Is working out while intermittent fasting safe?

Share on facebook
Share on pinterest
Share on twitter
Share on linkedin
Share on email
Share on print

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. I was very concerned, 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 IF and was told that I don’t have to fuel for my 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?

All good questions.

So, is working out while intermittent fasting safe?

The short answer.

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

The long game.

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!”

Excellent news! You want to run a marathon!

Now, you’re sitting there on the edge of your bed and you’re excited and super anxious to RUN THAT MARATHON, 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 bodies time to get used to running those additional miles and ADAPT to the distance.

If we were to step right out of bed that 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.

Same / Same.

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

If our bodies are used to training and working out while fueled with a pre-workout supplement, shake, or your trusty pre-workout snack, then your body will completely tank if you 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?

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 v. Fed

Before we move into the juice of this post, let’s first take a minute to differentiate between 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)

Basically, 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.

Now that we know what the fed state vs. the fasted state is, let’s talk a little more about how the body fuels itself during the fasted state while we work out.

Welp, the information is basically the same! If you are in a depleted (fasted) state when you decide to workout, your body will pull from it’s glycogen stores (liver) to fuel your workout. Once those stores are depleted, your body will pull from your fat stores to fuel your workout. But, be prepared for a small adjustment phase. It takes time to adjust to fasted workouts.

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)

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 and eat something.

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

Know your LIMITS!

That should go for everything, right! I mean, I’m not going to benchpress 300 pounds today because I feel like it. Like the marathon scenario, I need to work my way up to it.

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?

I just mentioned bench pressing 300 pounds. Let’ be clear, it’s NOT something on my bucket list, but if it was something that I wanted to do, I’d have to train and work my way up to bench pressing 300 pounds, right?

This same idea applies to working out in the fasted state.

Start small.

See how it feels.

You can’t expect to adapt to working out on empty right away, especially when you’re body is used to pulling fuel from a pre-workout to keep your body going.

You have to allow your body time to adapt to pulling fuel from what you have stored in your body.

Adjust accordingly.

Here’s the deal. If you are an intermittent faster, you don’t HAVE to workout in the fasted state. Read the post I have highlight below. I document how to workout while intermittent fasting. 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?

Lucky me, I guess?

Personally, it didn’t take me too long to adapt to working out fasted because I just swapped my pre-workout with black coffee. (Yes, I’m a black coffee psycho.) And, to be honest, I didn’t see much of a difference between my pre-workout supplement and my black coffee.

My workouts felt the same.

Read: The Benefits of Drinking Black Coffee.

At the point I went full-on fasted workout, my body was already adapted to very early morning workouts with that pre-workout supplement. Swapping out the pre-workout didn’t affect me that much.

BUT, I want to point out that it DID take me a long time to adapt to early morning workouts.

I had tried for YEARS to be an early morning workout person, with zero success. But, when I had 50 pounds to lose, and the only time I could get my cardio in was in the morning, I worked really hard to make that adjustment.

Read: My Weight Loss Story.

So, while there was little adjustment to working out fasted, there was a LOT of adjustment to working out in the morning,

Benefits, anyone?

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

Besides tapping into your fat stores to fuel workouts, for me personally, I found that I have fewer stomach issues when I workout fasted. 

I was a distance runner for years and suffered from the “runners trots” on every single run. It was terrible.

Something the size of a little chocolate chip in my system would turn into the freaking Niagara Falls five minutes into a run. I could never find a pre-workout protocol that would work with my stomach, and all runs ended the same. A mad dash to the toilet. 

The crazy part is that I started running completely fasted about a year ago and have had no issues since.

I ran two half marathons, one 10 miler and a 10K last year completely fasted and felt really good. I could even wait to use the potty when I get home! 

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! 

Clear things up?

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.

I mean, yeah, me going out and trying to run 26.2 miles TODAY with no training is unsustainable. I’d drop at the 3 mile mark because I haven’t properly trained and my body, at this time, is not adapted to running 26.2.

That’s not rocket science.

What it comes down to now, and what it will always come down to, is what works the best for you.

Only you can know and test your own limits.

If working out while fasting isn’t your jam, then don’t do it! Read my working out while intermittent fasting post. It is possible to be an intermittent faster and NOT workout while in a fasted state.

Keep Reading: Head over to my Weight Story Page to read more about intermittent fasting and my weight loss journey.


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!

Like it, Pin it!


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.

Share on facebook
Share on pinterest
Share on twitter
Share on linkedin
Share on email
Share on print

Leave a Comment

Your email address will not be published. Required fields are marked *