What is Fasted Cardio? Everything You Need to Know

Last Updated: January 14, 2022

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Are you a person that loves to knock out a workout first thing in the morning? There’s a high possibility that you’ve already been doing fasted cardio. We’re constantly looking for ways to trim down and get that ideal physique for ourselves. It might be because of an upcoming event such as your wedding or you could just be looking to improve your health. 

A recent fitness trend that has been quite popular on social media, with a fair amount of research done on it is “Fasted Cardio. Fasted Cardio is just that, doing low-to-medium intensity cardiovascular activity when your body is in a fasted state. 

All About Fasted CardioHow do we know we’re in a fasted state? According to Allison Childress, Ph.D., assistant professor at Texas Tech University for Nutritional Sciences mentions “A truly fasted state starts at about 8–12 hours after your last meal”. 

This instantly ringed a bell inside my head. As someone who does a 16:8, an intermittent fasting diet, this became interesting to me almost immediately. In a nutshell, my 16:8 diet is another popular diet where you eat during an 8-hour window and stay in a fasted state for the remaining 16. My Diet looks something like eating from 12 pm to 8 pm and then not having anything until the 16 hours are done.

As someone who normally went for a run or did weights at around lunchtime at work, the idea of doing my workout early in the morning sounded ludicrous. However, after seeing the benefits which I’ve outlined below I thought that I’d give it a go. I’ll go more into my story as we churn through the benefits and also the negative side of fasted cardio.

What Are The Benefits of Fasted Cardio?

1. Using Fat For Energy

There has been research done that when you run or do a workout in a fasted state that you’ll be burning your fat for energy. Our body primarily uses glycogen for its energy which comes from breaking down carbohydrates. 

When you wake up in the morning, having 8-12 hours since your last meal your body would have used up most of its glycogen stores for energy. When you don’t have your primary energy source, your body starts to look for the next best thing: Fat. 

A study has claimed that it is possible to burn up to 20 percent more fat when you’re doing cardiovascular activity in a fasted state. 

One thing that I personally enjoyed about doing fasted cardio was the fact that I could wake up in the morning and check my weight every day. Whilst checking my weight daily, I knew that my body would more or less be on its baseline of weight.

By checking it every few days being in the same fasted state I could clearly see my weight going down. This was a lot more motivating to me than seeing the fluctuating numbers on the scale when you’re constantly having large unpredictable meals.

2. Fewer Stomach Cramps

This is one that I thought I’d put in here as I know many people suffer from this after a meal or taking in too much water. Whenever I had a lot of water before my run or a meal too close to it I would get stomach cramps.

If you’ve had them before you know how frustrating they are when you’re trying to beat a PR (Personal Record) or perform your workout. By being in a fasted state my body would be empty of water and food which would help me avoid this annoying problem.

3. Avoid An Upset Stomach And Trips To The Toilet!

We’ve all been there, and it doesn’t take an expert in sports science to know that when you go for an exercise shortly after a meal or a smoothie that you could end up in the toilet. When you’re in a fasted state you know that your stomachs pretty much run empty, and you can avoid that rush to the public toilet during a workout.

4. You're Allowed to Have Coffee - But There's A Fine-print!

Black Coffee During FastsNow I know that this might contradict point #3 as Coffee can surely send you to the toilet. However, according to the USDA having black coffee is around 2-5 calories per cup which is enough to keep you in a fasted state. One thing to note is that you must have the black coffee straight. The moment you start adding sugar, milk or cream you’re going to break your fasted cardio. If you don’t like coffee, go green tea.

For those of us that struggle if we don’t have a coffee hit, this is a great benefit for you. You might also like to head to the toilet before your workout or run and coffee will surely help you with that! 

Now with all these benefits in mind, I thought that it would be an awesome idea to start doing my runs in a fasted state! Whilst this is my own personal experience and not everyone is the same what I’ve found is the results to be beneficial. 

I’m someone that has a very sensitive stomach and I’m prone to muscle cramps as I always seem to consume too much water without realizing. I started doing my runs first thing in the morning after having half a glass of water.

I loved it!

I was able to enjoy my run without all the subtle cramps and burps that I sometimes tend to get, even 2-3 hours after eating! Being on a 16:8 diet however, I wasn’t able to start eating until 12 PM. I would normally finish my run by around 7 am and then I’d have to get ready for work. 

At the start, I got really really hungry. However, I’ve noticed that as time went by my body became more accustomed to using fat for energy during these periods and I was fine not eating until 12 PM. 

At the end of the day, it’s not for everyone and you just have to do what works best for you. I love trying different things though, and I’m very happy with this diet and exercise regime right now. However, as with anything, there are drawbacks.

What's The Negative Side of Fasted Cardio?

1. Potentially Less Energy And in Turn, Decreased Intensity

At the start of the article, I mentioned that “Fasted Cardio is just that, doing low-to-medium intensity cardiovascular activity when your body is in a fasted state.”. Whilst this might serve as a warning – it’s more so geared towards the limitations of our machine, the human body.

I’ve done workouts in the morning where the night before me and my wife decided to lavish in some McDonald’s Nuggets and Cheeseburgers (It’s my curse!). What I’ve noticed with these workouts is that #1: I broke my fast by having late-night McDonalds and #2: I had A LOT MORE energy the next day to perform a workout in the gym. 

This is quite the opposite for me if I go to the gym and lift weights in a fasted state. Whilst cardiovascular activity such as running doesn’t pose this problem for me, lifting weights does. Some of you might only be lifting weights and not running or doing cardiovascular activity. For those of you that are like this, you might see a decrease in your energy levels when lifting weights.

At the same time, if you’re going for a long run – you will definitely start hearing your stomach grumble towards the end. 

2. Increased Bodily Stress

StressWhat I’m talking about here is increased cortisol levels. Cortisol is the body’s stress hormone. When you’re fasting your body has increased cortisol levels and at the same time, exercise increases it too. What does this really mean for us in layman’s terms though?

Well, increased levels of cortisol can lead to increased weight gain, muscle weakness, severe fatigue, and irritability. 

When I was training for my first trail marathon, I didn’t have a proper recovery plan and I just kept training days on end. What I noticed was that my resting heart rate went up and that I had a lot of trouble falling asleep. This was due to my increased cortisol levels.

This can be a topic all on its own but it’s basically something to look out for. Make sure that you’re always taking a proper amount of rest days and that you’re getting the right macronutrients that your body needs.


In the end and as in everything in life, it’s all about balance. Find out what works for you, some of you might have already been doing fasted cardio without knowing. Some of you might already be fasting but not exercising in the morning and may want to give this a go. 

Consider it as a tool to use and not as an end-all solution to your fat loss worries. Don’t buy into the fads that many companies are selling, do what works best for you. The beauty of fitness is that we can experiment with it as much as we want to tailor the best solution for ourselves. 

What’re your thoughts on fasted cardio? Why not let me know below.

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