The difference between calorie restriction and fasting – Fasting 27

Perhaps one of the most common questions we get is what the difference is between calorie restriction and fasting. Many calorie enthusiasts say that fasting works, but only because it restricts calories. In essence, they are saying that only the average matters, not the frequency. But, of course, the truth is nothing of its kind. So, let’s deal with this thorny problem. deathvalley

The weather in Death Valley, California should be perfect with a yearly average temperature is 25 Celsius. Yet, most residents would hardly call the temperature idyllic. Summers are scorching hot, and winters are uncomfortably cold.

You can easily drown crossing a river that, on average, is only 2 feet deep. If most of the river is 1 foot deep and one section is 10 feet deep, then you will not safely cross. Jumping off a 1 foot wall 1000 times is far different than jumping off a 1000-foot wall once.intermittentvscontinuous

In a week’s weather, there is a huge difference between having 7 grey, drizzling days with 1 inch of rain each and having 6 sunny, gorgeous days with 1 day of heavy thundershowers.

It’s obvious in all these examples that overall averages only tell one part of the tale, and often, understanding frequency is paramount. So why would we assume that reducing 300 calories per day over 1 week is the same as reducing 2100 calories over a single day? The difference between the two is the knife-edge between success and failure.

The portion control strategy of constant caloric reduction is the most common dietary approach recommended by nutritional authorities for both weight loss and type 2 diabetes. Advocates suggest that reducing daily caloric consumption by 500 calories will trigger weight loss of approximately one pound of fat per week.

The American Diabetes Association’s main dietary recommendation suggests to “focus on diet, physical activity, and behavioral strategies to achieve a 500–750 kcal/day energy deficit.” The ‘portion control’ advice to reduce calories has been fairly standard since the 1970s. This reduction is average calories should be spread consistently throughout the day, rather than all at once. Dieticians often counsel patients to eat four, five or six times a day. There are calorie labels on restaurant meals, packaged food, and beverages. There are charts for calorie counting, calorie counting apps, and hundreds of calorie counting books. Even with all this, success is as rare as humility in a grizzly bear.

After all, who hasn’t tried to portion control strategy of weight loss. Does it work? Just about never. Data from the United Kingdom indicate that conventional advice succeeds in 1 in 210 obese men and 1 in 124 obese women (4). That is a failure rate of 99.5%, and that number is even worse for morbid obesity. So, whatever else you may believe, constant caloric reduction does NOT work. This is an empirically proven fact. Worse, it has also been proven in the bitter tears of a million believers.

But why doesn’t it work? For the same reason the contestants of The Biggest Loser could not keep their weight off – metabolic slowdown.

Starvation Mode

The Biggest Loser is a long running American TV reality show that pits obese contestants against one another in a bid to lose the most weight. The weight loss regimen is a calorie-restricted diet calculated to be approximately 70% of their energy requirements, typically 1200-1500 calories per day. This is combined with an intensive exercise regimen typically far in excess of two hours daily.

This is the classic ‘Eat Less, Move More’ approach endorsed by all the nutritional authorities, which is why The Biggest Loser diet scores third on the 2015 USA Today’s ranking of best weight loss diets. And, it does work, in the short term. The average weight loss that season was 127 pounds over 6 months. Does it work long-term? Season two’s contestant Suzanne Mendonca said it best when she stated that there is never a reunion show because “We’re all fat again”.

Their Resting Metabolic Rates (RMR), the energy needed to keep the heart pumping, the lungs breathing, your brain thinking, your kidneys detoxing etc., drops like a piano out of a 20 story building. Over six months, their basal metabolism dropped by an average of 789 calories. Simply stated, they burning 789 calories less per day every day.

As metabolism drops, weight loss plateaus. Caloric reduction has forced the body has shut down in order to match the lowered caloric intake. Once expenditure drops below intake, you start the even more familiar weight regain. Ba Bam! Weight is regained despite dietary compliance with the caloric restriction and even as your friends and family silently accuse you of cheating on your diet. Goodbye reunion show. Even after six years, the metabolic rate does not recover .

All of this is completely predictable. This metabolic slowdown has been scientifically proven for over 50 years. In the 1950s Dr. Ancel Key’s famous Minnesota Starvation Study placed volunteers on a ‘semi-starvation’ diet of 1500 calories per day. This represented a 30% caloric reduction from their previous diet. In response, their basal metabolic rate dropped about 30%. They felt cold, tired, and hungry. When they resumed their typical diet, all their weight came right back.

Caloric restriction diets only work in the short-term, before basal metabolism falls in response. This is sometimes called ‘starvation mode’. Daily calorie restriction fails because it unerringly put you into metabolic slowdown. It’s a guarantee. Reversing type 2 diabetes relies upon burning off the body’s excess glucose, so the daily calorie-restricted diet will not work.

The secret to long-term weight loss is to maintain your basal metabolism. What doesn’t put you into starvation mode? Actual starvation! Or at least the controlled version, intermittent fasting.

Fasting triggers numerous hormonal adaptations that do NOT happen with simple caloric reduction. Insulin drops precipitously, helping prevent insulin resistance. Noradrenalin rises, keeping metabolism high. Growth hormone rises, maintaining lean mass.

Over four days of continuous fasting, basal metabolism does not drop. Instead, it increased by 12%. Neither did exercise capacity, as measured by the VO2, decrease, but is instead maintained. In another study, twenty-two days of alternate daily fasting also does not result in any decrease in RMR.

Why does this happen? Imagine we are cavemen. It’s winter and food is scarce. If our bodies go into ‘starvation mode’, then we would become lethargic, with no energy to go out and find food. Each day the situation gets worse and eventually we die. Nice. The human species would have become extinct long ago if our bodies slow down each time we didn’t eat for a few hours.Zaunder1

No, instead, during fasting, the body opens up its ample supply of stored food – body fat! Yeah! Basal metabolism stays high, and instead we change fuel sources from food, to stored food (or body fat). Now we have enough energy to go out and hunt some woolly mammoth.

During fasting, we first burn glycogen stored in the liver. When that is finished, we use body fat. Oh, hey, good news – there’s plenty of fat stored here. Burn, baby burn. Since there is plenty of fuel, there is no reason for basal metabolism to drop. And that’s the difference between long-term weight loss, and a lifetime of despair. That’s the knife edge between success and failure.

Fasting is effective where simple caloric reduction is not. What is the difference? Obesity is a hormonal, not a caloric imbalance. Fasting provides beneficial hormonal changes that happen during fasting are entirely prevented by the constant intake of food. It is the intermittency of the fasting that makes it so much more effective.

Intermittent Fasting vs Calorie Restriction

The beneficial hormonal adaptations that occur during fasting are completely different from simple calorie restriction. The reduction of insulin and insulin resistance in intermittent fasting plays a key role.

The phenomenon of resistance depends not only upon hyperinsulinemia, but also upon the persistence of those elevated levels. The intermittent nature of fasting helps to prevent the development of insulin resistance. Keeping insulin levels low for extended periods of time prevents the resistance.

Studies have directly compared daily caloric restriction with intermittent fasting, while keeping weekly calorie intake similar. A 30% fat, Mediterranean style diet with constant daily caloric restriction was compared to the same diet with severe restriction of calories on two days of the week.harvie1

Over six months, weight and body fat loss did not differ. But there were important hormonal differences between the two strategies. Insulin levels, the key driver of insulin resistance and obesity in the longer term, was initially reduced on a calorie restriction but soon plateaued. However, during intermittent fasting, insulin levels continued to drop significantly. This leads to improved insulin sensitivity with fasting only, despite similar total caloric intake. Since type 2 diabetes is a disease of hyperinsulinemia and insulin resistance, the intermittent fasting strategy will succeed where caloric restriction will not. It is the intermittency of the diet that makes it effective.

Recently, a second trial directly compared zero-calorie alternate-day fasting and daily caloric restriction in obese adults. The Caloric Reduction as Primary (CRaP) strategy was designed to subtract 400 calories per day from the estimated energy requirements of participants. The ADF group ate normally on eating days, but ate zero calories every other day. The study lasted 24 weeks.catenacci1

What were the conclusions? First, the most important conclusion was that this was a safe and effective therapy that anybody could reasonably follow. In terms of weight lost, fasting did better, but only marginally. This is consistent with most studies, where, in the short term, any decent diet produces weight loss. However, the devil is in the details. The truncal fat loss, which reflects the more dangerous visceral fat,was almost twice as good with fasting as opposed to CRaP. In fat mass %, there is almost 6 times (!) the amount of loss of fat using fasting

The other big concern is that fasting will ‘burn muscle’. Some opponents claim (without any evidence) that you lost 1/4 pound of muscle for every single day of fasting you do. Considering I fast at least 2 days a week, and have done so for years, I estimate my muscle percentage should be just about 0%, and I shouldn’t even have enough muscle to type these words. Funny how that didn’t happen. But anyway, what happened in that study? The CRaP group lost statistically significant amounts of lean mass, but not the IF group. Yes, there is LESS lean muscle loss. Maybe it has to do with all the growth hormone and nor adrenalin being pumped out.

Lean mass % increased by 2.2% with fasting and only 0.5% with CRaP. In other words, fasting is 4 times better at preserving lean mass. So much of that old ‘fasting burns the muscle’.

What happens to basal metabolism? that’s what determines long term success. If you look at the change in Resting Metabolic Rate (RMR). Using CRaP, basal metabolism dropped by 76 calories per day. Using fasting, it only dropped 29 calories per day (which is not statistically significant compared to baseline). In other words, daily caloric reduction causes almost 2 1/2 times as much metabolic slowdown as fasting! So much for that old ‘Fasting puts you into starvation mode’.catenacci2

Fasting has been used throughout human history as a tremendously effective method of controlling obesity. By contrast, the portion control strategy of daily caloric restriction has only been recommended for the last 50 years with stunning failure. Yet, conventional advice to reduce a few calories every day persists and fasting is continually belittled as an outdated, dangerous practice akin to blood-letting and voodoo. The study reports that “Importantly, ADF was not associated with an increased risk for weight regain”. Holy S***. That’s the Holy Grail, Man! The whole problem is obesity and The Biggest Loser is WEIGHT REGAIN, not initial weight loss.

Weight regain differed during fasting vs CRaP. The fasting group tended to regain lean mass and continue to lose fat, while CRaP group gained both fat and lean mass. Part of the issue was that the fasting group reported that they often continued to fast even after the study was done. Of course! It is easier than they though, with better results. Only an idiot would stop. One of the very fascinating things is that ghrelin (the hunger hormone) goes up with CRaP but does NOT during fasting. We’ve known forever that dieting makes you hungrier. It’s not a matter of willpower – it’s a hormonal fact of life – the ghrelin goes up and you are hungrier. However, fasting does not increase hunger. Fascinating. No wonder it’s easier to keep the weight off! You’re less hungry.

Calorie restriction diets ignore the biological principle of homeostasis – the body’s ability to adapt to changing environments. Your eyes adjust whether you are in a dark room or bright sunlight. Your ears adjust if you are in a loud airport or a quiet house.

The same applies to weight loss. Your body adapts to a constant diet by slowing metabolism. Successful dieting requires an intermittent strategy, not a constant one. Restricting some foods all the time (portion control) differs from restricting all foods some of the time (intermittent fasting). This is the crucial difference between failure and success.

So here’s your choices:

  1. Caloric Reduction as Primary: less weight loss (bad), more lean mass loss (bad), less visceral fat loss (bad), harder to keep weight off (bad), hungrier (bad), higher insulin (bad), more insulin resistance (bad), perfect track record over 50 years unblemished by success (bad)
  2. Intermittent Fasting: More weight loss, more lean mass gain, more visceral fat loss, less hunger, been used throughout human history, lower insulin, less insulin resistance.

Almost every medical society, doctor, dietician and mainstream media will tell you to use choice #1. I prefer to tell people to take choice #2.

106 Responses

  1. I am assiduous reader of your blog, I look forward to Thursday to read your new post.
    I was 128 kg and I lost 25 kg in 2 years and my weight does not decrease for 1 year.
    I am worried if I have my metabolism broken!

    I used to do only one meal a day, dinner around 1500, because I work and is comfortable like that, from Monday to Friday, on Saturday and Sunday I make a normal meal.
    Is it fasting or calorie restriction?

    I am 56 years old and I am very active, I exercise regularly.
    I would like an answer.
    Sorry for the grammar errors, I’m from Brazil and I’m using Google Translate.

    • Hey ANA

      Not answering for Jason here of course. But interested to know what you typical meals look like for a day/week?

      What sort of food do you eat? You say you exercise regularly, what does that look like?

      Sleep and stress? Good or not so good?


      • Stephen, thank you very much for your interest.
        During the two years of weight loss, I removed sugar and grains, consumed almost nothing industrialized, many vegetables, fish, chicken, no fruits, only avocado and coconut, no vegetable olives, I make LOUCARB almost ketogenic.
        I have no problem with stress, sleep is not ideal but has improved with Melatonin.
        Even though I have been in a long plateau for more than a year, I am persistent, I believe that at some point my body will react.
        In that time I acquired a lot of health and disposition, I do not use any medication, I go up the 12 floors of the building where I work in the morning and afternoon, and I still go to the gym.
        I’ve read a lot about autophagy so I’ve intensified the fasts, but I’m afraid I’m doing something wrong.
        Sometimes I change my mind and I fast for 48 hours twice a week.
        If you can help me, thank you!
        A big Brazilian hug!

        • Steve Summers

          I would suggest you start to eat more as 1500 cals a day with an active lifestyle is far from ideal. There’s a strong possibilty you’re suffering from a broken metabolism. I would up my cals to 2000 a day minimum and stop the excessive 48hr fasts. You should start to feel more energetic and your sleep should improve.

          • Steve – I read her dinner to take place at 1500 not that her calories are 1500. She’ll need to clarify that.
            Ana – do you track how much of which kinds of foods that you do eat? Perhaps a different balance of those foods would be beneficial? You mention you are almost ketogenic, how are you not ketogenic? If you are close to being keotgenic but your balance is off that could be the plateau. Perhaps you are eating good healthy fats but too many then your body isn’t feeding from your body fat. You will still get all those health benefits you mentioned and will easily be able to fast or have one meal a day due to minimal hunger .

          • Just found this amazing website! I’ve been on LCHF with 18 hr IF for about 4 months, significant weight loss, but not really seen a significant change in hunger level at the 18 hr IF mark. Is my RMR suppose to slowly drop, and should I be seeing (or will see) a reduction in 18 hr IF hunger level going forward?

    • sten bjorsell

      Maybe too much carbs in your single meal stimulates your insulin to not drop. Do a fasting insulin test. Lower carbs and be careful with proteins, no more than 0.7 g protein per kg of lean weight, measure again after 3 months both weight, waist and fasting insulin. Read and look up what Ron Rosedale says about too much proteins. Best of Luck!

    • Oi Ana, your diet looks poor in zinc at least. It is possibly not rich enough in fat and collagen, which can be remedied with bone broth, beef and butter. You may also try to see what it is like to have two meals one day, and zero meals another day. It is also possible that you are losing fat but building muscle, and that results in zero weight loss. What you need to monitor is not your weight but your waist circumference.

    • You sound like you follow the Eat Stop Eat type of intermittent fasting.. Brad Pilon (;topic=627.0;attach=2555)

    • I don’t believe in ‘broken metabolism’. You are still living your life, you consume and burn energy everyday. Just something in your diet is out of control. You may need to track your calories more accurate (so easy to go wrong here). And for breaking your plateau you may try prolonged fasting for more then 10 days. As you have experience with 48 hours fasting you will not have any problem with that. Only your fear can stop you from doing it. But please don’t worry, it’s safe. You can google it and find out for yourself.

  2. Hey Jason

    Another great post, you seem to be making a habit of it 🙂

    Totally agree with all your points relating to intermittent fasting vs calorie restriction. The mechanisms and hormonal effects on the body are very different, although for many people, there is a crossover. They fast AND by default reduce energy intake. but of course, they then fall into that trap of reduced metabolic rate and all the nasties that go along with that.

    However, and i know that this is an approach you advocate anyway, food choices are (or should be) central to this lifestyle, whether fasting or not. I find it very sad, yet not surprising, that so many people are jumping onto this IF bandwagon, restricting calories significantly once or twice a week, eating all the stuff that has caused them to be overweight on the other days, and wonder why nothing much is happening. Same old story of wanting a quick fix and seeking the holy grail.

    I’m a massive supporter of fasting, I’ve written about it extensively, and still get over half of my website traffic to my fasting posts. I did a lot of work with people really looking to create awesome physiques, and, under those circumstances, and with those goals, and for short periods of time, monitoring food intake can be helpful, and even essential to meeting those sort of body goals.

    BUT, we’re talking about lifelong health, leanness and wellbeing here, and calorie counting and forced restriction has no place in that context. Food choices, traditional foods, fermented foods to improve the gut biome, ideally exercise to increase lean muscle mass and improve insulin sensitivity too, adequate sleep, rest and recovery, all these things play a huge role in setting the internal environment so weight loss and improved hormonal signalling is a given, not a goal to be strived for.

    And fasting plays a great role in that. But, in my view, if the other things aren’t in place too, fasting becomes no more than a fad for people to try to use to get a quick fix on their health and body composition, something that has likely taken them many years to mess up. And they expect fasting to fi it overnight.

    As I said, great post, keep up the good fight 🙂

    Steve Reed

    • Hi! I am a bit perplexed. When Dr Fung talks about caloric restriction, isn’t 16/8 IF that I do everyday, 7 days a week like if I would do a calorie restricted diet ? I don’t count calories at all, I am careful with the carbs, I don’t restrict the fats but I don’t fast for 24 hours some days, I fast everyday for 16 hours and eat in an 8 hour period. Since I do that (the same thing) everyday, do you think my metabolism rate will decrease?

      Thank you

  3. Dr Fung, thank you for all the information you provide. It has been very helpful to me. So far, IF and LCHF combined has been the only strategy that has worked for me. However, I have been doing a water fast now for 4 days and have not lost any weight. Do you have any suggestions as to what the cause of this might be.
    I usually lose about 6 lbs on a 5 day water fast. Thanks.

  4. Just to reassure you Birgit, it is impossible to not eat for four days and not lose weight because whilst you are not consuming calories but must burn calories to live you will be losing those calories and so losing weight. It may be that you are retaining more water and so the gain in body fluid is disguising the fat loss.

    • Thank you, Martin. I am on my last (7th) day of fasting this go round. I have now eliminated all artificial sweeteners and reduced my salt intake. As a result, I have lost 2 lbs.

      Since starting IF with LCHF, I have lost 50 lbs, but have plateaued for the last 3 months. I plan to continue IF by rotating 7 days of water fasting with 2 weeks of alternate day fasting and LCHF.

      • If you were having sweeteners during your fast, then it was not a water fast. Even that small addition can halt weight loss on the scale. Next time, try just water or coffee, tea, and a little broth. The difference is monumental.

  5. Rajesh Pankaj

    Hi Dr. Fung, This is Rajesh from India, a big fan of your work. I have T2D since 2013, getting treated with Metformin 500 mg XR twice after each meal. I also tried SU-JOK therapy for almost a year with no significant improvement. But with all the restrictions, this September I became depressed thinking that I never getting out, I never be the same as earlier. Then I came across Prof Roy Taylor diabetes research online which opens my eyes. I immediately started my dies limiting it to more or less 800 Cal, mostly comprises of green vegetables(boiled n raw). At that point I was 71 Kg (waist: 37 Hgt: 5′ 7″) and avg RBS was 225-250(On Metformin). Following it for almost 8 weeks, weight drops to 62, RBS: 140-180(I stopped taking Metformin when started the diet), waist (on Naval): 34, Hip: 35. Hunger is never away nor ever my BS came to normal on my Glucometer. And then I it the Weight-Loss Plateau. I started researching again and found Diet Doctor and your blog. I must say its like blessing from a higher power. Not only it educated me in whole new way and open my eye to the reality, it relieves me that I can fill my tummy now without thinking about FAT. I am following a mix of yours dietary advice since last week and started 24 HRS fast twice a week starting this week. The only real problem I am stuck to is what to eat. If I opt out wheat Rice Legumes, there is almost nothing left to eat except green vegetables. You might know that regular Veg Indian diet main consist of Chapati(wheat Flour), Daal(Legumes) and Chaval(Cooked Rice). Could please suggest me “how, what and how much” should I eat? Thank you once again for the great work you are doing.

    UPDATE: I have completed 2 36 hrs fast and on first “Warrior Style” fast n My RBS Drops to to an average of 135. Cant How much happy I am to see this result ever since became a victim of this disease. I do not have words to Thank you Dr. Fung. Please Keep educate us. God Bless you Sir.

    • I’ve met a number of vegetarians [including Indian vegetarians] who practice low carb and/or intermittent fasting. I would stick to as much avocados, coconut, nuts, vegetables, and legumes that you can and avoid rice and wheat. Also, the higher the carb content in the diet then the more there needs to be more emphasis on fasting, exercise, and proper sleep. good luck

      • Rajesh Pankaj

        Thanks a lot Henry. That’s what I am following except No Fruits and No Legumes. I am missing Bread and Rice. Thanks Henry I will experiment if adding legumes would be beneficial for me. God Bless!!!

    • Namaskar, Rajesh.

      May I suggest that you try to eliminate or at least reduce the wheat and rice. Eat channa and beans as well as nuts such as almonds, (not cashews and peanuts), and drink full fat milk and plain lassi for protein. Tofu is a good vegetarian food with high quality protein and very few carbs, I don’t know if it is available in India. Also, increase your fat intake with ghee and coconut oil.

      I have had to experiment for several months to discover what works for me. Everyone’s situation is different.

      I wish you good luck.

      May Bhagwan shower his choicest blessings on you.

      • Rajesh Pankaj

        Thanks Birgit. Those are exactly same except Tofu, milk and Lassi. Instead I have boiled and roasted Soya Chunks once in 2-3 days. If anyone could advice that can bread (Flatbread/Chapati of any flour)be added to food or should I forget it forever?

        • Rajesh,

          For now, avoid all grains. When your health has recovered, you might try to experiment with incorporating limited amounts. The Diet Doctor has a number of recipes for low carb Bread. I have not tried any, but the pictures look good.

          • Rajesh Pankaj

            Thanks for helping me Birgit,

            When one can say his health is recovered? My RBS dangle around 150 mg/dl(without any diabetic medicine). No visual FAT on body anymore. Any suggestion?

        • I am DT2 for past 15 years. Started the 24 hrs fasting with only non-caloric stuff, water, diet ice tea, tea etc. I am on Metformin 1000 twice a day and Junevia once a day. Both of them are extended release. My concern is that If I an not eating anything for 24 hours, taking medicine may cause my blood sugar to go very down.

          Can anybody please please answer one simple question? Should I take my daily medicine while fasting?

          • Danielle

            I am not a doctor and this is not medical advice, but you need to talk with your GP about lowering your Junevia. It is not a good idea to fast on it in general terms but depends on your dose and your blood sugar readings so you need to review it with your doctor who prescribed the medication. Metformin is usually the last drug to be eliminated once your hbA1c is <6.8 but again you need to discuss specifics with your doctor. You need to fast so you need to find a doctor who is supportive of that. If you cannot find a doctor to support you fully in this then you need to simply inform your doctor that you will be doing extended fasting and tell him it is his professional responsibility to review your medications accordingly. Fasting is your choice and your right. Medication is your choice. Fasting on medication is potentially dangerous. Eating is not the solution, reviewing medication dosage is. Best Wishes

  6. So are the results as good if you do a daily 16-hour fast, instead of a 24-hour fast 2 days a week?

    • I would also like to know this. Or an 18/6, which is what I do every day. FWIW, I am a 32 year old female, 5’6″ tall, 135 lbs.

      • The precise details of the benefits of relative fasting schedules don’t seem well-documented at all. Right now scientists are just measuring fasting versus continuous eating, not type I fasting versus type II fasting.

        IMO, just do whatever fits your schedule. I do personally assume that longer fasts yield additional benefits, but they’re heard to schedule.

        • I agree. we are all different, and summer me is different from winter me. In summer I can go 36, do not even notice it. In winter I get cranky at around 20 or even 16 hours, and sometimes I just do a fat fast, where I eat a couple tbsp of ghee or coconut oil at 10am and 2pm (going dinner to dinner).

    • I have been doing 16-18 hours daily fasting for almost 2 years now with GREAT results. I eat 2 times a day: i break my fast (after a fasted workout) around 2 PM and i eat dinner at 7-8 PM (and some fruit snacking in between) and then i fast again from 8 PM to 2 PM. I feel great, i have low bodyfat percentage and i am pretty athletic (due to strength training), no muscle loss, no hunger etc… so from my own experience i can surely tell that 16-18 hours daily fasting works great.

    • sten bjorsell

      It will take at least 12 hours “run in time” before insulin falls enough to get the liver to empty glucose constantly. Once it starts emptying it can go on for 2 days and more for those with an enlarged liver, which keeps insulin from dropping further and reduces or prevents fat burning. Hence longer periods are more effective.

      Blood sugar can even go up during 2-3 days fasting for those with high insulin resistance/enlarged liver, until the stored glycogen eventually runs out.

      Note, as pointed out before, diabetics-2 normalizes blood sugar through a GBP operation! But not through the operation itself, only through the compulsory 5 day fasting before, during and after the operation. Insulin and therefore blood sugar drops to normal from the long fast and medication can then be stopped in the majority of cases. To prevent from relapse with a SAD diet their stomachs are largely cut away.
      But fasting in same way preceded by high fat low carb can be done for 5 days without too much trouble, without being locked away in hospital! And when eating is resumed with same high fat low carb food after, also hunger can stay away.
      But if a SAD lifestyle with 3 snacks a day and its blood sugar driving foods resumed, GBP is an alternative, that lasts for at least a few years.

    • You have to try a fast and see how it effects you and gives you the results you want or not . If not Do a different length fast. What works for one doesnt work for another. I just did back to back 48 hour fasts, eating one day in between. Iam now doing 24 hour fasts everyday until I do another 48 hour fast next week. My fasting BS finally came down to 92/ I really want it to stay there or a bit below. This am after 1 LCHF meal yesterday it was 102. Its progress. Shorter fasts didnt work for me. I dont believe anything below 20 hours is much of a fast anyway. Ive read people doing 12 hour fast??? In other words eating all of their waking hours?? 6am to 6pm? Just my opinion. But you should do what works for you, if its not working, change it up.

  7. Is there a practical way yet to measure insulin instead of blood glucose as was done in the two studies you mention? I’d love to be able to see what was happening on the insulin front in my body .

  8. The two choices you mention at the end of your post should be viewed as (1) a financial model, and (2) an effective model for health. The success rate for (1) when viewed in terms of health is less than 1%. However, as a financial model involving permanent cyclic failure it is one of the biggest producers of all time. The food, drug, diet, exercise, and even the research industry have invested billions of dollars in (1) and it has paid off beyond anything anyone could possible imagine. I mention the research industry because the Woman’s Health Initiative involved at least $700M which was just the latest attempt at spending huge amounts of money and hoping for a different result.

    The present reality for anyone with a chronic disease, and an associated obesity problem is that the majority of the medical profession will not help you. In fact, why should they? It threatens the financial model for all the key players. In fact, the financial reality is that a drastic reduction in chronic disease resulting from changing dietary patterns would probably measurably subtract from the national GDP. It would necessitate a massive economic adjustment. Some investment bankers are starting to get worried, however. Credit Suisse –

    With the exception of a minority in the medical profession, you will not get any help, and you are really on your on when it comes to obesity and actually curing or preventing chronic disease. The prevailing system is designed to make you a permanent and dependable chronically diseased cash cow.

    • You are so right. We are on our own and under continouous attack from the system to return to be cash cow again. That’s not going to work anymore when we have all this information thanks to Fung and so many others.

  9. Thank you Dr Fung! You are my health hero! I am so thankful for your work! I need to remember that with fasting, even when I am not loosing weight I am still healing my body. I have lost 30lbs this year thanks to you. Its very slow going but its going and not coming back.

  10. Dear Dr. Fung
    Is body temperature a good measure of “starvation mode”?
    I have been doing very well on an LCHF diet for 9 months, the last 4 also with added intermittent fasting. I lost 15KG and lowered my HbA1c from 7 to 5.7.

    However, the last month or so the weightloss stopped and I even started gaining a wee bit.
    Reading through your blog and book about Compensation, I thought perhaps my metabolism was slowing down – so I measured my temperature. I was very surprised. it was 95.5F (35.3C). So I started measuring constantly and noticed that I ranged from 95.5 to 97.7F (35.3-97.7 C). I almost never got up to normal 98.6F (37C), but eating raises my temperature.

    So I am wondering, is this a sign of metabolic slowdown and compensation? I don’t know what my temperatures were before I started the diet.
    What would be the best way to “fix” this. Eating more fat on eating days, and lengthening the fast?
    You wrote that for people who messed up their metabolic rate “Even after six years, the metabolic rate does not recover”. What would be a good way to recover?

    Anyway, anything added about the issue of body temperature would be most welcome.

    • sten bjorsell

      One simple check is fasting blood sugar morning after eating days. If over 100 you are probably eating too much protein, that converts to blood sugar. Longer fasting times will also help.

      • Phillip Actor

        GNG is NOT at all a reason as has been amply proven in recent science. It is a demand driven feature. The only real reason to not outdo protein versus fat in LCHF is because protein tastes much better, but only brings half as many calories per gram. Zero carbers do excellent on 2 kg of meat per day for a reason and they tend to be in ketosis permanently. Not possible is protein converted to blood sugar by magic instead of GNG 🙂

  11. OMG: “success is as rare as humility in a grizzly bear.” You are one funny and creative dude, Jason.

  12. Thanks Dr Fung for your wonderful posts. Not only do I learn, I laugh out loud!

  13. I just returned home from surgery for a complete hysterectomy. I have been morbidly obese for about 25 years or so, early signs of diabetes, creeping A1c, thyroid meds, blood pressure. I realize that I have been insulin resistant for much longer than that. I am 64 and have been given all manner of advice on taking the weight off since I was 10.

    In May I stumbled upon this wonderful community of people who are passionate about health first – i.e. the patient, and then statistics and other factors later. I began to fast for 24 hours three days a week. About a month later, with more reading and study, I cut out processed carbohydrates. About a month or so later I began to add a lot more healthy fats in to our diet.

    I bought the Obesity Code right away and gobbled it up faster than a Keebler elf could sell you cases of cookies. Although there was a lot of great information, I needed more. I was averaging a weight loss of around 1.25 pounds a week. My BG had come down to just over the “normal” range (90 or lower).

    In the meantime I had to find a new physician and I asked her questions about the diabetes. She handed me a little book published by NovoDisc (suspicious right there) and told me to follow the little book and eat 6 meals a day whether I was hungry or not. I read through the little book and the average carb load for a day was around 250 gms!. Well, that confirmed that the little book was not going to be my friend.

    With a new doctor came the battery of “base-line” tests. I had been off all meds for over 3 years and didn’t feel bad. My lipid panel came back well within acceptable limits and this was 6 weeks into the newly discovered fasting protocol.

    I had never really stopped bleeding monthly and mentioned it and more tests ensued. I had a very large fibroid and grossly enlarged uterus. Biopsy and lab tests and off the GYN Oncologist. As soon as the oncologist was scheduled I had also completed The Complete Guide to Fasting and decided to try to be in the best possible place when I went in for my consultation so I began a 14 day fast. The doctor announced that there had been a cancellation for surgery and I was scheduled in within 36 hours. The surgery went fantastic, there was NO pain, there was no sign or marker of any cancer, and I don’t exhibit any of the post-surgical symptoms everyone had warned me about.

    Through a change in the venue of the hospital I missed any meals on the day of surgery (post-op) and didn’t have breakfast the next day. When the nursing staff realized that I had not eaten there was a mild sense of panic. So I did order lunch and a small dinner. When I didn’t order breakfast (the most important meal of the day) the kitchen sent up a “diabetic” plate – consisting of 1 white flour pancake, 1 small sausage patty, 8 oz 1% milk, fruit cup with syrup, sugar free syrup and 2 oz of something that was supposed to be like butter, and 8 oz black coffee complete with sugar substitute and chemical “creamer. The sausage was tasty and the coffee fine. The rest I left.

    It is my belief that my pro-active decisions that began with being introduced to Dr. Fung saved me much grief and that my recovery is so much faster and easier because my body wasn’t fighting all the nasty things we have come to know as “food”. It was curious about Dr. Fung’s observations about the way that stress manifests itself because when I entered the hospital on day 11 my FBG was around 88 and coming out of surgery 4 hours later it was in the high 120s. This with nothing to eat or drink for over 24 hours – and no food for 11 days. It has taken almost a full week for my BG to start to recede to pre-op levels. Just an observation about my own experience.

    Thank you, Dr Fung, and all of you who contribute so much to this endeavor of advocating for real living.

    • Fantastic! Check out the Fung Shweigh group on Facebook as a support tool.

    • Chris, the food given to diabetic patients must constitute cruel and unusual punishment. Complete madness.

      Well done on your choices and progress.

  14. Excellent read. Favorite laugh, “Goodbye reunion show.” Thanks for all you do.

  15. What is IER and CER? I spent 10 minutes looking for an explanation and ended up having to trust the implication of the text compared with the graph.

  16. I have been intermittent fasting (and a few extended fasts) for 6 months and 3 days. Since I am human, I have also fallen off the wagon now and then. This falling off the wagon now and then has actually been a good thing. It has taught me a lot. One thing that I have learned is that sugar, especially fructose, makes me hungrier and destroys my will power, both. First, fructose, even in the form of fruits like grapes, destroys my willpower, and I do not mean just about eating. I struggle and usually win with getting to bed on time. If I were to have say a handful of grapes at 9:00 P.M., I will not be able to tear myself away from the idiot panel (used to be the idiot box, same crappy content, different delivery system).

    The sugar based sweets also make me hungrier. If I want to skip dinner, I cannot eat any sugar based sweets. Stevia does not do the same thing. I have no problems with stevia, even though it is a refined natural sweetener.

  17. I am not sure I fully understand. Let’s say I follow a 23/1 fasting protocol everyday, but also restrict my calories to a 40%-50% deficit everyday. Also, my only meal of the day is very low carb, high fat (keto). Will I slow my metabolism down over time?

    • No, because any energy you need can come from stored body fats once you deplete your glucose as fuel, metabolism has no reason to slow down.

      • Thank you, that’s what I thought I had understood. I feel great and my energy level is very high!

        • Pete and Michele, it’s exactly my case I put my doubt in the first post. Thank you!

      • sten bjorsell

        Agree, it is perfect at least to start. If weight loss stalls after a while reduce proteins. Check out Jimmy Moore and Ron Rosedale for more about it.

    • I echo this question. Having read every word that Dr. Fung has written, I still don’t know the answer. I suspect that total calorie intake doesn’t matter, but I would still like a definitive answer from Dr. Fung on this topic.

    • Your metabolism will NOT slow down after a substantial calorie reduction unless your insulin level is also high. Insulin controls the release of fat. Fasting with HFLC combined with a narrow eating window ensures that insulin stays low so your body can use fat as it fallback energy source. When fat can’t be used, the body’s only other choice is to reduce your energy output by putting the brakes on your metabolism.

  18. So what I get out of this is that daily caloric restriction is CRAP. Is Caloric Restriction as Primary really the official term? Did the geniuses who came up with this bother to read their oh so clever acronym?

    Thanks Dr Fung, I learn a lot from you and the humour makes the points stick.

    • Part of Dr. Fung’s witty style.

      He (as far as I know) chose the acronym deliberately for comic relief reasons.

      And the alternative theory he proposed, Hormonal Obesity Theory is…yes, it’s HOT.

  19. As has been referred to by a handful of people who have responded to this post – there is a third choice one can consider, apart from CRaP and Fasting (intermittent or otherwise), and this is the Low Carb, High Fat (LCHF) diet a.k.a. the Ketogenic diet.

    I began with fasting myself, and found this to be very challenging. But then I was introduced to the LCHF by other Fasters in a chat room I joined, and I found that I could achieve the same results by pursuing a Ketogenic or LCHF diet than I could with fasting.

    I have been on a LCHF diet now for almost 6 months now, and am very, very happy with the results.

    I still believe that fasting is an important and effective tool, certainly one that a medical professional should have in their arsenal – but it is not the only one. Even though I would describe myself as a ‘Keto-er’, and have been in a state of nutritional ketosis now for over 4 months (as tested through a breath analyser called ‘Ketonix’), I still occasionally perform a 36 hour fast at least once a month, and fast 16:8 at least once a week. I do this just to ensure that I give myself a ‘leg-up’ with remaining in Ketosis, as fasting is one of the easiest ways to promote the production of Ketones.

    The reality is that fasting is not for everyone, or everyone is not able or willing to fast and so find it difficult to do. I for one, certainly find following a LCHF or Keto diet much easier to maintain.

  20. Dear Dr. Fung,

    firstly.. THANK YOU! I think I have managed to read most of your blogs, watched your various videos on you tube and your books on Obesity and fasting and I had a question.

    I have been T2D since 2003 on standard medications and injection till 2008 with by weight going up to 98 kg. With some half-assed reading, primarily CraP, my weight came down to 82 kg and only on metformin until Dec 2015. From Jan, I began reading again with renewed vigor on how to get my weight (fat%) further down. But still on CRaP, LC diet and Intermittent fasting. (had not yet read your work), my weight came down to 74 Kg and stopped all T2D medications, including my heart doc Statin and Aspirin in 6 months.

    Then I got hold of your work and everything you have said about not to do. So now I been been fasting regularly since Aug 2016 (24 hr to upto 3 days).

    My Question: Unless its there somewhere and I have missed it, my question is on how to get my metabolism back up (reduced due to years of CRaP). Will Fasting do the trick? If yes, what protocol? Is there something that can be done

    Thank you in advance

  21. Hi,
    My question about all this intermittent fasting is: Is IF beneficial if you are not doing the low carb diet as well? I eat gluten free, but I do eat fruit and sometimes small amounts of grains. About 100 grams of carbs a day. Would intermittent fasting benefit me, or would it just stress out my body since I am not keto adapted? Thanks for any insights!

    • Bre, I started by changing to a low carb, high fat diet and found that I was easily and naturally going 12 – 14 hours without eating because eating a higher fat diet satisfied my appetite. I later moved easily into fasting because it was easy to add another 2 – 5 hours on top. So, fasting benefits anyone, but a diet that doesn’t make you feel hungry obviously makes fasting a lot easier.

  22. Fasting seems to work no matter WHAT your starting point–I have a girlfriend who is hopelessly wed to the SAD diet, so I turned her onto intermittent fasting instead of the continued uphill battle to try to clean up her diet. In spite of eating SADly, she has lost weight, and her blood sugars have improved. I imagine the lack of stress from me trying to correct her diet also played a role.

    She’s continuing to lose weight and improve as we speak, despite all the junk foods she eats. GO FASTING!!!

  23. Dear Dr Jason Fung,
    I have tried fasting for 24 and 48 hours, both felt good and increased my energi levels. However, during periods when I practiced intermittance fasting, I felt changes in my bowel flora, and I developed both vaginosis and yeast infection. Now I am a bit hesitant of returning to my fasting regimen. I wonder: could my problems be due to too much coffee on an empty stomach while fasting? Have you experienced these sort of problems among your clients? I have eaten a low carb high fat diet for almost five years.
    Thank you for your advice in advance!

  24. thebigpicture

    I do in general agree with fasting and a ketogenic diet, but it’s not as easy as it might seem.
    I’m overweight but not obese, have twice tried fasting beyond 24 hours and failed. I made it to about 36 hours each time before giving in. It’s a combination of hunger, emptiness, and a strange feeling of being tired and alert at the same time. You literally can’t sleep, nor can you do anything else. Eat a meal, and I instantly feel full, satisfied, and ready to either work or sleep.
    As far as ketogenic diet, again, not so easy. Refined carbohydrates are everywhere, they are cheap, abundant, and provide instant energy. To avoid them while you are grocery shopping, much less eating out, you simultaneously have to have extreme mental fortitude, as well as appear uppity and insular in social situations. Just one thanksgiving or christmas can wipe out a year’s worth of work, much less any other routine occasion when people are eating together.

    Another question nobody has ever answered: why are some people obese and not others? There are people who never fast, eat whatever they want, and never become overweight. This might be an exaggeration, but not by much.

    I highly value this blog, but the point is to individualize and find what works best for you. We simply aren’t all going to become healthy, thin people with great bodies. We aren’t all going to be able to follow the same regimen.

    • I think a 36-hour fast is success. I don’t even try to get past 16 – 20 hours.

      You’re right that refined carbohydrates are everywhere, but I now find it easy to ignore them. It is shocking when you realise how much of what we eat is sugar-laden junk and it explains the obesity rate. Slipping up at Christmas might set you back a couple of days, but I can’t understand why you say a year?

      Some people can get away with eating badly for a while, but it’s probably not doing them any good in terms of visceral fat and insulin. Very few people eat badly and don’t show the results eventually.

    • George Thomas

      It’s not that hard to dodge refine carbs. Sounds like you’re just making excuses.

  25. Hi Dr. Ginger

    I have been motivated by your articles to do fasting! But in my first attempt I fast with only water for 40-42hrs. I wasn’t feeling too weak but may be tired walking around. However (no) thanks to a diabetes exhibition in the mall I ended up checking my glucose level out of curiosity and it was 3.9. The sales lady who was selling the kits was somewhat surprised and asked me go eat something! With that remark it suddenly seemed I was too tired, weak and I broke the fast right away with bread. Then I felt I should have carried on few more hours perhaps taking rest not walking the malls.

    Question is should I just let my body tell me when to stop or was my glucose so low that it would have made sense to stop? I am not known diabetic, 50 yr old male Indian. Not obese, 65kg at 5’6″.

    Look forward to your (and others) comments! Thanks.

    • sten bjorsell

      Next time drink more water, it helps. If you eat less carbs and more fat for a few days before you start next fast you may not feel weak after 40 hours time as you have already adapted the body to burn more fat than usual. Fasting gradually becomes “pure” fat burning, once (diabetic) stores of glycogen and liver fats and ARE GONE AND proteins scavenged by autophagy has been used up. With LCHF as starting point you really get a flying fast start! Same fuel (fats) but from different sources.
      Wish you the best of Luck!

    • Christine S.

      Raj, normal fasting glucose levels are between 4.0 and 6.0. And glucose meters may not be that accurate – Canada follows the ISO 15197:2013 standard, which says that meter readings need to be within plus or minus 15% of actual! Other countries may follow different standards. I would say your blood glucose reading was just fine in that situation. A blood test in the lab is the only accurate measurement.

      I have Type 2 diabetes, diagnosed 12 years ago, and test multiple times a day, but I tend to go with how I feel more than what the meter says. Before I tried fasting for the first time almost 2 months ago, I was already eating on the lower carb side (less than 15%). Even so, my first fasts were less than 30 hours because I felt tired and hungry. For me it was my body adjusting to a major change in what was “normal” for it.

      I have tried a variety of fasting methods … fasting every other day (36 hour fast) for 3 days a week seemed to work, but was impractical. So, last week I did a water fast for 3 full days without any problem (84 hours, from dinner on day0 to breakfast on day4) – probably could have gone another day but I was bored and felt like eating. I had a few times when I felt a bit hungry, but I was able to ignore it. I had no drop in energy level over the fast. And the longer fasting period seemed to have a more beneficial effect on my glucose levels.

      Outside of fasting, I usually eat 3 meals a day (lower carb) with no snacking in between, and no eating after 7 pm. Sometimes only 2 meals. I have lost 16 pounds in 7 weeks, mostly fat from around the middle. My waist circumference has decreased by just over 2 inches. My fasting glucose readings are slowly coming down. I still take metformin, but I stopped doing the evening injection of long-acting insulin. I have about 25 or so more pounds to lose, but my primary goal is to get the fat out of my liver (found last summer during an abdominal ultrasound done for another reason) and to get my blood glucose into a normal range.

      Note however, we all have different health status and circumstances, and your experience may vary. My impression from reading the variety of comments to Dr. Fung’s posts is that if you are a vegetarian, or normally eat a higher carb diet, it could take you longer to adjust to fasting.

      • Just discovered there is a study result posted as table 3 in Dr. Jason article of 1 May 2015 (Fasting 4). The subjects’ average glucose on day 2 exactly same as what I observed in my case!

        That’s reassuring.

  26. Sorry the name got auto corrected, I didn’t notice!

  27. Thanks Sten, Christine!
    Yes my main concern was if I had not eaten the sugar level was low enough to cause some serious trouble such as fainting. I think I can stretch myself bit more next time. I am not vegetarian but not much of fats in my diet right now. I’ll keep your comments in mind. 84 hours seems like long long time impressed you pulled it off!

    Good luck with your efforts

  28. Thank you very much for this article, it was a very interesting read. I visit this website on a regular basis to learn more about fasting and other topics concerning metabolism.
    I’m an intermittent faster (/low carb) from Germany and I’ve got a question. When people fast for 3 or 4 days a week, usually the weekly average intake of calories will be decreased automatically. Would it be better if one tried to avoid this to happen and increase the daily calorie intake to even out the zero-calorie days? Or is there any calorie deficit one shouldn’t surpass?
    Right now I’m fasting 3 or 4 days a week (often 2 days in a row, so not quite ADF) and I’ve been doing strength training for about a year now. On feeding days I usually eat about 2000-2500 calories because I need to lose about 5 kilograms (thanks to my vacation). I can’t find a well justified answer whether I should increase my intake or not. I don’t really care about building muscle mass right now, but more about upping my body fat percentage by losing some fat mass.
    An answer would be very much appreciated. (and sorry for any grammar and spelling mistakes)

    • Weight loss will occur when the body is faced with a calorie deficit AND low insulin level. Low insulin level allows your body to use stored fat to supply the energy difference. If you body requires 2,000 calories each day, and you reduced consumption to 1,500, the remaining 500 calories would be supplied from fat.

      As far as exercise is concerned, I would suggest that you consider the resulting calorie burn as a weight loss bonus. By not eating replacement calories you also reduce muscle glycogen levels sooner and get into fat burning sooner. Exercise is best used to retain muscle mass. Compared to fasting, it is a very ineffective method for losing weight.

      • Thank you very much for the reply. Then I will continue with my fasting schedule. I just wondered whether my deficit was too large. I feel good and I love to fast on consecutive days. I’ve been doing this for some months now (more for health reasons than weight loss) and I didn’t want to change to some other schedule.

  29. I need to correct something there: of course I want to lower my body fat percentage… Sorry about that.

  30. Elizabeth Fen

    I am T2D for the past 7 years. As Dr Fung had mentioned my medication had only gone up.I started with metformin500and750 ,now it’s 850 twice a day.i have not been regular about exercise or dieting. My weight 70kg went down to 62 kg and remains 65 kg ever since I becameT2D. I am 5’6 and doctors say my weight is o.k. I have started the intermittent fasting with 14 hours just to get my body used to the idea. Is it necessary to go off carbs totally. During my meals I consume some carb.what could be the best fast plan for me .i would like to know the location of Dr Fung s clinic too.

  31. Hi dr Fung!
    I am on my third week of eat-stop-eat protocol (dinner-dinner ), and I feel increased hunger and fatique the day after fasting. I even experience some carb cravings ( LCHF for 8 months, without cravings) Funny thing that during fasts everything is just fine. But the next day I have zero energy…
    Can it be caused by depleted liver glycogen? I rarely eat more than 80gr total carbs per day, usually even less. At the moment this is my only theory…

    • @Tamara, I have exactly the same results you describe, and this after multiple extended fasts, my feet (especially) and hands are constantly cold. I find I have to wear wool socks at night. For instance today is my third straight day of not eating…at all.

      Distilling, from all of Dr Fung’s blogs / Youtube vids and his two book, there are carbs and then there are refined carbs. While 80g of carbs/day is nowhere near ketogenic if it is comprised of salads and not potato chips that should suffice. There are those on here that believe the ‘true way’ is, essentially, the Induction phase of the Atkins diet, under 10g carbs/day. For myself, I try very hard to stick to, what used to be called, complex carbs. Not speaking for Dr Fung, just my experience over the last 7 months.

      • I no longer aim for ketosis, I stick to LCHF because it is so great not to be tempted by sweets and junk food. Its like my body works the way its supposed to.
        But the after-fast fatigue is bad. Another week past by, and its still bad….

  32. Absolutely epic post Jason! My clients ask me all the time about “stavation mode”. I’ll point them here.

    Btw, the “Zauder” 2000 study on your “Metabolic changes over 4 days of fasting” image should read “Zauner” (with an N) unless I’m mistaken.

    Cheers and keep up the good work!


  33. Even more interesting is the effect of prolong fasting with correct refeeding.

    In this interview Dr Valter Longo compares the difference between chronic calorie retriction vs short term fasting/refeeding…see 1:10:00


    Dr Fung, you should contact the biggest looser and put your plan against there’s. So that America can see which is the better way. Of course it would probably ruin their show…

  35. I really missed this week’s blog . I also miss his comments.

  36. Ketogenic diet

    Really good article. Thx for the same. My Qs is, during fasting, the uric acid level is increased and body will be getting into ketosis mode. Can the person who is having high uric acid level, follow the intermittent fasting?

    In intermittent fasting, which window will give best result like 16:8, 20:4 or 23:1…

    Please advise and thanks in advance

  37. First, lest I forget, you probably need a copy editor. Few, if any, word processors or text editors can deduce language structure (verse correct spelling)

    I have two questions.

    Dr Joseph Kraft showed that type 2 diabetes could actually be determined upwards of a decade before any indication showed up in a routine fbg test by using a glucose tolerance test instead. This effect was even put to graph in earlier blog posts where fasting blood glucose is relatively flat until BAM, it goes vertical years into the chart. This is precisely what Dr Kraft discusses. If people took action at the first sign of drifting into the second or third category of the glucose tolerance test, they may avoid diabetes altogether.

    My question, in this vein, is this. Where fasting’s main deliverable is to break insulin resistance, will people who start fasting, per your recommendations, ever get back to the category 1 results on a glucose tolerance test? There is a subtleness in this question. People who read your blog and buy your books know how to lower their A1C simply by, well, following your advice, little to no refined carbs and alternate daily fasting. However, this metric is different, isn’t it?, from the body’s ability to metabolize a fixed amount of liquid glucose in a set time.

    My second question has to do with broken metabolism. You speak often on the ills of CRaP and the resulting Biggest Loser years later results, vis-a-vis RMR. You have stated here and elsewhere that fasting increases one’s metabolism via adrenaline and HGH. My question is, is that rise at day 5 (protein conservation phase), permanent or temporary? Can fasting cure a ‘broken’ metabolism?

    Tying those two questions together, when my doctor saw I took my A1C from 8.5 to 5.8 he congratulated me on successfully managing my T2B with diet and exercise. He had neither read your books nor ready Dr Roy Taylor’s study results. By US nomenclature my A1C has been drifted from the 5.8 a year ago to 5.5 today as depicted in quarterly tests. Should one expect maintaining those numbers to become progressively harder over time, as in ‘chronic and progressive’ which doctors believe T2D to be.


    • I can’t answer for fung, but I was diagnosed with T2D two years ago. The day it happened I was admitted to an emergency room with a fasting glucose of 200+ the morning of my admission. Sadly, I can’t remember my a1c at admission, but it can’t possibly have been good.

      I’ll spare you the long version of what I’ve been up to over the past two years, but suffice to say, my last a1c test was 5.1. Over the past two months I have never measured a fasting glucose above 94 and the single highest blood glucose I’ve managed to catch following a meal that I remembered to check was 104 pp at 1hr. It’s possible I’ve missed abnormal morning bg readings or pp readings, but the a1c would suggest I probably haven’t.

      I started on metformin two years ago, had it increased and invokana added and then had the metformin removed, the invokana halved and now I’ve been on absolutely no drugs for 7 months. the few times I’ve decided to test my glucose response recently have been completely normal as far as I can tell.(Thanksgiving 2 slices of pie plus two cups of sorbet consumed 1hr after main meal both 1hr and 2hr pps following that dessert where below 100 although hand devices do have an error of 15%).

      I haven’t actually done a proper blood glucose challenge of the 75g pure glucose fluid variety yet, but I plan to do so within the next year once I’ve managed to reach a normal body fat% and hold it for a few months. Given my progress so far, I think it’s fair to say that, at least for me, T2D is probably not progressive and may not even be chronic. Medically speaking you ‘reverse’ T2D rather than ‘cure’ it, but if I have an a1c that’s better than most Americans, t2d or not, my fasting bg is fine and I’m dropping weight at a rate that compares with rates seen by gastric bypass patients, it’s probably a decent indicator that the disease doesn’t have to be progressive or irreversible.

      I dunno if any of that is helpful for you, but I hope it is. Also, for what it’s worth, you may want to research the ‘carbohydrate intolerance’ hypothesis and the ‘alternative hypothesis’ for obesity. Those ideas tie in very closely with some of the stuff at the forefront of suspected causes of diabetes and if they’re correct, they provide very direct courses of action for reversing T2D by addressing the root causes of the disorder. My own experiments on my own body have lead me to believe very strongly that ‘carbohydrate intolerance’ is real keeping it in mind can give you the power to take back control of your body’s metabolism and health.

      For the record, I started this whole ordeal at 465 pounds and am now around 280 in just under two years time.(340 to 280 over the past 5 months following more aggressive carbohydrate restriction) I’m considering a ‘normal’ body fat% to be 25% as I’m a 29 year old male, my current number is somewhere between 30 and 32%.

  38. […] this blog post Dr. Fung points out that meal timing is more important than the average calories […]

  39. […] and even lose weight. (Here is a quick breakdown article on the benfits of fasting. Here is an awesome article by Dr. Jason Fung on the difference between fasting and calorie […]

  40. Thanks for the great post Dr Fung. One question I have is in regard to the increased metabolic rate that fasting produces. Have any studies been done to see how long the body maintains this increase? Reduced calorie diets decrease metabolic rate even up to six years. But I have seen nothing about how long the increased metabolic rate continues. Thanks!

  41. Hello Dr Fung,

    I have a question about caloric intake on Intermittent fasting vs fasting.

    Here is my understanding so far: When fasting for 36, 48 hours, or however long, one would abstain from all caloric intake during that period. The body does much to maintain itself by burning fat for energy, producing growth hormone, etc. When breaking the fast, one would resume regular caloric intake enough to satisfy for one’s daily macros, rather than enough to compensate for the lack of caloric intake during the fast. So over a week during which one would fast, one’s weekly caloric intake would be reduced significantly compared to a week without fasting due to missing the caloric intake during that fast.

    In the advice I’ve come across regarding daily intermittent fasting, say the 16:8/20:4 or whatever daily fast ratio, I’ve read that during the feeding window, one must eat all the calories necessary to satisfy one’s daily calories/macros within that window. This has been the recommendation for those on a ketogenic diet as well.

    In regards to maintaining a ketogenic diet with the goal cutting body fat percentage, I’m having trouble understanding why the two fasting situations require different levels of caloric intake. In ketosis, just as in a fasting state, whether for 16 or 48 hours, the body is burning fat for energy. If one’s goal is to lose body fat, why is it necessary to eat all of one’s macros within the eating window for intermittent fasting, but not necessary to eat the calories missed after a longer fast? Is that recommendation misguided?

    Thanks a lot for anything that could help clear my confusion.

    • MachineGhost

      The trick is to incur negative energy expenditure without decreasing TEE that would negate it. So if your goal is to lose body fat and not maintain or gain, you could still use a calorie deficit when refeeding. Since the body cannot access more than around 31.4 +- 3 calories per pound of fat per day under non-fasting SAD conditions, I would target a deficit no larger so TEE won’t be decreased.

  42. Jan Hansen


    I’ve followed your advice, and my bloodsucker has improved, but ! I’ve been intermittent fasting i 4 weeks, and I’ve cut out all carbs and sugar, but it seems like my sugar storage is unlimited? How long does it take to get rid of it? I’m not taking medication, but I do take Gymnema, Chromium and Cinnamon that is working to a certain extend.

    Thank you for all your information/education, is it desperately needed !
    I’m Danish, and you won’t believe what the Danish diabetes association is recommending to eat, it’s sick ..

  43. Intermittent fasting was exactly what made a world of difference for me and my health quest. Short story:
    – Diagnosed with diabetes in 2013, weighing 240 pounds, A1C at 9%.
    – By adjusting a diet, replacing sugary and starchy food lost about 24-25 pounds over the course of 2 years.
    – Discovered Intermittent Fasting in the spring of 2016; lost 50 pounds over the next 7 months. Sitting at 165 pounds now, A1C dropping steadily and is under 6 now. Off hypertension medication as it’s not needed anymore; Metformin went down from max dose to a min (500mg a day), and most days not taking even that now.

    I wish I’d found this approach earlier. Would make my life easier sooner. Not complaining though! 🙂

  44. Dr. Fung, I can’t thank you enough for the wealth of information on your website. It has been lifesaving! I am a kidney transplant patient (9 years now!) who has been struggling with NODAT T2D. Prednisine really kicks my ass and over the years my total daily insulin dose (basal + rapid) has increased to 108 units. I have been on LCHF + IF for 3 months (23 hrs fasting with a 1-hr feeding window). Lost a lot of weight and waist girth and got my insulin down to 40 IU. Then I went on my first 5-day fast. Totally discontinued insulin. Still got daily prednisone BG spikes despite no food at all but they got better. Broke my fast after 5-days and found that on my referding day a split dose of 10 IU of Levemir was sufficient to keep my prednisone BG spikes under 7.0 mmol/L. Wow! What’s not to love! Many thanks!!!

  45. Dr. Fung, I have been on the LCHC Diet with the intermittent fasting for 15 days. I lost 5 pounds so far but am having problems with my bowel movements. I have relaxall every day morning and evening to regulate me. I fast 16 hour fasts and 24 hour fasts. Not sure if I am doing them correctly. I have the bullet coffee in the morning with cream, butter and coconut oil. Then drink green tea in the morning until about lunch time. I have broth after that two or three times until the evening. I have a low carb high fat dinner.
    Can please you please tell me if this is the way it should be done. I have not tried 36 hours yet but am thinking about trying. I have a lot of weight to lose (60 pounds).
    Any advice that you can give me would be helpful.

  46. Hari Ganesan

    Hi doc ,
    Have been seeing all your videos through facebook and whatever resource i have. I believe whatever you had said made a lot of sense to me . But i have one big question . i could not get your email id to ask this question . So kindly take time to answer this, i would be very happy to see a reply from you even though i understand your have a very tight schedule.

    If We Fast for longer periods one of my doctors here who is an gastroentrologist suggests that i would develop gastricitis and duodenogastric reflux. Please shed some light over this . Thank you in advance and am waiting for your reply to start with my diet regimen.

  47. I hope someone can help me/ clear something up. Apologies if it has already been answered and would appreciate a point in the right direction.

    I am a 34 Y/O male, 6 ft and weigh about 189lbs with roughly 18% body fat. Im trying to loose BF and eating a LCHF/Keto diet, Intermittent fast Every Day, weight train in gym in the morning 3 times p/w, but am struggling to lose any weight. In fact i have noticed a slight increase in weight over the past month 🙁

    I am not sure what is happening!? I was a victim of calorie restriction in the past but am trying hard to not reduce calories past my BMR. My workouts are intense and i do them at 5am and my first meal is at 11am so i am doing a 16/8 I.F. I track my macros via My Fit Pal so i know i am staying in range of my total calories. The 16/8 fits in well with my lifestyle/commitments.

    I cant for the life of me work out why i am not dropping any weight and I would really appreciate someones point of view on this.

    Thanks in advance!

  48. Hi!

    I’ve been following this blog and the YouTube videos. I started with fasting blood sugars of 225 and am now regularly around 85 mg/dL.

    Last week I tried going from alternate day fasting to a 3 day fast. My blood pressure went up and my sugar on day two was 63. Is this common? What could be causing that? I meet with my doctor to get my A1C re-checked this week. It would be great to have any insight on this issue.

    Thank you!

  49. David Nyman

    OK, I promise you I’m not trolling just confused. I’m a successful IF-er myself, so I’m a supporter not an opponent. But I don’t understand the point about BMR. Of course, if body mass falls, so does the requirement for energy input to maintain body mass, other things being equal. And I fully accept that eating frequently as opposed to IF produces very different hormonal outcomes (and beneficial in the case of IF, else I wouldn’t be doing it). But IF increases BMR? What are the measures? In my case, a lifetime’s exercise habit has left me with a resting heart rate of 40, typically around 45-48 when seated during the daytime. Is this evidence of a “raised” BMR? And yet I have no problem maintaining body weight and composition and my energy levels are high. On the other hand, on the few occasions over the years *while still maintaining compressed eating* my average calorie intake has crept up, on has gone the weight. Reduce the calories, off it came and has been maintained at that level for years at a stretch. Just sayin….

    • Rustybeth

      David Nyman: BMR is affected by your body composition, and it takes more energy to maintain muscle than it does to maintain fat. Your BMR is made up of many factors, the biggest of which is your brain’s operation, then you have body functions, and then you have cell maintenance. So the loss of fat as body mass falls is not going to significantly impact your BMR to the detriment of your metabolic rate. The expression of growth hormone during the fasting state will stimulate muscle activity and even growth (by harvesting amino acids from the metabolized fat stores), and thus you will keep functioning at a high enough level to keep you going through the proverbial neolithic winter so that you can survive until food is abundant again. Some people, probably you, don’t have the metabolic slowdown and hormone complexities that cause the immediate weight issues from CRaP diets. You also seem to have a well balanced regulatory system for keeping your weight stable. I commend you on having great genes.

  50. Hey!

    First off, I love your material! I have read the latest book, and am catching up on your blog posts now.

    I have a question for you, what do you think of the latest news that compares IF to restriction and has data that contradicts itself, and this post:

    Your data makes more sense, but wanted to see what you think.

  51. Yeah, calorie restriction is bogus. It’s failure rate is nearly 100%! And we’re to believe it’s only because “fat people aren’t trying”. I totally relate to always being hungry on a calorie-restriction diet. I fasted for a few days and since then, I almost never have hunger pangs no matter how little I eat. I’m excited to fast again and set up a regular routine of a couple days of fasting per week. Most importantly, hoping to see plenty of weight loss!

  52. Tom Nguyen

    The “CRaP” strategy! How befitting.

  53. “What happens to basal metabolism? that’s what determines long term success. If you look at the change in Resting Metabolic Rate (RMR). Using CRaP, basal metabolism dropped by 76 calories per day. Using fasting, it only dropped 29 calories per day (which is not statistically significant compared to baseline).”

    I just read your book on fasting, and there you claim that basal metabolism INCREASES with fasting. Would like clarification on this.

  54. Hi Dr. Fung,
    I was just looking at the Catenacci et al. – the study that ADF to CR. I was wondering whether the statistically significant differences in baseline body weight between the two groups (114.0 kg for CR, 94.7 kg for ADF), total fat mass (48.8 kg for CR, 38.5 kg for ADF) and trunk fat mass (26 kg for CR and 20.9 for ADF) might have influenced the outcome, particularly regarding the type of weight regained. Also wondering if the fact that the overall daily caloric daily intake was substantially and significantly lower for the ADF group compared to the CR group might influence the results? In the end, is this the best comparison of ADF and CR?

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