many people are re-discovering this dietary intervention. It can carry huge benefits if it is done right: weight loss, increased energy, reversal of type 2 diabetes and many other things. Plus, you’ll save time and money.

In this beginner’s guide you can learn everything you need to know about intermittent fasting.

 

  1. Learn more
 
 
 

Introduction – a natural part of life

Intermittent fasting – isn’t that starvation?

No. Fasting differs from starvation in one crucial way. Control. Starvation is the involuntary absence of food. It is neither deliberate nor controlled. Fasting, on the other hand, is the voluntary withholding of food for spiritual, health, or other reasons.

Food is easily available, but you choose not to eat it. This can be for any period of time, from a few hours up to days or even weeks on end. You may begin a fast at any time of your choosing, and you may end a fast at will, too. You can start or stop a fast for any reason or no reason at all.

Fasting has no standard duration, as it is merely the absence of eating. Anytime that you are not eating, you are fasting. For example, you may fast between dinner and breakfast the next day, a period of approximately 12-14 hours. In that sense, fasting should be considered a part of everyday life.

Consider the term “break fast”. This refers to the meal that breaks your fast – which is done daily. Rather than being some sort of cruel and unusual punishment, the English language implicitly acknowledges that fasting should be performed daily, even if only for a short duration.

Fasting is not something queer and curious, but a part of everyday, normal life. It is perhaps the oldest and most powerful dietary intervention imaginable. Yet somehow we have forgotten its awesome power and ignored its therapeutic potential.

Learning how to fast properly gives us the option of using it or not.

To get started, either watch our brief video course on fasting, or keep reading below.


309,255 viewsDo you want to keep watching? Part 2: How to maximize fat burning

 

 

 

How does intermittent fasting work?

Balancing eating and fasting

At its very core, fasting simply allows the body to burn off excess body fat. It is important to realize that this is normal and humans have evolved to fast without detrimental health consequences. Body fat is merely food energy that has been stored away. If you don’t eat, your body will simply “eat” its own fat for energy.

Life is about balance. The good and the bad. The yin and the yang. The same applies to eating and fasting. Fasting, after all, is simply the flip side of eating. If you are not eating, you are fasting. Here’s how it works:

When we eat, more food energy is ingested than can immediately be used. Some of this energy must be stored away for later use. Insulin is the key hormone involved in the storage of food energy.

Insulin rises when we eat, helping to store the excess energy in two separate ways. Sugars can be linked into long chains, called glycogen and then stored in the liver. There is, however, limited storage space; and once that is reached, the liver starts to turn the excess glucose into fat. This process is called De-Novo Lipogenesis (meaning literally Making Fat from New).

Some of this newly created fat is stored in the liver, but most of it is exported to other fat deposits in the body. While this is a more complicated process, there is no limit to the amount of fat that can be created. So, two complementary food energy storage systems exist in our bodies. One is easily accessible but with limited storage space (glycogen), and the other is more difficult to access but has unlimited storage space (body fat).

The process goes in reverse when we do not eat (fasting). Insulin levels fall, signaling the body to start burning stored energy as no more is coming through food. Blood glucose falls, so the body must now pull glucose out of storage to burn for energy.

Glycogen is the most easily accessible energy source. It is broken down into glucose molecules to provide energy for the other cells. This can provide enough energy to power the body for 24-36 hours. After that, the body will start breaking down fat for energy.

So, that the body only really exists in two states – the fed (insulin high) state and the fasted (insulin low) state. Either we are storing food energy, or we are burning it. It’s one or the other. If eating and fasting are balanced, then there is no net weight gain.

If we start eating the minute we roll out of bed, and do not stop until we go to sleep, we spend almost all our time in the fed state. Over time, we will gain weight. We have not allowed our body any time to burn food energy.

To restore balance or to lose weight, we simply need to increase the amount of time we burn food energy (fasting). In essence, fasting allows the body to use its stored energy. After all, that’s what it is there for. The important thing to understand is that there is nothing wrong with that. That is how our bodies are designed. That’s what dogs, cat, lions and bears do. That’s what humans do.

If you are constantly eating, as is often recommended, then your body will simply use the incoming food energy and never burn the body fat. You’ll only store it. Your body will save it for a time when there is nothing to eat. You lack balance. You lack fasting.

Learn more

Time-restricted eating – a detailed intermittent fasting guide

How fasting affects your physiology and hormones

Video:

Is Obesity Caused by Too Much Insulin? – Interview with Dr. Ted Naiman

 
 

Benefits of intermittent fasting

Fasting’s most obvious benefit is weight loss. However, there are a myriad of benefits beyond this, many of which were widely known in ancient times.

The fasting periods were often called ‘cleanses’, ‘detoxifications’, or ‘purifications’, but the idea is the same – to abstain from eating food for a certain period of time for health reasons. People imagined that this period of abstinence from food would clear their bodies’ systems of toxins and rejuvenate them. They were more correct than they knew.

Some of the purported physical benefits of fasting include:

Advantages

Fasting offers many important unique advantages that are not available in typical diets.

Where diets complicate life, fasting simplifies. Where diets are expensive, fasting is free. Where diets can take time, fasting saves time. Where diets are limited, fasting is available anywhere. Where diets have variable efficacy, fasting has unquestioned efficacy. There is no more powerful method for lowering insulin and decreasing body weight.

Here are even more reasons to try it, along with more details: The 7 practical benefits of fasting

Success stories

  • Keto and intermittent fasting: "I am completely blown away by the changes"
  • How Gino reversed his type 2 diabetes by doing the opposite
  • Intermittent fasting: down 42 pounds in 14 months

 

Different ways to fast

Shorter fasts (<24hrs)

Fasting offers infinite flexibility. You can fast for as long or short as you like, but here are some popular regimens. Generally, shorter fasts are done more frequently.

16:8

This involves daily fasting for 16 hours. Sometimes this is also referred to as an 8-hour eating ‘window’. You eat all your meals within an 8-hour time period and fast for the remaining 16 hours. Generally, this is done daily or almost daily.

For example, you may eat all your meals within the time period of 11:00 am and 7:00 pm. Generally, this means skipping breakfast. You generally eat two or three meals within this 8-hour period.

20:4

This involves a 4-hour eating window and a 20-hour fast. For example, you might eat between 2:00 pm and 6:00 pm every day and fast for the other 20 hours. Generally, this would involve eating either one meal or two smaller meals within this period.

Learn more about shorter fasts

 

Longer fasts (>24 hours)

24-hour fasts

This involves fasting from dinner to dinner (or lunch to lunch). If you eat dinner on day 1, you would skip the next day’s breakfast and lunch and eat dinner again on day 2. This means that you are still eating daily, but only once during that day. This would generally be done two to three times per week.

5:2 fast

Dr. Michael Mosley popularized this variation in his book ‘The Fast Diet’. This involves 5 regular eating days and 2 fasting days. However, on these two fasting days, it is permitted to eat 500 calories on each day. These calories can be consumed at any time during the day – either spread throughout the day, or as a single meal.

36-hour fasts

This involves fasting for the entire day. For example, if you eat dinner on day 1, you would fast for all of day 2 and not eat again until breakfast on day 3. This is generally 36 hours of fasting. This provides more powerful weight loss benefit. The other great benefit is that it avoids the temptation to overeat dinner on day 2.

Extended fasting

You can fast almost indefinitely. Generally for fasts greater than 48 hours, I recommend a general multivitamin to avoid micronutrient deficiency. The world record for fasting is 382 days, so going 7-14 days is certainly possible.

I discourage people from fasting for more than 14 days due to high risk of re-feeding syndrome.

Learn more about longer fasts

 

 

Common questions and answers about fasting

Questions and Answers About Fasting

Who should NOT fast?

You should not fast if you are:

  • Underweight (BMI < 18.5)
  • Pregnant – you need extra nutrients for your child.
  • Breastfeeding – you need extra nutrients for your child.
  • A child under 18 – you need extra nutrients to grow.

You can fast, but may need supervision, under these conditions:

  • If you have diabetes mellitus – type 1 or type 2.
  • If you take prescription medication.
  • If you have gout or high uric acid.

Won’t fasting put me into starvation mode?

No. This is the most common myth about fasting. In fact, the truth is just the opposite. Studies conclusively show that fasting increases basal metabolic rate. Learn more

Can I exercise during fasting?

Yes. You should continue all your usual activities, including exercise, while fasting. You do not need food to provide energy for exercise. During this time, your system will burn body fat for energy. Excellent! Learn more

What are the possible side effects?

There can be a number of possible nuisance side effects. Here’s what to do if you encounter them:

  • Constipation is common. Less going in means less going out. You don’t need medications unless you experience discomfort. Standard laxatives can be used to help.
  • Headaches are common and tend to disappear after the first few times on fasts. Taking some extra salt often helps mitigate such headaches.
  • Mineral water may help if your stomach tends to gurgle.
  • Other possible side effects include dizziness, heartburn and muscle cramps. Learn more

A more serious side effect is the refeeding syndrome. Fortunately, this is rare and generally only happens with extended fasts (5-10 days or more) when one is undernourished. Learn more

Why does my blood sugar go up during fasting?

This is due to hormonal changes that occur during fasting. Your body is producing sugar in order to provide energy for your system. This is a variation of the Dawn Phenomenon. Learn more

How do I manage hunger?

The most important thing to realize is that hunger passes like a wave. Most people worry that hunger will continue to build until it is intolerable, but this does not happen. Instead, hunger comes in a wave. If you simply ignore it and drink a cup of tea or coffee, it will often pass.

During extended fasts, hunger will often increase into the second day. After that, it gradually recedes; and many people report a complete loss of hunger sensation by day 3-4. Your body is now being powered by fat. In essence, your body is ‘eating’ its own fat for breakfast, lunch and dinner and therefore is no longer hungry. Learn more

Won’t fasting burn muscle?

No. During fasting, the body first breaks down glycogen into glucose for energy. After that, the body increases fat breakdown to provide energy. Excess amino acids (the building blocks of proteins) are also used for energy, but the body does not burn its own muscle for fuel.

It would be a long stretch of the imagination to think that our bodies store energy so carefully in the form of glycogen and fat only to burn muscle when it is needed.

Fasting has been practiced for thousands of years without difficulty. In my experience with over 1,000 patients on various fasting regimens, exactly zero have complained that they have noticed significant muscle loss. Learn more

What are your top tips for intermittent fasting?

Here are the nine top tips, briefly:

  • Drink water
  • Stay busy
  • Drink coffee or tea
  • Ride out the hunger waves
  • Don’t tell anybody who is not supportive that you are fasting
  • Give yourself one month
  • Follow a low-carb diet between fasting periods. This reduces hunger and makes fasting much easier. It may also increase the effect on weight loss and type 2 diabetes reversal, etc.
  • Don’t binge after fasting

Learn more practical fasting tips

How do I break a fast?

Gently. The longer the fast, the more gentle you must be. For short duration fasts, eating too large a meal after fasting (a mistake that we have ALL done, myself included) will usually give you a stomach ache. While this is not serious, people learn quickly to eat as normally as possible after a fast.

Isn’t it important to have breakfast every morning?

No, it’s not. This is an old misconception based on speculation and statistics, and it does not hold up when it’s tested. Skipping your morning meal just gives your body more time to burn fat for energy. Since hunger is lowest in the morning, it is often easiest to skip it and break your fast later in the day. Learn more:

Can women fast?

Absolutely. The only exception is women who are underweight, pregnant or breastfeeding. Other than that, there is no reason not to fast. Women have problems during fasting, but so do men. Sometimes women do not get the results they want, but that happens to men, too.

Women have fasted for thousands of years without incident. Studies show that the average weight loss for women and men who fast is similar. Learn more about women and fasting

Isn’t fasting the same as reducing calories?

No. Not at all. Fasting reduces the time you spend eating and addresses the question of ‘when to eat’. Calorie reduction addresses the question of ‘what to eat’. They are separate issues and should not be confused with each other.

Fasting does reduce calories but it’s benefits extend far beyond that. Learn more

Will I lose weight?

Absolutely. It is almost inconceivable that you will not lose weight if you do not eat.

I call fasting ‘The Ancient Secret of Weight Loss’ because it is one of the most powerful dietary interventions for weight loss, yet it has been almost completely ignored it in recent years. Learn more

More Q&A

Many more questions and answers about intermittent fasting
 


 

How to get started


Now that you know all the essentials of fasting, how do you get started? You just follow these steps:

  • Decide what type of fast you want to do
  • Decide upon the length of time you want to fast
  • Start fasting. If you do not feel well, or if you have any concerns, then stop and seek help
  • Continue all your usual activities outside of eating. Stay busy and live normally. Imagine you’re “eating” a full meal of your own fat
  • Break the fast gently
  • Repeat

Yes. It really is THAT simple.

Extra video preparation

If you feel that you want more preparation first, then watch our intermittent fasting video course.

If you want to, you can jump straight to the get-started sections of the course:

  • The left one below is about how to do a 24-hour fast (it’s surprisingly simple).
  • The right one is a real challenge, a 7-day fast (are you up for it?).
How to Get Started with Fasting – Dr. Jason Fung
The Power of a 7–Day Fast – Dr. Jason Fung

 
 
 

Resources to learn more

 

The fasting video course

 

Interviews

 

Presentations

  • The Cause of Obesity
  • Therapeutic Fasting
  • The 2 Big Lies of Type 2 Diabetes
 
 

Q & A

Ask Dr. Jason Fung

Books

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Check out Dr. Fung’s books The Obesity Code and The Complete Guide to Fasting on Amazon.

Website

Dr. Fung’s website: IDMprogram.com

About Dr. Fung

Who Is Dr. Jason Fung?
 
 
 

More

Low carb for beginners

How to lose weight

How to reverse type 2 diabetes

Comments

What are your experiences with fasting? Do you have questions that are not answered above? Please feel free to leave a comment below.

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118 Comments

  1. Laura
    Yes drink lots of water or you will be dehydrated and that causes other issues
  2. Jess
    Barbara, I find that carbs and sugars make me cranky. It's hard to fast easily without going on a low carb, low sugar, high fat diet first. So if you eat the normal diet and just try to fast, you probably will be hangry. But if you have been LCHF and keto for awhile, it's manageble. I'm 41 and was only able to lose 50+ lbs after having two kids by doing keto and fasting. I ended up being at the weight I was in my teens!:) Nice.
  3. All
    I'm astonished by the stupidity of some of the people leaving comments here. Wow people come on. You all sound illiterate. Stick to high carb diets, obviously low carb living is too much a stretch for you.
  4. Katelijne Dendooven
    Hello I am.just starting today. Is it ok to drink fresh veggie juices during the fasting ?

    Thank you

    Reply: #122
  5. karthik
    In many places, I read that the body starts reacting (burning fat, ketones production, increasing GH, etc) start only after 24-36 hours. If I eat before 24 hours (as in 16-8 fasting or 24-hour fasting), won't this process prevent burning the fat? After 24 hours, if the fat burning would start in hour 25, but then if I already eat food at hour 24, then will the fat burning never happen? I'm just trying to find the logical answer.
  6. Beaner
    If you are already eating low carb and in ketosis,as I am,you can go all day without eating anything because your body is using its fat stores for fuel..I've gone 2-3days without eating because my body no longer gives me hunger signals.
    When I do want to eat something, I only eat some protein and I'm good.I drink alot of water and some beef broth during fasts and I never seem to be actually hungry.
    Fasting is something your body adapts to very quickly.
    Reply: #109
  7. Nana
    I am on several medications daily. Blood pressure medications, aspirin, vitamins, supplements etc. I take them every morning around the same time with water. Would that be considered a problem with a Intermittent Fast? Would a 16-8 fast or 2-24 hour (or 2:5 ) system work best?
  8. Janewithay
    Hi. I have been doing a version of this diet for nearly a year now - albeit unknowingly.

    I started out doing a low carb high protein diet last fall. It worked well at first and I dropped about forty pounds by the New year. Then I hit a plateau. Nothing seemed to budge it - nothing. People kept telling me to go on a carb binge and that would somehow trick my body into losing weight. It didn't. As predicted, it put two-three pounds back on me. Because I was going to my doctor for monthly weigh in's I would find myself fasting two days before just to carve some pounds off before my visit. What I discovered by doing this was that I had more focus, energy, and a significant weight loss - sometimes up to five pounds. I started playing around with it until I found a formula that works well for me.
    I generally fast Mon and Tuesdays and then refeed gradually on Wednesday and Thursday. By Friday, I have a two-four pound weight loss. I maintain that by eating a good, low carb diet on the first weekend. Then I repeat the same process a week later. The only difference is I concentrate on the second week to maintain the weight loss from week one. I do this by actually allowing myself to eat a bit more than I do the previous week and by increasing my activities. Sometimes I manage to lose another pound or even two.
    It sounds complicated but it's not really. Week one is to lose weight. Week two is to lock in the weight loss. Thus far I have lost 84 pounds and am no longer obese. I am still on the lower end of being overweight though so I would like to lose another 15 pounds.
    For the record, I drink coffee with cream, even on a fast. And I am not a saint; if I am about to pass out from fasting then I eat something! (Like an egg or a piece of hard cheese) Plus, if someone wants to go out for dinner or cocktails then I adjust my fast. I have to live this life and personally, I am not going to be one of those zealots who always says no to everyone, and most especially, to myself. Living in a constant state of denial is not mentally healthy, imo.
    PS: Breakfast is for sissies! lol! Never eat it ... never crave it ... don't want it ... Glad I'm not going to curl up and die because I just said NO to breaking my fast before 12:00. But don't tell my doctor. lol .. she's not on board with the no breakfast movement at ALL!

    Replies: #112, #123
  9. Janewithay
    The beef broth is excellent in helping to ward off leg and feet cramps, too!
  10. Fay
    Hi, I want to get leaner but i DONT want to lose weight. I am already on the slim side so my aim is to lose the belly fat i have (after 2 pregnancies) and build muscle (i weight train 5 days a week). So when i do the 16/8 method should i be eating the same amount of calories i normally would in order not to lose weight and build muscle?
  11. Latif
    I am in my 40s, keeping good health without any health problem.
    I am fasting 2 days in week. Waking up early morning before the down and having 5 to 7 nos of dates and one glass of pure water, continue the fasting without drinking of eating till sun set. Then break the fast with one glass of water and some dates. After an hour have my dinner.
    This what Prophet taught in Islam
  12. Lisa B
    just curious. so then what do you do for week 3 and 4?
  13. Peter
    Peter B
    I am on day 6 of my first water fast, my weight at the start was 109 kg and now its 105 kg. On the third day I checked my blood keytones and they were 4.1 and have ranged from 3.9 to 4.9. I take a potassium supplement, 2 teaspoons of Himalayan rock salt and some apple cider vinegar dissolved in water during the day. I drink at least four litres a day. I feel different, slightly out of sorts but not hungry. I have gone to several dinners with friends and family and they think I'm mad but google fasting and see various esteemed professors of medicine and scientific researchers laud prolonged fasting for the health benefits. My granddaughter, aged three, is somewhat bemused as I do not attempt to steal chips from her plate when she is not looking and actually offers them to me now. I plan to several long fasts this year as my aim is to get my weight to 85 kg.
  14. Pat Clark
    Just to clarify, do I do the 16/8 schedule every day or just a limited number of days?
  15. Eric
    If you consume vitamins during the fasting period, will it break the fast? Should you only consume vitamins during the 8 hour time to eat if you were doing the 16:8 intermittent fast?
    Reply: #119
  16. Faye T
    Hi I have started Keto diet since the 5th Jan and it’s nearly two weeks now and have lost 2.7kg. I notice those days where I woke up late meaning I did a fast from 7pm till 10am, the weight loss is better. And lately because I woke up early I tend to eat breakfast so the result isn’t as good. One problem I have had since started the Keto diet is I have lower back sore both sides, tight muscles around the ankle and shoulder area. What could this be ? I read drinking a lot of water and get hydrated is necessary. In the past I was never a heavy meat eater, so all this soreness could be caused by the change of my diet now ? Appreciate some help ! Thanks
  17. Casey
    Does fasting automatically make you lose weight regardless of how much you eat in the 16/8 method? Can we consume the same amount of food in the 8 hour window we would have without the fast?
  18. Karl
    Hi, I do crossfit and do the WOD's at around 6:30pm to 8:30pm. And I do the 16:8 fasting with my fasting state at 8pm-12noon and fed state at 12noon-8pm. As soon as my workout is done, I am basically on my fasting state. Is it okay to eat my dinner early at around 4-4:30pm? Will it affect the fasting and my workout? Or should I adjust my time table and need to go with the post-workout dinner? Thanks!
  19. Eric
    I found some information on other sites about this subject. It appears that taking vitamins during the fast will not break it. However some side effects like nausea may occur when taking vitamins on an empty stomach. I also found that vitamins are absorbed better when consumed with fats. So basically, taking vitamins will not break a fast, but its probably better to take them with food so the nutrients are absorbed better, and avoid side effects when taking them on an empty stomach. Does anyone else have more information about this?
  20. Farah
    Is coffee with a little but of cream ok while fasting? It’s what keeps me going during my fasted period. Also can I change fast times throughout the week? I work day and night on tues and wed so need to push my eating window so I can eat when off night job. Any input would be fabulous. Thank you!
  21. Sara
    I have been doing low carb for 5 months and I lost 10kg, but now I am eating a little more and my weight is creeping up. What is happening?
  22. johnny gato
    Don't know if anyone replied to you, but drinking any veggie juices is a no-no no matter what you do. Fruit juices are worse than veggie, but both are just stripped down to water, minimal vitamins and SUGAR.
  23. Robin
    Thank you so much for sharing your experience! It sounds similar to mine, although i have yet to see the results you have seen. For the past 2.5 years, I as following a low carb plan, restricting myself to 60 carbs a day. Like you, I plateaued and was stuck for quite awhile. My second week into te LCHF challenge, I lost five pounds and then got stuck again. Slowly my body transitioned into a fasting mode and lately, I find myself doing the 24 hour fast. Lost an additional 3.5 this week and an inch in the belly. The best part is, I'm not hungry at all and not missing breakfast at all. I don't tell many people about the fast, because like your doc, they are just not believers; but I am.
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