Is working out while intermittent fasting safe?


In my 6 Unexpected Side Effects of Intermittent Fasting(IF) post, I mentioned that working out while intermittent fasting was probably one of my biggest concerns when I decided to give IF a try. Is working out while intermittent fasting safe?

Pre & post-workout fueling was literally stitched into the fabric of my everyday life for DECADES, so when I started intermittent fasting and was told that I don’t have to fuel for my fasted morning workouts, I was not only skeptical, I was a non-believer.

I was absolutely, 100% convinced that my workouts would suffer from the lack of a pre-workout supplement.

I mean, how could my body possibly sustain a workout with no immediate food fuel to pull from? And, what about my poor muscles? Wouldn’t my body start burning muscle to compensate for my lack of fuel?

Turns out, those myths have been debunked.

In this post I’m going to:

  • answer the question “is it safe to work out in the fasting state?”
  • review WHY you need to ease into working out in the fasted state.
  • explain the difference between the fasted workout and the fueled workout and how the body fuels your workouts in the fasted state.
  • go over why working out in the fasted state does NOT result in a loss to your muscles.
  • review steps to take to help your body adapt to working out in the fasted state.
  • go over some benefits that I experience while working out in the fasted state.
  • provide some guidance for those crazy athletes.

Let’s get started.

Is it safe to workout while fasted?

Short answer is “yes” (for most people), but it can take some time to adapt.

Consider this.

For those just getting started, I like to think about working out in the fasted state this way.

Let’s say, theoretically speaking, that one morning you wake up and decide, “Hey. I want to run a marathon!”

Now, would you:

  1. get right out of bed, put on any old sneakers you own, step outside and go for a 26.2 mile run? or, would you;
  2. lookup training programs, or even training groups to join, so that you can properly train over a period of months to run a full marathon?

I’m going to go with option #2, because option #1 is COMPLETELY insane!

Patience and prep.

Training to run a marathon takes time and prep. You need to find the right sneakers to wear on your long runs, you need to find the right clothes to wear that won’t tug, pull or chafe, the right fuel, and most importantly, you need to TRAIN.

You need to train your body so that it can properly adapt to running 26.2 miles.

By properly training, and adding those few miles to our runs week after week until race day, we allow our bodies time to get used to running those additional miles and ADAPT to the distance.

If we were to step right out of bed one morning and attempt to run a full 26.2 miles, there is a 100% chance of failure. But, since we trained and allowed our bodies time to adapt to the extra miles, we were able to use that training to get us across the finish line.

This same exact concept applies to working out while intermittent fasting (or in a fasted state).

The key to working out while fasted is to take time to adapt.

If our bodies are used to training and working out while fueled with a pre-workout supplement, shake, or that trusty pre-workout snack, then our body will completely tank if we try to do the SAME workout right away without the fuel it was so used to receiving.

It’s like running a marathon without any training. You allowed your body ZERO time to adapt to working out in the fasted state, so why do you think you’ll be successful at working out in a fasted state right out of the gate?

Well, I know why. We’re impatient. I was, too. I wanted to see fasted workout success right out of the gate, but that just didn’t happen. It took at least two weeks for my body to adapt to working out while fasted.

So, how do we do that? How do we adapt?

I’m going to talk about that a bit further down in this post, but first we need to clear a few things up before I do.

Fasted State v. Fed State

Let’s quickly review how our body pulls fuel to sustain out workout in the fed state and the fasted state.

The fed state.

According to Dr. Jason Fung, author of The Obesity Code, in the fed state, when you are eating, insulin levels are high. During that time, it makes sense to derive your energy from the food that you are eating. So what happens is that we shut down burning of stored food energy in the form of fat and glycogen. (Source)

What this means is that, in the fed state, we are pulling our energy from the food that we are eating and will continue to pull energy from this fuel source (food) until we stop eating and our bodies switch to the fasted state. 

The fasted state.

According to Dr Fung, as you fast, insulin levels fall. This is the signal to switch energy sources from food to stored food. You pull stored energy out from the liver (glycogen) and if that is not enough, body fat. (Source)

In other words, in the fasted state, when there is no longer any food energy to pull from, the body will then switch its fuel source to pull energy (glycogen) from the liver. Once glycogen stores in the liver are depleted, the body starts to pull its energy from body fat.

Fasted fueling.

Knowing what we know based on what Dr. Fung just told us above we can now determine HOW our body will fuel itself during a fasted workout.

As mentioned above, if you are in a depleted (fasted) state when you decide to workout, your body will pull from its glycogen stores (liver) to fuel your workout. Once those stores are depleted, your body will pull from your fat stores to fuel your workout.

That’s right! Once you have used up all of your fuel stores in your liver you’re body will start to use your stored fat to fuel your workouts.

Amazing, right!

We start to burn our own fat to fuel our workouts!

But, be prepared for a small adjustment phase. It takes time to adjust to fasted workouts.

Note: When I started working out in the fasted state I felt like I took 1000 steps back and that first fasted workout felt the very first workout I ever did in my life. It was grueling. But, it passed.

See quote from Dr. Fung below. The adaptation phase is normal. We just need to exercise a little patience.

Per Dr. Fung – 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. (Source)

But, my muscles?!

Another common misconception is that the body will start breaking down muscle to use as protein for fuel.

Dr. Fung explains, It seems that there are always concerns about loss of muscle mass during fasting. I never get away from this question. No matter how many times I answer it, somebody always asks, “Doesn’t fasting burn your muscle?

Let me say straight up, NO.

Muscle gain/ loss is mostly a function of EXERCISE. You can’t eat your way to more muscle. Supplement companies, of course, try to convince you otherwise. Eat creatine (or protein shakes, or eye of newt) and you will build muscle. That’s stupid. There’s one good way to build muscle – exercise. So if you are worried about muscle loss – exercise. It ain’t rocket science. Just don’t confuse the two issues of diet and exercise. Don’t worry about what your diet (or lack of diet – fasting) is doing to your muscle. Exercise builds muscle. Clear? (Source)

I’m not going to get into this topic in too much detail, and I do realize that this is a legitimate concern for many people, but I believe what the good doctor says and have seen no difference in my muscles since I started working out while fasted. Unless you’re trying to “bulk up” this should not be a concern for you.

How to adapt to working out in the fasted state?

Before I get started on this little section, I need to insert a little disclaimer.

If you feel sick, nauseous, lightheaded, or just overall ill, STOP what you are doing. Sit down, rest, and eat something if you absolutely have to. There is a very good chance that you (just like I did) tried to jump out of the gate too quickly and started with a workout that was just too much.

Take it easy and start a bit slower.

Note: Just because people can workout fasted doesn’t mean that it’s right for everyone (I’ll touch on that in a bit.).

Getting Started.

Let’s take the marathon scenario and work with that. Adapting to working out in the fasted state is just about the same as adapting to anything, right?

Start small.

Always aim to start light and short when you start working out in the fasted state. Your body has very likely become accustomed to pulling its fuel from food or drink so it will be very shocked when that fuel source is no longer there.

Just like training for a marathon, you start small (low mileage) and slowly add to that mileage week after week. Think those thoughts when you start working out while fasted.

See how it feels and go in with the understanding that those first few workouts will be tough.

Note: Don’t feel discouraged if you feel like you’ve taken a few steps back when you start working out while fasted. This is normal and it will pass. Give yourself some time and some room to grow.

Adjust accordingly.

Listen to your body. If you feel awful, then scale back. If you are starting to get adapted, then feel free to add on!

I’m a big fan of listening to my body and will make adjustments accordingly.

Note: If you are an intermittent faster, you don’t HAVE to workout in the fasted state. Read the post I have highlight below. I review different workout scenarios for intermittent fasters. This lifestyle isn’t a “one size fits all” lifestyle. Changes can be made to fit YOUR lifestyle, too!

Read: When is the best time to workout while intermittent fasting?

What I do.

It took me about 2 weeks (or so) to adapt to working out in the fasted state.

Note: Pre intermittent fasting I would consume a pre-workout supplement to fuel my early morning workouts. When I started intermittent fasting, I switched to black coffee as my fuel supplement. I noticed little difference in the “energy” department when I switched from my pre-supplement to black coffee. Once I adapted to working out while fasted, I felt completely the same.

In my early stages of working out in the fasted state, I did the following:

  • reduced the length of my cardio workouts from 45 minutes to 30 minutes;
  • reduced the intensity of my workouts to a lower intensity workout;
  • listened to my body and made adjustments as needed;
  • increased intensity over a 1-2 week period once I started to adapt to working out in a fasted state.

In total, this took about 1-2 months and I can very happily say thatm to date, I have no issues working out while fasted.

Read: The Benefits of Drinking Black Coffee.

Benefits of working out in the fasted state.

If you decide that working out in a fasted state will work for you, you may start to notice benefits of those workouts that you hadn’t noticed before.

Besides tapping into your fat stores to fuel workouts, for me personally, I experienced the following benefits from working out while fasted: 

  • reduced stomach issues while running;
  • increased energy during and after my workout;
  • increased focus while working out;
  • reduced “healing” time after a hard workout or run;
  • feel more motivated to complete daily tasks;
  • an overall feeling of calm while feeling completely energized.

You may experience your own set of benefits when you start working out while fasted, and they might be completely different than mine! After you adapt to working out while fasted, take a moment and see what you are benefitting from!

For the crazy athletes.

Now, NOW. If you’re a crazy cardio addict or a powerlifter, you may want to do a little research about doing these types of workouts in a fasted state.

There are several endurance athletes on the boards that I follow that run extreme distances completely fasted. This seems crazy to me, but as they mention on these boards, they started their fasted training schedule on the lighter end and built up the miles as their body adapted to running distance in a fasted state. 

I assume the same holds true for powerlifters, ect. My guess is that these folks really have to pay attention to what they eat during their eating window to ensure they get enough fuel from that meal to get them through these workouts in the fasted state. Not sure. But, do your research if this is you! 


I think the biggest misconception about working out in a fasted state is that:

  • a) the body will start to “burn your muscles” in search of a fuel source, and;
  • b) working out fasted is not sustainable.

The reality is anything can be unsustainable if you haven’t properly prepared for it, right?

Think about the tips I’ve provided in this post. Know your limits. Listen to your body and be smart.

If you start small and take it slow, you will be adapted in no time.

Keep Reading!


If you’re new to intermittent fasting and need some help getting started, go ahead and sign-up for my FREE 3 Day Short Course! It’s a self-paced email course that you can read at your leisure. Sign-up below!

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The information on this site is not intended or implied to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on the stuckassdown blog.


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