What is ‘fasted cardio’ and is it useful for endurance performance?

Nutritionist and endurance athlete Mick Chapman explains the concept of fasted cardio, and breaks down when it can be useful for athletes.
What is fasted cardio? Photo by Fitsum Admasu on Unsplash

Editor’s Note: In this series on nutrition and performance, Sydney-based endurance athlete and registered public health nutritionist Mick Chapman delves into a range of topics related to running, fitness and the energy sources we put into our bodies. In this first post, he takes on the concept of ‘Fasted Cardio’.

You may have seen the term on someone’s Strava page — an activity titled: 60 mins of fasted cardio. Or maybe you’ve heard it tossed around at the gym, or out running with friends. 

But what exactly is fasted cardio? 

Essentially, fasted cardio is the personal choice to intentionally not eat food (or accidentally forgetting to fuel) before a workout or training session. 

This, more often than not, occurs in the morning as there is usually a longer time period where you haven’t taken in food due to sleep.

This may be a good way to reduce the feeding window in your day if your goal is overall weight loss. And if viewed through this lens, fasted cardio is just another tool in the nutritional toolbox.

However, it’s perfectly fine if you don’t ever use this particular tool, as there isn’t anything magical about its benefits. Let’s take a closer look. 

Fasted cardio often happens (unintentionally) in the morning, when athletes don’t eat before training. Photo by Fil Mazzarino on Unsplash

What’s the appeal of fasted cardio?

First, let’s understand the principles of what’s happening in the body.

Carbohydrates are the main fuel source for the human diet — unless we decide to modify that pathway. That’s where fasted cardio comes in. 

During fasted cardio, your body has less carbohydrate stores, and so it primarily uses fat as an energy source. 

It’s really important to note the difference here between fat oxidation (using fat as an energy source) and fat loss (actually contributing to losing weight). 

They are two separate things and people often confuse them.

The main drawcard to fasted cardio is the ‘fat burning’ idea. This is just fat oxidation (using fat as a fuel source) during your cardio sessions. But importantly, this doesn’t always equate to fat loss. 

If weight loss is your main, overarching goal, a meaningful calorie deficit over time is more important than fasted cardio. 

One key reason is this: after a fasted cardio session, it’s very likely your body will revert to using carbohydrates as its main fuel source, as quickly as your very next meal post-training. Consequently, everything balances out without us even noticing. 

But what about fasted cardio for performance?

The concept of fasted cardio for endurance performance has garnered a lot of interest from coaches and athletes. 

People often refer to it as ‘training low’ or being in a ‘glycogen depleted state’. 

The thought process behind its potential benefit is this: if we have a higher availability and storage of fat fuels in our body than we do carbohydrates, why not try and tap into that reserve? 

Unfortunately, it’s  not as simple as turning one fuel source off, and another on. They work in different ways.

Carbohydrates are the primary fuel source for performance. Their stores usually run out with a longer exercise bout, at around the 60-90 minute mark. This is why ‘Carb Loading’ and race nutrition tends to focus on topping these stores up consistently before and during an event, to minimise depletion. 

When you ‘Bonk’ or ‘Hit the wall’ in a race, chances are you’ve run out of the right (carbohydrate) fuel. Photo by Quino Al on Unsplash

If you’ve ever bonked in a race, you are experiencing first-hand your muscle glycogen becoming very low. You can think of fat as the fall back fuel source; the Plan B, so to speak. 

Fat is then used as a fuel source, but it isn’t as quick to metabolise and can’t keep up with your pace, hence the bonk. Your body is telling you to slow down or stop.

How to use fat-fuel effectively in training

When thinking of using fats in training (training low), there are two ways of doing it. Either training in the morning before eating, or by completing two sessions in a day.  (The latter can work because sometimes it’s hard to restore glycogen levels in between these workouts). 

Research shows that maximal fat oxidation rates occur at moderate intensity exercise corresponding to 59-64% of VO2 max in endurance-trained individuals. 

Fat oxidation is almost zero when at or above 90% VO2 max. That means, as soon as intensity increases past a moderate point (roughly 6 out of 10 on the effort scale), fasted cardio loses more and more of its benefit to our performance. 

Performance benefits from fasted cardio (and fat oxidation) max-out at 6/10 effort level. It’s not recommended for race day. Photo by Alessandro Venturi on Unsplash

When you need that last minute ‘burst’ or sprint in a race, or are trying to hit 5th gear in a workout, fat fuel isn’t going to cut it. Carbohydrate fuel is a far better option.

Therefore, while fasted cardio (or being in a glycogen depleted state) might be suitable for lower-intensity training sessions (think recovery runs and moderately paced tempo runs), it’s not recommended for race day. 

Nor is it recommended for higher intensity sessions or strength-based training. It may even inhibit performance outcomes for these sessions and lead to feelings of discomfort.

What’s the bottom line?

There are many factors at play when eating and exercising, especially for endurance training and racing. My advice is simple: do what works for you! 

If fasted cardio works for you, that’s great; but try not to implement it in training any more than twice per week. Keep in mind that an overall carbohydrate intake is still recommended for endurance athletes (even if one or two sessions are fasted). 

If you are struggling to get through a session or get light headed, don’t sacrifice the session because someone told you you need to be fasted. 

Get in touch: For any references or to read more on this topic, you can reach me on: mchapmanph@gmail.com or on IG @chapp.o.

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