What is fasted cardio & is it useful for endurance performance?

Fasted cardio is the personal choice to intentionally not eat food (or accidentally forget to fuel) before a workout or training session. This more often than not occurs in the morning as there is usually a larger time period where you haven’t taken in any 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. If viewed through this lens, it 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. 

What’s the appeal?

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

Carbohydrates are seen as 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 so 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) & fat loss (actually contributing to losing weight). They are two separate things and people often get them confused.

The main drawcard to fasted cardio is the whole ‘fat burning’ idea. This is just fat oxidation (fat as a fuel source) during these sessions. But importantly, this doesn’t always equate to fat loss. A meaningful calorie deficit over time is more important than the idea of fasted cardio.

After a fasted cardio session, It’s likely our bodies revert back to utilising carbohydrates as a fuel source with the next meal post-training so everything balances out without us even noticing.

But what about for performance?

The concept of fasted cardio for endurance performance has had a lot more athletes and coaches discussing this theory over the years. It is often also refer to as ‘training low’ or being in a ‘glycogen depleted state’. The thought process behind it is that we have a higher availability and storage of fat fuels in our body than we do carbohydrate, so why not tap into that reserve? It’s not as simple as turning one fuel source off or on. They work in different ways.

Carbohydrates are still recognised as 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 focus on topping these stores up consistently before this happens. If you’ve ever bonked in a race, you are experiencing first hand your muscle glycogen becoming very low. Fat is then utilised as a fuel source but it isn’t as quick to metabolise and can’t keep up with your pace- hence the bonk to tell you to slow down or stop.

When thinking of utilising fats in training (training low), there are two ways of doing it. Either training in the morning before eating or a second session in a day (as sometimes it’s hard to restore glycogen levels between depending on meals). It’s documented that maximal fat oxidation rates are reached 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 meaning that as soon as intensity increases past a moderate point, fasted cardio loses more and more of it’s benefit to our performance. When you need that last minute ‘burst’ or sprint in a race, not having carbohydrate fuel to rely on is not in your best interest. 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.

Bottom line

There are many factors at play when eating & exercising, especially for endurance training and racing. 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 a weekly overall carbohydrate intake is still recommended for endurance athletes (even if one or two of your 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.

For any references or to read more on this topic, reach out and I’ll happily provide the articles that helped inform this blog.

What about fuelling whilst racing? More on that in a future post.