Fasting and exercise

Cycling Class at the Gym

Is it possible to exercise while fasting? This is a common question we hear all the time and the simple answer is ‘Yes’.

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?

Let’s think logically about what happens when we 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. Much of it is used to repair proteins and excess amino acids are turned into 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.

So what happens during a fast?

macro-oxidationIt’s just the food-storage process in reverse. First, your body burns the stored sugar, then it burns the stored fat. In essence, during feeding you store 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-polysccharideIf 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.

Glycogen and ‘hitting the wall’

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 three times a day, it’s like you go shopping for food three 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_finish_feb82_0-300x300So 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’.

Enter fat-burning mode

So, how do you get around that? Glycogen stores are not enough to power you through the entire IronMan race. However, at the same time, 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.

Training in the fasted state

Studies are starting to demonstrate the benefits of such training. For example, one 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 the 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-1What does this mean in plain English? It means that our bodies have the 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.

How elite athletes can use this

Legendary exercise physiologist and physician 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-skinnyYou can be certain that these elite level athletes would not be eating low carb and training in the fasted state 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? No way.

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.

Endurance sports

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-768x512Consider 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 two weeks to become fat adapted, though. During the first two weeks of fasting, you may need to take it a little easy, but you should quickly recover after that.

Jason Fung


Intermittent Fasting for Beginners

Popular videos about fasting

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Earlier with Dr. Jason Fung

Obesity – Solving the Two-Compartment Problem

Why Fasting Is More Effective Than Calorie Counting

Fasting and Cholesterol

The Calorie Debacle

Fasting and Growth Hormone

The Complete Guide to Fasting Is Finally Available!

How Does Fasting Affect Your Brain?

How to Renew Your Body: Fasting and Autophagy

Complications of Diabetes – A Disease Affecting All Organs

How Much Protein Should You Eat?

The Common Currency in Our Bodies Is Not Calories – Guess What It Is?

More with Dr. Fung

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

His book The Obesity Code is available on Amazon.

The Obesity Code

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



  1. Ken klassy
    If you fat adapted and lifting weights on a regular basis while In a fasted state(say after 36 hours), what is the body's response to needed protein? In other words, Normally you have protein post work out to repair muscle damage---if you do not have protein post workout will you still be able to build muscle?
  2. Kenneth
    I had the same question, so I tested it. I've done intermittent fasting periodically over the past 2 years, but of the 18 & 24 hour duration. Last summer I started a regular weight lifting routine to put some meat back on my bones after 2 1/2 years of weight loss. I'd lost over 200 pounds with a consistent low carb diet. I had 2 different routines working different muscle groups. I would do two days lifting and then take a recovery day. For the first several weeks I slowly increased strength and bulk. Then, near the end of the summer I decided to try a longer fast - 7 days. I continued the weight lifting routine through the fast. On my first two lifting sessions I had all my strength. Those were the first 2 days of the fast. I completed my sets of 6 or 8 lifts to failure just as I had done before the fast. I took my off day. On may next two sessions, day 4 and day 5 of the fast, I was notably weaker. I could not come close to completing my sets. I was getting 4 or 5 reps in before failure. I ended the fast early at day 6. I ate lots of protein on that day and on subsequent days. I continued lifting weights. In the week following the fast all my strength came back and then some. By the end of the week I'd moved to heavier lifts on all of my routines. I noticed a significant difference in my appearance. I could clearly see the newly added bulk.

    I'd read that fasting increases testosterone and HGH. I was curious to see how this would work with weight lifting. These were my results. Take this for what it is worth. These were just my observations from a one time N=1 experiment. I plan on trying it again soon. I'm planning another longer fast later this month.

  3. Jennifer Freeman
    When you say fast, do you mean completely not eating food? Or not eating carbs, only protein and fat?
    Reply: #11
  4. Andrew
    I am a competitive weightlifter and have been interested in IF for a awhile now. In your article I see you say there a lot of benefits for endurance sport athletes who do IF, do you know if there is any research into advantages/disadvantages for strength athletes/sports? Thanks.
  5. 1 comment removed
  6. Jo
    I am training for my first marathon. If I am doing intermittent fasting, do you recommend I fast on my recovery days, or on the days I exercise?
  7. Chris
    I’m doing the Keto diet. I started about 4 months ago. I notice I am gaining weight since I’ve been exercising. I only eat omad but I notice my appetite has also increased. It seems exercise and a bad nights sleep increases my appetite. Are there any suggestions that could help me?
  8. Lukkas
    So what's the situation with Anabolic exercises? Once the glycogen is gone, how is it replenished during an extended fast?

    I know fasting is supposed to be protein sparing, so it's not gluconeogenesis is it?

    Is it somehow converted from bodyfat?

    How long would it take the body to recover glycogen while fasting in hours or days?

    Reply: #9
  9. Lukkas
    Urg, I meant to say Anaerobic exercise.
  10. Lukkas
    I meant to say Anaerobic exercise there.
  11. Felipe Piacesi
    His talking about eat no calories. No carbs or other macronutrient.
  12. Felipe Piacesi
    Doctor Fung, could you post the study reference citaded in this paragraph: "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."
    Thank you very much.
  13. Samson
    Hi Dr Fung, I started the Keto diet about3 weeks ago and IF about 2 weeks ago. I am currently on OMAD. Dropped about 15 pounds during this period. I wanted to exercise as well to help tone my body. I eat between 6-9 pm and work out around 1/2 pm in a fasted state. However I noticed that I am gaining weight for the past couple of days. I am 100% sure that my macros are within limits. I am measuring everything and following the recipes on Diet doctor. It has just been two days but I am worried that all that weight might come back! Please advise!
    Reply: #14
  14. Kristin Parker Team Diet Doctor

    Hi Dr Fung, I started the Keto diet about3 weeks ago and IF about 2 weeks ago. I am currently on OMAD. Dropped about 15 pounds during this period. I wanted to exercise as well to help tone my body. I eat between 6-9 pm and work out around 1/2 pm in a fasted state. However I noticed that I am gaining weight for the past couple of days. I am 100% sure that my macros are within limits. I am measuring everything and following the recipes on Diet doctor. It has just been two days but I am worried that all that weight might come back! Please advise!

    Dr Fung recommends not doing OMAD or 24 hour fasts more than 3 times a week for this reason.

  15. Sinisa
    Hi Dr Fung, Do you have any advice on fasting headache. How to prevent them or deal with them when it happens.
  16. Darren
    In my experience strength goes down big time if trying to continue strength training during a longer fast of 72hrs +. Aerobic efficiency seems to increase, but intensity duration still decreases. Basically you can work at a low to mid level aerobic rate for longer, but can't seem to work at the higher end of the aerobic spectrum for as long. As for the anaerobic spectrum, the duration and intensity is greatly diminished.

    Flipping back to my experience with strength training, I found that dropping the load to between 60%-80% of 3rm, and decreasing both reps and sets, is doable. Normally on my main exercises I would train at between 85%-95% of 3rm, and at between 60%-70% of 3rm for assistance exercises.

    If doing OMAD however, I find that training intensity can be maintained throughout.

    If wanting to incorporate a slightly longer fasting window and still maintain strength training, I would suggest timing it to coincide with a lower intensity training period of your training cycle. Something like a 3 week progressive overload followed by a 1 week deload period, or even complete rest, incorporating only low intensity aerobic exercise such as walking, works well.

    The fact is, you can happily take two weeks off from strength training without any loss of strength. Although this is dependant on how long you have been training for to begin with. People relatively new to strength training may only be able to have a week off without seeing any loss in strength.

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