What You Should Know About Fasted Cardio

Cardiovascular exercise should be a part of everyone’s fitness regimen, regardless of what their goals are. Many people associate cardio with fat loss alone, but the benefits go far beyond that. In addition to the cardiovascular system benefits, your respiratory system can be enhanced as well. 

As with all aspects of training, there are discussions about what type of cardio is best as well as when to do it. In regards to the latter topic, is it better to do cardio first thing in the morning while you’re in a fasted state, or should you eat at least one full meal before hitting the elliptical or going on a run?

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The Belief Behind Fasted Cardio

Let’s look at both sides of this controversial coin before we dive into the topic any further.

First, we’ll look at the advocacy behind fasted cardio. The belief is that if you get up and do your cardio first thing before eating a meal or taking any supplementation, then your body will not have any calories to use for energy and kickstart your metabolism.

Therefore, it will have to resort to using your excess fat storage in order to help propel you through the session. As a result, you’ll burn body fat and get leaner over a period of time.

Related: Calculate Your Basal Metabolic Rate (BMR) & Calorie Requirements

Many people in the fitness community credit Bill Phillips for popularizing the movement for fasted cardio. He proposed the method in his popular 1999 book, Body-For-Life by recommending three 20-minute High Intensity Interval Training sessions a week, first thing in the morning. His book went on to become a New York Times Bestseller, and thousands of people shared photos of their transformations using the program over the decades since.

In the other corner are those who speak against fasted cardio.

The guidance behind this belief is that your body will actually resort to using muscle first instead of your fat storage. Additionally, if you eat a healthy meal first, it will actually kickstart your metabolism before you start the cardio session.

As a result, you will improve your performance during that session, and you’ll burn more fat throughout the day because your body has already been fed and primed before you started that workout. We won’t argue in favor or against fed cardio workouts here, but let’s explore more about fasted cardio and if it would be a good choice for you.

Related: The Best Cardio for Fat Loss: A Science Based Approach

What Does Research Tell Us?

There has been enough interest in fasted cardio that studies have been done to either prove or disprove its effectiveness.

One study that was published in the British Journal of Nutrition in 2016 looked at trials that compared the metabolic effects of aerobic exercise up to two hours long or less. Some of the subjects performed the cardio fasted while others ate a meal first. Fat oxidation was greater in the fasted subjects versus those that were fed. Thus, the study concluded that “aerobic exercise performed in the fasted state induces higher fat oxidation than exercise performed in the fed state.”¹

Going back even further, a 2014 study determined that body composition changes occurred regardless of whether the cardio was performed fasted or after being fed first. While the study didn’t determine one was superior over another, the take away for those that opt for fasted cardio was that it still proved to be effective.²

It May Not Work for Everyone

It's important to note that there are studies that have proven fasted cardio may not be the best choice for everyone.

A 2012 study by Cambridge University focused on 12 active men in which some performed cardio after an overnight fast and without breakfast, and others ate a meal first. That study concluded that while there was no difference in energy intake or performance, the group that ate first had a more suppressed appetite later in the day, meaning they were less likely to eat excess calories that they didn’t need.³

What Type of Fasted Cardio is Best?

When Phillips first spoke of fasted cardio, he suggested his HIIT workouts because of how the body has to respond to the various levels of effort being given throughout the workout. Over the years since, the other major topic of debate in the fasted versus fed discussion is whether HIIT or Low Intensity Steady State (LISS) training would be best if you were to choose training fasted.

A 2020 study looked at the effects of fasted HIIT cardio on 32 obese women that participated, and what they found was some of the women reported a decrease in performance while others reported no changes of significance. There were also no major changes in fat oxidation. The study went on to determine that endurance athletes wouldn’t benefit from doing HIIT workouts in a fasted state, either.⁴

Related: Training Talk: Should You Do HIIT or LISS Cardio?

What Should YOU Do? 

At the end of the day, while it's encouraged to consider what the research shows, you should also do your own research on your own body to find what works for you.

The one takeaway that has been adopted by most in the fitness community is that doing HIIT workouts in a fasted state isn’t a good idea. The fat oxidation isn’t much greater, if at all, and you could negatively impact your lean body mass as a result. So, if you opt to perform fasted cardio, it should be in a LISS fashion.

What does that look like in application? Measure your weight and bodyfat percentages and keep those numbers somewhere. Keep those numbers close because you’ll need them in four weeks. You should be following a regular weight training schedule and balanced nutrition program as well.

Starting the next day, when you get up, drink 8-16 ounces of water before you do anything. Water doesn’t impact your fasted state, and you will need the hydration for your muscles. 

Then, go on a moderate speed walk or ride a cardio machine such as an elliptical or exercise bike for 15 minutes. Repeat this for each day you train in a week. So, if you train four days a week, then do a fasted cardio session four days a week as well. Just make sure you take at least one day off from all training regardless of how advanced of an athlete you are.

After one week, bump up those sessions from 15 minutes to 20 and go for another week. The following week, go up to 25 minutes, and then do 30-minute sessions for a fourth week. At the end of the fourth week, measure your weight and bodyfat again. Did your numbers improve? If so, then fasted cardio would work for you. If not, then you should repeat the self-imposed study again, but this time, eat your first healthy meal of the day before you take part in the cardio session.

Conclusion

Science has proven that fasted cardio can be beneficial when applied in a sensible manner. Fasted cardio can have its place in a fitness program, regardless of whether you’re a beginner that is just now starting to train, or an advanced athlete that is looking to reach a new level of personal excellence.

If you decide to try fasted cardio for yourself, low to moderate levels of effort would be better for more people than going all out in a HIIT session. If you’re trying to lose that last bit of bodyfat to reveal a sculpted physique, then fasted cardio may be the key that finally unlocks your full potential.