Fasted Cardio: Is It Good for Fat Loss or a Waste of Time?

by Matt Weik, BS, CSCS, CPT, CSN

Has anyone ever asked you to do cardio on an empty stomach? Like many things popping up right now, fasted cardio is still a hot topic in the fitness and weight loss world.

Like many other health trends, it is a highly debated topic with both fans and skeptics going toe-to-toe on who’s right and who’s wrong. Some people consider it a fast and effective way to lose stubborn body fat, while others think it is a complete waste of time and effort, and the only thing you’re doing is burning away your lean muscle tissue.

Who’s right? Who’s wrong? Let’s dive a little deeper into the topic and discuss the benefits to see if it’s the right weight loss method for you to engage with.

What is Fasted Cardio?

Fasted cardio is a type of exercise that you do after an extended period of fasting. For example, if you eat dinner at 7pm and then wake up at 6am the next morning, you’ve fasted for 10 hours — and that’s when your body is most likely to have used up its glycogen stores (the energy it gets from carbs).

What this means is that your body can now shift over to burning fat as fuel, so fasted cardio can potentially help you burn more fat in the long run.

While many fitness professionals go by this theory, it is still quite a debated topic in the fitness industry. Is it effective or not? The answer still remains contested, and it is ultimately your decision whether or not you want to exercise on an empty stomach and in a fasted state.

But let’s actually unpack some of the benefits associated with fasted cardio to get a better understanding and see if you feel the juice is worth the squeeze.

The Benefits of Fasted Cardio

The whole idea behind fasting is that if you fast on an empty stomach, preferably overnight, your body will run out of glucose, which is the main source of energy, and hence your body will shift over to using stored body fat for fuel.

It was found in a review that in several studies, fasted cardio led to improved metabolic performance after the training session was complete. Although, that same review noted that for prolonged aerobic activity, eating pre-workout increased performance. Therefore, you need to weigh what your goal is. If it’s burning fat, fasted cardio may be the way to go. If performance is the goal, then having a pre-workout meal is ideal. Only you can decide what is most important for you and help you get closer to achieving your personal goals.

More research is needed to make solid claims, but fasted cardio also has some other benefits. Here are some potential benefits of fasted cardio:

  • If you like to work out on an empty stomach, fasted cardio can be a good option, especially if you have a sensitive stomach or feel more energetic without eating before a workout.
  • If you have a tight time schedule, fasted cardio can save you time by saving you from preparing, eating, and digesting a meal before working out.
  • If you are accustomed to intermittent fasting, fasted cardio lets you exercise before you consume food for the day.

Will Fasted Cardio Help Improve Fat Loss?

Being able to effectively burn body fat loss is directly linked with your ability to burn more calories than you consume daily. When it comes to fasted cardio, research is mixed on whether it actually promotes fat loss or not. That’s probably not what you wanted to hear, but let’s look at the research findings.

In one study, 20 young girls were put into two separate groups — one group did 1 hour of fasted steady-state cardio, while the other did 1 hour of nonfasted steady-state cardio. Both groups worked out 3 days per week for 4 weeks while following a diet that focused on being in a daily caloric deficit. In the study, researchers did not find any difference in weight loss or body composition between the groups.

There are some studies, however, that do promote fasted cardio being an indicator of fat loss during a workout.

According to a review of 27 studies that were published in the British Journal of Nutrition in 2016 noted that aerobic exercise, if performed in a fasted state, can lead to higher fat oxidation than exercise performed in a fed state.

This research shows that while fasted cardio can burn more calories than fed cardio, the difference it makes to a daily calorie expenditure in a 24-hour time span is really insignificant.

Fasting can make it challenging to complete a workout, whereas eating before could help you to complete your training session.

According to a 2018 review of 46 previously published studies from the University of Limerick in Ireland, it was found that when participants consumed food before a cardio session, they could work out for longer periods of time. Working out in a fed state resulted in longer aerobic workouts, showing that while fasted cardio may have several other benefits, it does not always lead to longer or better quality workouts.

Is Fasted Cardio Safe?

Now that we have a brief understanding of the benefit of fasted cardio, and the reality behind it, let’s discuss whether it’s safe. If you are generally healthy, then it is safe to incorporate short or moderate-length steady-state fasted cardio programs into your workout routine.

Although, if you exercise for an extended period of time or plan on engaging in high-intensity workouts, fasted cardio can be challenging or risky due to the potential side effects of dehydration or low blood sugar, which can lead to dizziness, lightheadedness, shaking, or even passing out.