Is It Safe to Exercise While Fasting? The Truth about Energy and Nutrients

In this video, Dr. Ekberg addresses the question of whether it is safe to exercise while fasting. He explains that the body obtains energy from different sources, such as glycogen, body fat, and protein. When you eat, the body processes and stores the nutrients for later use, so the energy you use during exercise does not come directly from the food you just ate. Dr. Ekberg discusses different scenarios, depending on whether you are fat-adapted or carb-dependent, the duration and intensity of the exercise, and the length of the fast. He emphasizes the importance of understanding your body and using common sense when exercising while fasting.

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How does this happen?

Key Insights:

  • Energy does not come from the food we just ate, but from nutrient pools in the body (glycogen, fat, protein)
  • Feeling energized after eating is due to the stimulation of hormones, not the immediate utilization of food
  • The safety of exercising while fasting depends on the type and duration of the exercise, as well as your metabolic adaptation
  • Fasting and aerobic exercise are generally safe if you’re fat adapted, as your body primarily uses body fat for energy
  • Fasting and anaerobic exercise may require the use of glycogen, which is limited if you’re not fat adapted
  • Weight lifting can be done while fasting if you’re fat adapted, but replenishing protein afterward is crucial for muscle repair
  • If you’re not fat adapted, intense exercise while fasting may lead to muscle breakdown and feeling lousy
  • Understanding how the body works and using common sense is essential when exercising while fasting


Hello health champions. Is it safe to exercise while you’re fasting? What really happens in the body when you’re doing fasting or intermittent fasting? Where does the energy come from? Today I’m going to answer all those questions plus a few questions you didn’t even know you had but you’ve been wondering about ever since you were a kid. Coming right up.

Hey, I’m Dr Ekberg, I’m a holistic doctor and a former Olympic decathlete and if you want to truly master health by understanding how the body really works make sure you subscribe and hit that notification bell so that you can keep learning about how to get truly healthy.

In order to talk about whether it’s safe to exercise and fast, we first have to understand where does the energy actually come from and we have to get rid of a few myths you’ve always been told. Things like if you don’t eat any breakfast you won’t have any energy, make sure that you get frequent snacks so you can boost your energy, and you have to eat protein immediately after a workout because otherwise, you won’t be able to build muscles and you should eat so that you feel better. But if you feel better after you eat, was it actually the food you ate that made you feel better or was it something else? Let’s talk about the facts.

Whenever you eat something, it comes down to your esophagus and it’s going to sit in your stomach for two to five hours. Your body is going to work on it, it’s going to churn it, it’s going to massage this, it’s going to add enzymes and hydrochloric acid, but you’re not going to utilize it because it’s not ready for that. The only thing you can absorb to some degree in small amounts are things like water, alcohol, and glucose. That stuff goes out a little bit faster. But if you eat a whole food, a whole meal, there’s going to be no alcohol obviously and there’s going to be very very little glucose in there. So if you eat fruit, then a tiny percentage of that fruit is glucose, free glucose. Nothing else, no fat, no protein, and no molecule bigger than a single molecule is going to get absorbed. That’s going to happen much later in the small intestine. So it sits for two to five hours in the stomach, then when it’s ready it moves into the small intestine. That’s this squiggly convoluted thing here and that’s where most of the nutrient absorption takes place. But that’s 4 to 12 hours after you ate the food.

So here’s the big question, how come then that you feel better after eating? How do you feel satisfied? How do you feel energized when you’re not using any of that yet? Because the food stimulates things, it creates an expectancy in the body. So you’re stimulating hormones and you’re starting to break down glycogen to raise blood sugar and feel satisfied because the body knows, hey, I can use up this glycogen because you just ate, so I’m going to be able to fill up the stores in a little bit. And if all of this is true, which it is, then what about the nutrients for exercise? What about all the calories and all the energy and all the stuff you need for exercise?

Well, the same thing is true. It does not come from the food you just ate. Nutrients are absorbed in the small intestine 4 to 12 hours after you eat the food. And what that means is that the energy that you’re using in this moment, whether it’s just for everyday activity or whether it’s for exercise, doesn’t come from the meal that you just had. It comes from nutrient pools. And you have glycogen, that’s the extra glucose, the extra carbohydrate you eat gets put together into glycogen and stored in the muscles and liver and that’ll last you 12 to 24 hours. You have fat that will last you 1 to 12 months. If you’re super skinny, you still have about a month’s worth of fat that you could live off. If you’re obese, you could make it up to a year. Some vitamins that are water-soluble will last you for a few days or weeks, but others, even if they’re water-soluble or fat-soluble like B12 or vitamin D, will last you for years. The body has a way of storing them. And protein, which you don’t want to use for energy but you could, will last you for weeks.

So we have to rethink this idea about eating something to get energy. Your body asks you to eat, you create hunger so that you can keep up with your energy expenditure. But what you’re eating now is going to be processed and stored and used later. You are not going to run out of fuel in the next couple of hours or days. If that was the case, then humans would never have survived. Whenever we were without food for a day, then we would just kind of wither down and lay down and die. And we wouldn’t have the energy to go out and catch more food.

The only reason that we have this idea of eating and getting energy and feeling better in this moment is that we have trained a behavior into our system. We’ve become carb-dependent. And in a sense, we’ve created a drug habit. We’ve created because any time that you eat something and it changes how you feel, it’s kind of a drug. It’s a form of stimulation. And if we train the body into a rhythm, that becomes a habit. Now we have a drug habit. Of course, it’s not like a heroin habit or anything, but it’s still a drug habit. And that is why we believe that we have to top off our energy stores every couple hours. That’s not the case.

Now getting back to the question of the video, is it safe to exercise when you’re fasting? Well, it’s going to depend. It depends on the type of exercise you’re going to do. It’s going to depend on the duration of the exercise, are you going for seconds, minutes, or hours? The duration of the fast, how long have you fasted and how long do you plan to continue fasting? And also very important, how have you trained your metabolism? Is your body fully fat-adapted? Does it know how to use fat for fuel primarily or is it still carb-dependent?

Now let’s come back to that question of where does the energy come from and let’s look at a few different scenarios. First scenario, you’re planning to do some aerobic exercise. That’s low-intensity exercise. You can breathe deep normal breaths, you don’t have to huff and puff. But those deep breaths provide enough oxygen to provide you with energy. Let’s say that you didn’t have breakfast, so you’ve only gone 12 to 16 hours without food.

Now, first case, you are fat-adapted. That means that you’ve trained your body into using fat for energy and you’re going to use body fat and glycerin for fuel. So body fat, the fatty portion of that tissue, is going to provide 95% or so of all the fuel you need. And you’re going to need a little bit of glucose for the brain and your body is going to convert glycerin into glucose for the brain. Glycerin is part of a triglyceride, that’s just how your body stores fat. And because you are fat-adapted, you’re not really going to shock your body. You’re not going to shake up anything, it’s just going to continue burning fat the way it normally does. So you can go forever, pretty much. I know there’s no such thing as forever, but until you get a blister or until you need to sleep or until you run out of body fat, you can keep doing this and not really stressing your body.

It’s a little different if you’re carb-dependent. If you haven’t trained your body to use fat primarily, now even at low levels, you will use some carbohydrate. You’ll use a good amount of body fat still, but you will use some glycogen. And because you have some glycogen and because it’s low intensity, you can go a really long time. But not as long as this scenario because after 12 to 16 hours, your glycogen stores are running pretty low and you’re using up some of the glycogen during the exercise. And when you run out, that’s not a good thing. Now you’re going to feel really bad, but your body is going to break down protein to try to come up with some glucose when you run out of glycogen. If you’re fat-adapted, you could use glycerin for glucose, but if you’re not, then you’re going to resort to breaking down protein. And now you’re going to have some muscle wasting as a result.

Next scenario, you decide to do some more intense exercise, some anaerobic training. That means you’re going to push it until you start huffing and puffing because normal breath or deep breaths can’t keep up. So what this means is even if you’re fat-adapted, you will need to use some carbohydrate. Anything above that aerobic threshold is going to have to be supplied by glycogen. But because you’re so good at burning fat, that fat is still going to be the predominant portion of your energy supply. But you will need to break down some glycogen. You will have plenty of energy because if you’ve had this lifestyle for a few months and you’re fat-adapted and you’ve done some exercise, your body has learned to build up normal glycogen stores even if you’re on a low-carb or a keto diet. And that means if you decide to do some high-intensity interval training, then it’s not going to be a problem. Your body is going to know exactly what to do. It’s going to use fat, it’s going to use glycogen. And as long as you don’t go on forever, then it’s not going to be an issue.

Now, what if you’re not fat-adapted? Then you’re going to be burning more glycogen than body fat. And again, these ratios are approximate. All I’m saying is that if you’re fat-adapted, you’re going to tend to use more body fat than glycogen. If you’re carb-dependent, then it’s vice versa. But it’s always going to depend on the intensity of the exercise. The more intense it is, the more glycogen you’re going to have to use. And also, if you’re carb-dependent, if you’ve trained your body to expect carbohydrate every few hours and you’ve gone 12 to 16 hours without food, now your glycogen is gonna run pretty low. So if you go exercising, you’re gonna run out of glycogen much, much sooner. And as a result, you’re gonna feel really, really weak. You’re gonna feel bad. And you’re going to conclude that you have to eat carbohydrate in order to do anaerobic training. And this is one of these myths out there that you can’t do intense exercise if you don’t eat a bunch of carbs. Well, that’s only true if you’re not adapted to the alternative.

What about lifting weights? Can you do that when you’re fasting? If you’re fat-adapted, no problem. You use primarily body fat for most of your energy. But any time that you do something heavy, you push yourself, you do a heavy set and your muscles start burning, that’s lactic acid, which means that you broke down some glycogen. And again, that’s not a problem because if you’ve done this for several months, you’re going to have plenty of glycogen to start with, even on a keto diet. Here’s the thing that you want to keep in mind, though, is weight lifting does break down muscle. You can break down muscle in a bad way if you’re starved and your body is trying to make glucose. But in a good way, that’s kind of the purpose of weight lifting, is that you’re stressing the muscles, you’re creating some micro tears, and then afterwards, your body repairs the muscle and it’s stronger than it used to be. So by stressing the muscle, it gets stronger. But in order for you to be able to repair it, you need human growth hormone. And that’s why the body makes extra growth hormone when you’re fasting and when you’re stressing the body. But you also have to remember to replenish the protein after the workout. So if you had fasted for 16 hours and you go to a workout and you create all these positive things, but you don’t eat, let’s say you go another 48 hours without food, that’s not a good thing because your body is going to try to repair these muscles, but it’s not going to have the resources to do it. So you’re going to be in a deficit and the body’s going to try to borrow and scramble, which is not ideal.

What if you’re carb-dependent? Well, the first several pieces here are going to be exactly the same. But just like with the anaerobic exercise, you’re going to run out of glycogen much faster. You’re going to feel much weaker, you’re going to feel lousy, and you’re going to conclude that it’s a bad idea to lift weights unless you eat carbohydrates.

Now let’s go back to the aerobic scenario but let’s say that you’ve gone much longer. You’ve gone at least three days, maybe five or seven days. It doesn’t really matter because now you’re in a steady state. You’re fully adapted and things are just going along. So if you’re fat-adapted, then it’s the same thing as if you had gone 12 to 16 hours because you’re going to be burning body fat and glycerin. And you can go as long as you have body fat to burn. If you started out as carb-dependent, though, it’s going to be very different because in 72 hours, you have burned through your glycogen and your body has no choice but to become fat-adapted. You’re not going to be as stable and feeling as good as if you’ve done it for a long time, but your body has no choice. It’s going to burn fat. The difference, though, is that you’re going to be feeling lousy with any form of exercise. Why? Because it doesn’t matter if you exercise or not, you’re going to feel lousy after three days of fasting if you’re carb-dependent. Your body is just not used to it. It’s having a hard enough time just coping with staying alive while not eating. It’s not dangerous, you’re just not going to feel good because you’re in a transition phase. And if you add exercise to it, that’s just going to make matters worse.

What about anaerobic exercise? What about intense exercise? Can you do that if you’ve gone 72 hours or more? If you’re fat-adapted, then you’re going to be using still body fat more than glycogen. But you’re not going to have the full stores of glycogen because you’ve gone 72 hours or more without food. So you’re starting off with less glycogen. There’s less resources in the body overall than if you had eaten a ketogenic diet all along. So can you do a HIIT exercise under those circumstances? I would say yes, but I would use common sense. I would make sure I take it maybe a little bit easier or at least that you do it very, very brief. Make sure that you just do a few seconds at a time and that the whole workout only lasts a few minutes. Could you do something longer like an Ironman? Well, you would probably survive it, but I wouldn’t recommend it because an Ironman is very, very demanding. It’s high intensity, it’s anaerobic, it lasts for hours and hours and hours. So it’s a huge demand. You’re going to be breaking down your body, you’re going to be using up all the resources it has and if your resources are not fully topped off, then I just don’t think it’d be a great idea. You probably wouldn’t perform all that well, you’d probably make it through but it wouldn’t be a very productive or necessarily a healthy thing to do.

If you’re not fat-adapted but you’ve gone 72 hours, now you’re becoming fat-adapted. But again, you have almost no glycogen. You’re going to feel absolutely lousy and I would not recommend that you do any anaerobic training if you’re in that phase. If you go on three days and you’re becoming fat-adapted, then your body is having a hard enough time as it is.

What about weight lifting? Can you lift weights after three days of fasting? If you’re fat-adapted, your energy is going to come primarily from body fat. You’re going to use glycogen for the extra hard stuff when your muscles are burning. That’s lactic acid from breaking down glycogen. But you start off with less glycogen, so I wouldn’t go too hard or too long, even though it’s not going to be a big problem.

Here’s the main key: like we said before, the purpose of heavy weight lifting is to stress the muscles so they break down, so they can rebuild and get stronger. And you could do that after three days of fasting, as long as you don’t overdo it. But you have to make sure that you eat something afterwards or don’t do it because if you’ve gone 72 hours without, you’re in autophagy, your body is trying really hard to conserve protein. And if you just break down more protein, it’s going to put a tremendous strain. You’re not going to be able to repair properly and that whole weight workout was really just destructive.

And if you’re not fat-adapted, then in 72 hours, you’re becoming fat-adapted. You’re in the process, you have almost no glycogen left over. So if you work out with weight lifting, you’re going to break down even more muscle. You’re going to break it down from tearing fibers from the workout, you’re going to break down even more muscle from trying to make glucose for the anaerobic strain and you’re going to feel really, really terrible. So simply, I would say don’t do it.

You have to remember that it is safe, but you have to understand what’s happening in the body. If you’ve just been fasting 12 to 24 hours, no problem. If you’re doing longer fasts, then it’s extra important you understand what’s going on. You go easy and you use some common sense. Watch lots of videos from this channel so you start understanding how the body works and then put that together and don’t be stupid.

If you enjoyed this video, you really need to take a look at that one next. Thank you so much for watching. I’ll see you next time.