Fasting and hunger


Does fasting increase your hunger to unimaginable and uncontrollable dimensions? This is often how fasting is portrayed, but is it really true? From a purely practical standpoint, it is not.

From my personal experience with hundreds of patients, one of the most consistent, yet surprising things reported is the reduction, not an augmentation of hunger. They often say things like, “I thought I would be consumed by hunger, but now I only eat a third of what I used to, because I am full!” That’s great, because now you are working with your body’s hunger signalling to lose weight instead of constantly fighting it.

The number one, most common misperception of fasting is that it will leave us overwhelmed with hunger and therefore prone to severe overeating. Thus you get pronouncements from ‘experts’ like “Don’t even think about fasting, otherwise you will be so hungry that you will stuff your face full of Krispy Kreme donuts”. These ‘experts’ often have zero experience with fasting either personally or with clients, so this is classic ‘lecturing birds how to fly’ behavior. So what actually happens with hunger?

Approximately 4-8 hours after we eat a meal, we start to feel hunger pangs and may become slightly cranky. Occasionally they are quite strong. So we imagine that fasting for a full 24 hours creates hunger sensations 5 times stronger – and that will be intolerable. But this is exactly what does NOT happen.

Hunger – a conditioned response

Hunger is, in fact, a highly suggestible state. That is, we may not be hungry one second, but after smelling a steak and hearing the sizzle, we may become quite ravenous. Hunger is also a learned phenomenon, as demonstrated by the classic experiments of Pavlov’s dogs – known in psychology as Pavlovian, or classical conditioning.

Pavlov2In the 1890s, Ivan Pavlov was studying salivation in dogs. Dogs will salivate when they see food and expect to eat (unconditioned stimulus – UCS) – that is, this reaction occurs naturally and without teaching. In his experiments, lab assistants would go in to feed the dogs and the dogs soon began to associate lab coats (conditioned stimulus -CS) with eating. There is nothing intrinsically appetizing about a man in a lab coat (yummy!), but the consistent association between the lab coat and food paired these two in the dog’s mind.

Very soon, the dogs began to salivate at the sight of the lab coats alone (having now been conditioned) even if food was not available. Ivan Pavlov, genius that he was, noticed this association and started to work with bells instead and before you know it, he was packing his bags to Stockholm to get his Nobel Prize and taste some of those oh-so-delicious Swedish meatballs. The band ABBA, unfortunately had not been formed yet. By pairing bells and food, the dogs began to anticipate food (salivate) at hearing bells alone without the food. This was the Conditioned Response

The applicability of this Psychology 101 lesson to hunger is obvious. That is, we can become hungry for many reasons – some of which are natural (smell and sizzle of steak) and others which have become conditioned into us. These conditioned responses can be very powerful and cause great hunger. If we consistently eat breakfast every single morning at 7:00, lunch at 12:00 and dinner at 6:00pm, then the time of day itself becomes a conditioned stimulus for eating. Even if we ate a huge meal at dinner the night before, and would not otherwise be hungry in the morning, we may become ‘hungry’ because it is 7:00. The Conditioned Stimulus (time of 7:00) causes the Conditioned Response (hunger).

We’re conditioned to constantly think about food

Similarly, if we start to pair the act of watching a movie with delicious popcorn and sugary drinks, then the mere thought of a movie may make us hungry even though we have already eaten dinner and would normally not be hungry. The movie is the conditioned stimulus. Food companies, of course, spend billions of dollars trying to increase the number of CS that will make us hungry. The Conditioned Response is hunger – for popcorn, chips, hot dogs, sodas, etc.

Food at the ballgame! Food with movies! Food with TV! Food in between halves of kids soccer! Food while listening to a lecture! Food at the concerts! You can eat with a goat. You can eat on a boat. You can eat in a house. You can eat with a mouse. Conditioned responses, every one.

How to combat this? Well, intermittent fasting offers a unique solution. By randomly skipping meals and varying the intervals that we eat, we can break our current habit of feeding 3 times a day, come hell or high water. We no longer have a conditioned response of hunger every 3-5 hours. We would no longer become hungry simply because the time is 12:00. Instead, we would still get the unconditioned response of hunger, but not the conditioned one. That is, ‘you get hungry because you are hungry’, rather than ‘you get hungry because it’s noon’.

Similarly, by not eating throughout the entire day, we can break any associations between food and anything else – TV, movies, car rides, ball game etc. Here’s the solution. Eat only at mealtimes and at the table. No eating at your computer station. No eating in the car. No eating on the couch. No eating in bed. No eating in the lecture hall. No eating at the ball game. No eating on the toilet. (OK, that last one is gross, but I’ve seen it!).

MuffinsAtCONOur current Western food environment, of course, strives to do the opposite. There is a coffee shop or fast food restaurant on every corner. There are vending machines in every nook and cranny of every building in North America. In every conference, even at the Canadian Obesity Network, each break time is greeted by fattening muffins and cookies. Ironic and funny if not so heartbreaking. (Yes, we are doctors that treat obesity. Oh look, muffin! I’ll just eat it in the lecture hall even though I’m not really hungry!)

Breaking the habit

One key advantage of fasting is the ability to break all these conditioned responses. If you are not accustomed to eating every 4 hours, then you will not start salivating like Pavlov’s dog every 4 hours. If we are conditioned this way, no wonder we find it increasingly difficult to resist all the Mcdonald’s and Tim Horton’s stores while walking around. We are bombarded daily with images of food, references to food, and food stores themselves. The combination of their convenience and our ingrained Pavlovian response is deadly and fattening.

In breaking habits, you must understand that going cold turkey is not often successful. Instead, it is far better to replace one habit with another, less harmful habit. For example, suppose you have a habit to munch while watching TV – chips or popcorn or nuts. Simply quitting will make you feel that something is ‘missing’. Instead, replace that habit of snacking with a habit of drinking a cup of herbal or green tea. Yes, you will find this weird at first, but you will feel a lot less like something is ‘missing’. So, during fasting, you may, instead of completely skipping lunch, drink a large cup of coffee. Same at breakfast. Or perhaps replace dinner with a bowl of homemade bone broth. It will be easier in the long run. This is, of course, the same reason that people who want to quit smoking often chew gum.

Social influence can also play a large role in eating. When we get together with friends, it is often over a meal, over coffee, or some such dietary event. This is normal, natural and part of human culture worldwide. Trying to fight it is clearly not a winning strategy. Avoiding social situations is not healthy either.

So what to do? Simple. Don’t try to fight it. Fit the fasting into your schedule. If you know you are going to eat a large dinner, then skip breakfast and lunch. One of the easiest ways to fit fasting into your life is to skip breakfast, since that meal is very uncommonly taken with others and, during working days is easy to skip without anybody noticing. This will quite easily allow you to fast for 16 hours (16:8 protocol). Also, unless you go out to lunch every day with the same crowd, lunch is also quite easy to miss without anybody noticing during the work day. This allows you to ‘slip in’ a 24 hour fast without any special effort.

OvereatingSo, in essence, there are two major components to hunger. The unconditioned biological stimuli – that is, the part that will normally stimulate hunger naturally (smells, sights, and tastes of food) and the conditioned stimuli (learned – movie, lecture, ball game). These CS do not naturally stimulate hunger, but through consistent association, have become almost as powerful. That is, the movie, the TV, the sight of McDonalds, the sound of a jingle etc. They have become hopelessly intertwined but they are by no means irreversibly so. Simply change out the response (drink green tea instead of eat popcorn). Fasting helps to break all the conditioned stimuli, and thus helps to reduce, not enhance hunger. Hunger is not so simple as your stomach being ’empty’.

So – here’s the real question – does fasting lead to over-eating? This was answered in a study published in 2002. 24 healthy subjects underwent a 36-hour fast and then caloric intake was measured. At baseline, subjects ate 2,436 calories per day. After a 36-hour fast, there was an increase in caloric intake to 2914 calories. So there was a degree of over-eating – almost 20%. However, over the 2-day period, there was still a net deficit of 1,958 calories over 2 days. So the amount ‘over’ eaten did not nearly compensate for the period of time fasting. They conclude the “a 36-hr fast..did not induce a powerful, unconditioned stimulus to compensate on the subsequent day.”

Here’s the ‘Stop being so long winded, Dr. Fung. I’m busy so spare me the details’ bottom line – NO, fasting does not lead to overeating. No, You will NOT be overwhelmed with hunger. YES, you can fast. It’s OK.

Jason Fung


Intermittent fasting for beginners

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More with Dr. Fung

Dr. Fung has his own blog at He is also active on Twitter.

His book The Obesity Code is available on Amazon.

The Obesity Code

His new book, The Complete Guide to Fasting is also available on Amazon.



  1. Megan
    I love that you mention Tim Horton's. Yay Canada! Great article. Thank you.
  2. 1 comment removed
  3. Janet
    If you experienced it fist hand, is it still misconception?
  4. Peter
    My experiences show this is not quite right at least as far as losing weight is concerned.
    After fasting I always gain weight. This has happened with short 24hr fasts and longer ones. The gain for longer fasts tends to occur over the following 2 - 4 days. As far as I can tell, Dr. Fung is right in that it is not overeating (calories measured) that is the problem but maybe some "temporary" lowering of the metabolism ?
    I am considering doing some extended personal trials trials on this but in the meantime, I would be interested in any thoughts from the good doctors.
  5. Mira
    Please-please-please somebody answer me to this question. Why does food makes me so tired and sleepy after fasting? I try to eat lchf and as my break fast meals I tried big meals and small light ones. The effect is always the same - super tired and sleepy! What can I do about this? I dont want to increase my caffeine consumption and it doesn't feel like a right way to go anyways. Please share your advice!
    Reply: #8
  6. R.
    It is incongruous that most of the allusions in this article are meat-based when coming from someone who calls himself the "Diet Doctor".
  7. Mark Smith
    Not true, I fast for 18 hours+ water type drinks only and I am way more hungry on my days off.
  8. Lisa
    I have found the same. I broke my fast after 90 hours, ate simply and lightly but felt exhausted within about half an hour of doing so. Perhaps our bodies adjust to not having to put energy into digesting and then all of a sudden it needs to? I am not an expert and really just guessing.
  9. Ro
    been on every other day for a couple of months. on keto for 5 months. at 18/6 every day since the beginning. not a single snack or graze. on the 24 hour for 5 months I am always hungry between 12 and 6 pm if on the every other day 24 hour. It's not emotional, triggered, or rumination it's my stomach growling and h u n g e r. I also drop after the fast and then the next day when I do 18/6 I gain back 50 percent of whatever I dropped. the explanations are too simplistic. i am not around anyone or anything that is triggering anything. I don't graze. there is a HUGE difference between eating 2 or 3 times, alone per day, and once hunger wise for me. yeah it's crazy that folks graze up to 10 times a day. snacking should be outlawed. I would like to see the decade studies on this or even a 6 month to 12 month on who remained in and out. what is sustainable?
  10. Steve
    This is not true. I have been on 16/8-18/6 for two months and I am still extremely hungry around the clock. I am starving all night long - it gives me trouble sleeping. I am starving all day long even during feeding hours. I am still waiting for the hunger to stop. I don't eat cheat meals, but IF is making it extremely difficult for me to not binge eat.
    Reply: #11
  11. Kristin Parker Team Diet Doctor

    This is not true. I have been on 16/8-18/6 for two months and I am still extremely hungry around the clock. I am starving all night long - it gives me trouble sleeping. I am starving all day long even during feeding hours. I am still waiting for the hunger to stop. I don't eat cheat meals, but IF is making it extremely difficult for me to not binge eat.

    If you find this unsustainable, examine what you are eating during the eating window. It should be very low carb and high fat. That will help stabilize the blood sugar and insulin which will bring hunger under control/

  12. 1 comment removed
  13. Teresa
    Well, I have done the 16/8 cycle, only ate between 1600-1800 Cal's, 25 or lower I carbs an I actually lost 35 lbs in 3 1/2 months. I was never hungry, in fact I was less hungry. I did what they call the lazy keto which worked for me. I got off for a few months, but I have started back with no problems so far. Good luck to everyone!
  14. Teresa
    Well, I have done the 16/8 cycle, only ate between 1600-1800 Cal's, 25 or lower in carbs and I actually lost 35 lbs in 3 1/2 months. I was never hungry, in fact I was less hungry. I did what they call the lazy keto which worked for me. I got off for a few months, but I have started back with no problems so far. Good luck to everyone!

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