Fasting and Exercise – Fasting 23

posted in: Exercise, Fasting | 56

Is it possible to exercise while fasting? This is a common question we hear all the time. People think that food gives them energy and therefore it will be difficult to fast and exercise at the same time. Some people with physically demanding jobs feel that they could not fast and work properly. What’s the truth?

Well, let’s think about this logically for a second. When you eat, insulin goes up telling your body to use some of that food energy immediately. The remainder is stored as sugar (glycogen in the liver). Once the glycogen stores are full, then the liver manufactures fat (DeNovo Lipogenesis). Dietary protein is broken down into component amino acids. Some is used to repair proteins but excess amino acids are turned to glucose. Dietary fat is absorbed directly by the intestines. It doesn’t undergo any further transformation and is stored as fat.

Insulin’s main action is to inhibit lipolysis. This means that it blocks fat burning. The incoming flood of glucose from food is sent to the rest of the body to be used as energy.Macro oxidation

So what happens during a fast? Well, it’s just the process in reverse. First, your body burns the stored sugar, then it burns the stored fat. In essence, during feeding you burn food energy. During fasting, you burn energy from your stored food (sugar and fat).

Note that the amount of energy that is used by and available to your body stays the same. The basal metabolic rate stays the same. This is the basic energy used for vital organs, breathing, heart function etc. Eating does not increase basal metabolism except for the small amount used to digest food itself (the thermic effect of food).glycogen-polysccharide

If you exercise while fasting, the body will start by burning sugar. Glycogen is a molecule composed of many sugars all put together. When it comes time to use it for energy, the liver simply starts breaking all the chains to release the individual sugar molecules that can now be used for energy.

As mentioned before, short term storage of food energy (glycogen) is like a refrigerator. The food energy goes in and out easily, but there is limited storage. Long term storage (fat) is like a freezer. Food is harder to get to, but you can store much more of it. If you eat 3 times a day, it’s like you go shopping for food 3 times a day and any leftovers get stored in the fridge. If there is too much for the fridge, it goes into the freezer.

Julie Moss 1982 Iron Man Triathlon

So what happens during fasting and exercise? Well, the body simply pulls energy out of the ‘fridge’. Since you have enough glycogen stored up to last over 24 hours on a regular day, you would need to do some serious exercise for a long time before you could exhaust those stores.

Endurance athletes occasionally do hit this ‘wall‘, where glycogen stores run out. Perhaps there is no more indelible image of hitting the wall as the 1982 Ironman Triathlon where American competitor Julie Moss crawled to the finish line, unable to even stand. Athletes also term complete exhaustion of short term energy stores ‘bonking’. I know some of you may think ‘bonking’ refers to other activities done on all fours, but this is a nutritional blog!

So, how do you get around that? Glycogen stores are not enough to power you through the entire IronMan race. However, you know at the same time, that you are still carrying vast amounts of energy in the form of fat. All that energy is stored away and not accessible during exercise. But the only reason it cannot be used is because your body is not adapted to burn fat.

By following a very low carbohydrate diet, or ketogenic diet, you can train your body to burn fat. Similarly, by exercising in the fasted state, you can train your muscles to burn fat. Now, instead of relying on limited by easily accessible glycogen during competition, you are powered by an almost unlimited energy drawn directly from your fat stores.

Studies are starting to demonstrate the benefits of such training. For example, this study looked at muscle fibres both before and after training in the fasted state. This means that you fast for a certain period of time, usually around 24 hours and then do your endurance or other training. The combination of low insulin and high adrenalin levels created by the fasted state stimulates adipose tissue lipolysis (breakdown of fat) and peripheral fat oxidation (burning of fat for energy).  Other studies had already shown that breakdown of intramyocellular lipids (IMCL – fat inside the muscle) is increased by training in the fasted state. Six weeks of training in the fasted state also induced a greater increase of fatty acid binding protein and uncoupling-protein-3 content in muscle.  TrainFasted

What does this mean in plain English? It means that our bodies have the wonderful ability to adapt to what’s available. When we fast, we deplete much of the stored sugar (glycogen). Our muscles then become much more efficient at using fat for energy. This happens because muscle ‘learns’ how to use the fat as energy by increasing the amount of proteins that metabolize that fat. In other words, our muscles learn to burn fat, not sugar.

Looking at muscle cells before and after exercise in the fasted state, you can see that there are more muscle bundles, but also that there is a deeper shade of red, indicating more available fat for energy.

Legendary exercise physiologist and physician Tim Noakes of Cape Town, South Africa has led the way in understanding the benefits of low carbohydrate diets for elite level athletes. Many national level teams (such as the Australian cricket team) are now applying these lessons to crush their competition. Legendary NBA players such as LeBron James, Kobe Bryant and Carmelo Anthony are turning to low carbohydrate, high fat diets to slim down and prolong their careers. lebron-skinny

You can be damn certain that these elite level athletes would not be doing this Low Carb mumbo jumbo and training in the fasted state malarky if it had any detrimental effect on their athletic performance. Quite the contrary. Hall of Fame NBA player Steve Nash does not eat simple carbs at any cost. Drinking sugary Gatorade? Not bloody likely to help.

Another study looked at the effects of a 3.5 day fast on all different measures of athletic performance. They measured strength, anaerobic capacity and aerobic endurance. All of these measures did not decrease during the fasting period.

The body simply switches from burning sugar to burning fat. But, for endurance athletes, the increase in available energy is a significant advantage, since you can store infinitely more energy in the form of fat rather than sugar. If you are running ultra marathons, being able to utilize your almost unlimited fat energy instead of highly limited glycogen energy will mean that you won’t ‘bonk’ and might just win you that race.

During the period where you are adjusting to this change, you will likely notice a decrease in performance. This lasts approximately 2 weeks. As you deplete the body of sugar, your muscles need time to adapt to using fat for energy. Your energy, your muscle strength and overall capacity will go down, but they will recover. So, LCHF diets, ketogenic diets and training in the fasted state may all have benefits in training your muscles to burn fat, but they do require some time to adapt.FuelTanker

Consider an analogy. Imagine that our bodies are fuel tankers. We drive these large tankers around, but only have a limited amount of gas in the gas tank. After the gas tank runs out, we are stuck on the side of the road calling for help. But wait, you might say. That’s ironic. You are carrying an entire tank of gas, but ran out of gas. How is that so? Well, that gas is not accessible.

In the same manner, we carry around huge stores of energy as fat. But our muscles are trained to run on sugar, and they run out of energy, so we need to continually refuel despite the large tank of fuel stored as fat.

So, what’s my best advice on physical exertion and fasting? Don’t worry about it. Do everything you normally do during fasting. If you normally exercise, or even if you don’t, you can still do it during fasting. Whether you fast for 24 hours or 24 days, you can still exercise. Your muscles may take up to 2 weeks to become fat adapted, though. During the first 2 weeks of fasting, you may need to take it a little easy, but you should quickly recover after that.

56 Responses

  1. Charlie Chambers

    Is this in reference to Intermittent Fasting or water fasting >24hrs.

    • See parts 4 and 5 of this series, which go into more detail about what happens on longer fasts.

      • Hi can u give me some advice when I do fasted training I feel really exhausted and when I break my fast I feel even more tired why is this?i normaly break my fast with oatmeal/bananas or Greek yoghurt I normaly am fasted while doing hiit cardio training

  2. Absolutely correct. I run 50K races (31 miles) in 5-6 hours with no food whatsoever and maybe a couple of nuts for breakfast. I have plenty of energy, especially in the 2nd half of the race. This became possible only after I switched my diet to low-carb….

  3. Greetings Dr. Fung…thanks again for another informative post. My question is related to protein and strength training during a prolonged fast. If we are not eating for days or weeks and doing weight training or HIIT do we get protein, for muscle repair, from the autophagy process? In a previous article you mentioned that protein is recycled, I believe. You have also previously cited studies showing little or no muscle loss. Also how about BCAA’s use during a prolonged fast for muscle rentention while strength training and fasting?

    Again, many thanks to you and Megan for your great work!

  4. I have exercised many times while fasting up to 5.5 days. I have not experienced any detriments I can see. However, exercising will make me hungrier (and that’s true while fasting or not).

    Steve, I personally wouldn’t try BCAAs, only because they’ll make it difficult to fast. I tried taking potato starch (relatively low carb) while fasting for one 3 day fast, and only I think once a day. The potato starch made me ravenous and it was through pure willpower that I was able to continue to fast. I’ve gone back to only having coffee, tea, “fake” teas, water and bone broth while fasting. And for bone broth, I have it only at night, and I still think it can trigger hunger. However, since I’m going to bed pretty much after I drink it, it’s not too bad.

    For me, fasting is primarily mental, although there is some physical hunger at times too. If I mentally decide to eat, I’m done. The fewer calories I can consume while fasting, the better I am usually. (Though I had a horrible 5 day fast after the first of the year — I was physically quite hungry on the fourth day — before having bone broth — to the point where I considered quitting then. I made it one more day though, through sheer will power.)

  5. Dr Fung
    Thank you for a very informative post, but you keep mentioning that fasting triggers an increase in adrenaline. Well, in the study that you first mentioned in part 4 and table 3 from that study, adrenaline (which is also known as epinephrine) if anything slightly decreases whereas NORadrenaline (which is also known as norepinephrine) was demonstrated to skyrocket. Now these two molecules have slightly different physiological effects in terms of blood pressure and metabolic rate but both increase glucose in the blood. Would you like to clarify this issue?

    • I had the same observation as well. A more precise way of wording that may have been to say there is an increase in the measured Adrenergic response during fasting. See

    • Han,
      These both hormones called Catecholamines, sure rise when fasting, and your question about blood glucose raising is because they increase the process of turning fat to glucose called gluconeogenesis, so this is a positive effect, meaning that you´re burning fat for converting it to sugar.

      • Sergio
        Gluconeogenesis does not make glucose from fat, it makes it from amino acids, so if you are burning fat you cannot make glucose, and yes, they are both catecholamines but Dr Fung quoted a study in part 4 (link above) and illustrated in table 3 that adrenaline is increased. If you examine the table and read the study, you will realize that adrenaline is slightly decreased and noradrenaline is the one to skyrocket. My comment was about people confusing adrenaline (aka epinephrine) with norepinephrine (aka noradrenaline)

        • Gluconeogenesis does make glucose from fat. It takes a triglyceride and pulls out the glycerol, and makes glucose from that.

          Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. From breakdown of proteins, these substrates include glucogenic amino acids (although not ketogenic amino acids); from breakdown of lipids (such as triglycerides), they include glycerol (although not fatty acids); and from other steps in metabolism they include pyruvate and lactate.

  6. Dr Yuri Nikolayev (Russian) has supervised thosands of fasting experiments in his practice. He recommended an active life when someone would embark on a fast. In one of the clinics he worked, often times would appear celebrities and writers who avidly claimed for the mental clarity of the fasted state, giving them space to produce significant work…

  7. Dr.Fung

    Thanks. Wondering if one can exercise in the morning while on fasting regimen i.e. 18 :6 .If so what would happen to Fasting Blood Sugar which is already high in the morning.

    Would appreciate your response.

  8. I could not agree more. I have been a cyclist all of my life. I am 68 years old now. My biggest regret in regards to athletic performance and the processed junk that is marketed to cyclists and endurance athletes in general is that I fell for it all of those years. I used to eat every two or three hours – power bars, P&J sandwiches, and of course Gatorade. Your energy goes up and then it goes down, not to mention having your teeth rot out.

    A typical ride for me is 50 or 60 miles, and a I do a couple of centuries in the fall. The best move I ever made was to get into nutritional ketosis and go test it by riding all day without eating. Only drink if thirsty because that is another hoax. How does it come out? Your basic energy level stays the same all day long. There is no trouble with endurance performance whatsoever, and there is no such thing as bonking. Bonking is basically a manufactured condition invented by the sports food industry. Forced hydration, another myth, was invented by the Gatorade people and other imitators.

    An African bushman chasing down an antelope on the African savanna during the mid day, when temperatures were highest, did it without Gatorade and power bars, not to mention Nike shoes and hydration packs. If humans were not genetically adapted to go long periods of time without eating or drinking for that matter the human race would not even exist. What we are not adapted to is eating processed garbage every two hours. Noakes points out that prior to the early 70’s endurance athletes were trained to restrict food and water during their events. It all changed with Gatorade and the result was a bunch of over hydrated dead athletes. The US Army even bought off on this pseudo scientific hydration crap in the 90’s until they ended up killing several recruits in basic training. They ended up changing their policy because you do what your drill sergeant orders, even if it kills you.

    • Excellent insights Samuel!

    • anastacia fearless

      Well said. I learnt a lot from your comment and from this post of Dr. Fung. I am now on day five of total water fasting and have been able to exercise without feeling faint at all. I will fast until my body tells me to stop. Yes, the corporations have been marketing us a whole lot of hype about wasting away and warning us off healthy fats. Hi to everyone who is finding a new but ancient way.

  9. I know you’ve said it before that there should be no detrimental effects exercising whilst fasting, but I can not do interval training whilst fasting or I feel nauseous and dizzy, and really struggle, always cutting my excercise much shorter and feel horrible as well.

    If I just do moderate exercise (like cycle to/ from work, 45 mins each way, swim for an hour, walk the dogs for hours and hours), I have no problem, other than feeling extra hungry afterwards, like Bob said.

    If I try to do bootcamp/ kick boxing /HIIT style exercise (ie heavy breathing, lots of sweat, powerful full body work out) in less than 30 mins I feel nauseous and dizzy. I never have this problem if I’ve eaten before exercising. Having some chicken broth isn’t enough. I don’t need much food before, 1/2 an avocado is generally just enough – but it of course breaks my fast.

    Anyone have any suggestions or any idea why this happens? What can I do?

    Dr. Jason Fung: We are all individuals, so if you find exercise and fasting incompatible, then you should stop. However, most people do well.

    • Hey Kirstie – might have to do with the body needing a sudden supply of glucose to fuel high-intensity exercise which can’t be mobilised so quickly? I can’t do Crossfit when I fast but have no problems running or swimming. Other Crossfitters have similar problem although there are always a handful who love to workout on an empty stomach in the morning before they re-fuel… It’s so individual…

      • It really is individual, I have done Crossfit at the end of a three day fast for 30 min, the only reason I didn’t continue was because I was not used to it, din’t know how my body would react, but now, when I fast for several days, if I have time I do Crossfit. I usually only do water fasts.

    • Mary Cameron

      I get very dizzy and cannot even walk great distances when water/herbal/green tea fasting. I have been dx’d with agressive high grade ovarian cancer and am on Day 5 of a fast, hoping I can dodge this IIb cancer. I am also taking hydrogen peroxide during this fast. If I can’t tolerate the H2O2 I will level out the dose. Pleased to say I got into the Ketones > Glucose quickly. Die you cancerous bastard. And may my cells have use for anything left over.

  10. Interesting article, but you’ve hit one of my pet peeves–using the word “infinite” when a large finite quantity is being described. The article states “you can store infinitely more energy in the form of fat rather than sugar.” If that were true we could all stop eating because we could go forever on our current body stores of fat.

  11. Dr. Fung.

    A question. The article states that metabolic rate doesn’t change during fasting. I had heard that the metabolic rate slowed down during periods of fasting/starvation, or does that only happen after a longer period of time than you are talking about (or, is what I had heard incorrect)?

    • Dan, I believe that metabolic rate goes up (due to growth hormone increasing for instance) when fasting. The theory is that if you go on calorie restriction (that is, you eat but eat less than what’s required), after a time, your body says “Oh no! I’m not getting enough food, so I’ll decrease my metabolic rate”. And this is apparently what happens, as evidenced by many studies. On the other hand, if you fast (eat basically no calories), your body basically assumes you have to go find food, so it increases your metabolic rate. It’s either find food or die (in some manner of speaking — today, we tend to have a lot more fat on us than we did many years ago).

      Starvation is a different story. When you’re fasting, you’re not starving per se. That is, you have enough body fat to survive on for a while, weeks or months. You won’t begin to starve until you’re relatively emaciated.

  12. Great article once again Dr. Fung. One quick question. It’s a good thing our bodies can adapt to fat burning relatively quickly, but I was wondering about efficiency. If I’m doing a lot of HIIT, can the oxidation of fat produce energy at the same (or better) rate as sugar? Are there any studies that compare this? The anecdotal evidence by some of the comments suggests yes, but it would be interesting to know what’s been tested.

    • I believe that Dr. Jeff Volek and Dr. Steve Phinney did some pretty interesting tests on this. Google them.

  13. I am on day 5 of a fast and this post is timely. I just did a resistance workout and felt like taking all my weights up- an indication of feeling well, no issues. However I have been LCHF for a couple of months and maintained a Glucose:Ketone ratio around 1, as well I was 22:2 intermittent fasting every day. One meal per day and was approx. 1000 cal deficit. So I am fat adapted and now I am finding no issues with longer duration fasts.
    During this fast I am taking probiotic and prebiotics to help balance gut metabolism and even with that I found my blood glucose dropped to 3.8 last night. Considering last quarter of 2015 I had a HbA1c of 16.1 and fasting glucose 20.2 it is definitely good news. I am off diabetic meds as well now. All thanks to managing lifestyle with the helpful advice from blogs like Jasons. I have dropped 15 kg so far – approx 1 kg per week.

    • Brad, did you find that the prebiotics made you hungry for the fast? I tried that for one fast, taking potato starch and drinking juice from sauerkraut, but I found the hunger to be too strong (this is on a 3.5 day fast). It was all I could do not to break the fast. Usually, fasting gets easier the longer I fast (though not always). So, I stopped taking prebiotics for fasts longer than a day. Now, I have one cup of coffee with cream, one cup of tea (no cream), water, and some bone broth at night. That’s it, but I do take powdered magnesium. I go back to prebiotics (usually, only potato starch and green plantains) and probiotics (fermented foods, sometimes yoghurt) when the fast is over.

  14. Mark Collins

    When I fast using 16:8 I feel a huge benefit in my strength and Crossfit performance. It is the opposite to what I think should happen. It is like night to day. It is almost like I have taken a drug. I am not sure if the benefit is the increased growth hormone or the process of autophagy. I also feel less inflammation and recover better from my workouts. During my eating window I usually get in 2500-3000 calories with about 150g of carbs. I tend to get my carbs from potato, sweet potato and fruit. Since I have started fasting I have stopped taking protein supplements and sometimes do not eat for 1-2 hours post workout.

  15. In reference to your “gas gauge” analogy, here’s an article from Marty Kendal about how to use your BG meter as a gas gauge to determine whether or not your “fuel tank” needs refilling or not. Is it true hunger, or just a mood?

    While I’m on the subject of food as mood, this was discussed on JM’s latest LC Conversations today:

    Your book arrived yesterday, and it’s as easy-reading as your blog. You do have a way of cutting to the heart of the matter!

  16. i am a filipina, a polio victim at 3, 41 yrs old and has a type 2 diabetis. My husband is only working as an extra family driver. I have two kids that i really love. Diabetis affects me a lot financially, physically and mentally. My life has changed since diagnosed by the desease. I want to help my husband in raising our kids. I am poor. We are poor. But i want to try your program. Please. God bless you, Sir

    • Maricel, please try it, you’ve got nothing to lose and everything to gain. Since protein stays the same and carbs are replaced with fat, your food bill shouldn’t really change and if you get into fasting, might even reduce 🙂 Good luck, your family will thank you for sticking around as long as possible!

  17. Dr. Fung,
    Thanks for the post. Shouldn’t this be # 23 in the fasting series (The biggest Loser Diet was #22)?

  18. Dr. Fung, this does not relate to your post, but I wanted to direct your attention to an Opinion piece in the NYTimes concerning insulin companies. It might be useful to see if you could write a response to this writer, because her argument is for cheaper insulin, not less insulin, for Type-2 Diabetes. The dietary changes that you talk about would certainly make the whole thing cheaper – even free -since no insulin would need to be purchased at all.

  19. I trained fasted for many years before my body said no more. Women need to be careful with long fasted training. Cortisol will be on overdrive and yes, you will lose weight but your sense of well being will go down the tube. Chronically elevated cortisol put my body into state of adrenal fatigue. Ammenoreah, high anxiety, hypoglycemia, irritability and heart palpitations. I think training fasted can be done if done early in the AM but pushing it past a certain point can have negative effects. A small piece of protein to steady blood sugar can do wonders before a training session. Listen to your body and don’t push something that isn’t working for you. Fasted training can work for people but is not a cure all for everyone. PS – I am not a novice athlete.

  20. “You can be damn certain that these elite level athletes would not be doing this Low Carb mumbo jumbo and training in the fasted state malarky if it had any detrimental effect on their athletic performance.”

    It’s impossible that an athlete could make a dietary or training error?

    Couldn’t this exact same argument be made for those athletes eating pasta or drinking Gatorade?

    • if an athlete makes a dietary or training error it is very possible that he/she is not an elite level athlete

  21. Dr Fung thanks again for the article!

    a quick question, lets say I became fat adapted after 1 month of LCHF diet and fasting and exercising in fasted sate.
    If I go back to my previous diet, more carbs, for 3-4 days (holiday, social event,…). Will I lost my fat muscle adaptation, ketogenic metabolism? How quickly? If yes will it be again 1 month do become again fat adapted?

    Thank you!

  22. Interesting series on fasting and fat loss. I am still wondering what are the ways by which we can change the body weight set point. Anyone has an idea?

  23. I would be careful with this advice, as even those professional athletes mentioned who went low card do usually need steady carbs to meet up with the demands of physical activity especially during the course of the season when they are playing most often. The low carb was done during the offseason and later modified as the regular course of season begins for them to include some more carbs.

  24. I did 2 fasts, 5 days and 6,5 days where I tried to keep up my exercise regime. The first attempt was horrible, the second alot better but I still was seriously lacking energy. Tomorrow I start the 3rd with a few new tricks I learned from Jack Kruse. Let’s see how it goes.

  25. Jeremiah

    quick question:

    in regards to fasting during workouts, what are your thoughts about the advice that people need to ingest BCAAs before and after workouts when you fast? Will taking BCAAs help or is it just wasted money? Thanks.

    • BCAAs are pretty much a waste. Of the 3 amino acids typically present, only leucine is significant for muscle. BCAAs also stimulate insulin which then eliminates the benefits of fasted training. Some people say that a BCAA after training can help prevent catabolism but if worried enough to take a BCAA, you might as well just eat some protein which naturally contains leucine.

  26. Is there any risk with thyroid function due to prolonged limited access to carbs?

  27. Every Monday I eat at 6 p.m. The next morning without breakfast I ride my bike between 32 and 44 miles for at least an 18 hours fast. Then I have a protein based lunch. I do the same thing Wednesday in to Thursday. Saturday I eat late in the afternoon then fast 24 hours until the same time Sunday. I also take a long walk on Sundays. Not counting other nights I fast at least 60 hours a week, two 18 hour stretches and one 24.

  28. Hello, terrific post.
    I have a question regarding the 2 week adjustment period. If I fast for say 5 days and feast for 2 days (with considerable carb intake on feast days) or ADF and eat more carbs then the standard LCHF diet calls for, will I then not reap the benefit of burning primerally fat and not suger? And if so will how many carbs are Ok for me to eat to burn primaraly fat instead of suger, while exercising heavily, 3-4 times a week.
    If it means anything for better answers, I’m, 6 feet, 215 pounds, very lean and train power lifting 3-4 times a week. The goal is to have maximum amount of energy on predetermined competions days.
    Thanks again for a great blog. If you or some avid reader if this blog as insight on this topic I would appreaciate it greatly.

  29. Hello, love this blog
    I have a question regarding the week adustement period. If I practice ADF following a longer duration fast of ca. 5 days. Will I burn primeraly fat and not sugar of I eat a higher than normal carb intake vis a vie LCHF diet? If not, how many carbs can I ingeast on feast days in order to stay in ketosis (or burn fat primeraly?)
    It it matters for a more accurate answer, I’m 215 pounds, very lean and train power-lifting 3-4 times a week. The goal with this diet is to maintain good health but also reach peek energy levels on days of competition.
    Thanks again for a great blog. If you or possibly some avid reader (well versed on the subject) I would greatly appreciate your feedback.

  30. Can I vegetarian become adapted to fat burning? For ethical reasons, i no longer want to eat meat, but I’m gaining weight as a vegetarian. I just discovered fasting, and am hoping I can fast and get the benefits of low insulin without having to increase protein.

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