Does “Fasted Cardio” Help You Burn More Calories from Fat?

Posted on Oct 21, 2015 in Uncategorized

Does Fasted Cardio Help You Burn More Calories from Fat?

From the look of our responses to the Monday Mythbusters question in which 97% of respondents answered NO, this was interpreted as a very simple question. Unfortunately, nothing is that simple, right 😉 The answer to this question, as it is written, is actually…YES. The deception, however, is in the details…

First, a little physiology…

There are 3 basic substrates that the body uses to fuel movement: Carbohydrate, Fat, and Protein. It IS broadly accepted that our bodies will preferentially draw more from fatty acids to fuel low-to-moderate intensity aerobic exercise (<70% HRmax), and more from glucose or carbohydrate stores to fuel high intensity exercise (>70% HRmax). Protein will be significantly broken down to use as fuel only under extreme circumstances.

If you’re not familiar with the concept of “fasted cardio”, the theory suggests that your body will preferentially use a greater amount of fatty acids to fuel low-to-moderate intensity aerobic exercise when glucose availability is at its lowest point, which occurs following an overnight fast. And, in fact, a number of studies have shown that this is TRUE.

However…the real question that you need to ask is “will this strategy help me lose body fat over time?” Many assume that if “fasted cardio” does what it’s supposed to physiologically, that regularly performing cardio activities in a fasted state will result in greater body fat loss over time. After all, does it really matter what substrate you burn during a workout if doesn’t result in you looking better naked? This is where we begin to poke holes in this strategy.

A 2014 study published in the Journal of the International Society of Sports Nutrition showed that body composition changes were similar after 6 weeks of either fasted or fed low-to-moderate intensity cardio. The authors, Brad Schoenfeld and Alan Aragon concluded that it’s basically a “toss up” and that individual preference should guide your decision on how to fuel for your “cardio” workouts. In other words, eat something so that your stomach isn’t tossing and turning while you labor away on the elliptical.

In the big picture, it’s important to remember that long-term fat loss is a consequence of thermodynamics. You need to expend more calories than you consume. Exercise can help by increasing daily calorie expenditure in a few ways.

• You burn calories directly while you train.
• You can manipulate your workout so that you burn more calories in the 24 to 48 hour period following your workout.
• You can use exercise to build muscle, which up-regulates your resting metabolic rate.

The best way to optimize these benefits is to incorporate higher intensity cardio, which burns more calories per minute, and strength training to build muscle. Both strength training and high intensity interval training have been shown to maximize both calories burned while training and excess post-exercise oxygen consumption (EPOC) or the 24-48 boost in thermogenesis you get from exercise. Moreover, each of these strategies has been shown to result in greater total body fat loss over time compared to low-to-moderate intensity aerobic exercise.

High intensity training requires that you fuel appropriately to provide sufficient energy to complete the activity. Although it may be possible to train your body to function in a fasted state, it is NOT possible to achieve your best effort, which will ultimately limit your calorie expenditure.

There is also some evidence that pre-fueling for your workout will also increase EPOC, and help you build muscle more effectively by preventing muscle protein breakdown for energy use. Two more reasons “fasted cardio” fails in the long haul.

In summary, although there may be a physiological premise for performing low-to-moderate intensity cardio in a fasted state, if your goal is to lose body fat you WILL be better off eating in preparation for your workout instead. Show up to train FED, feel energized while you train, and reap the rewards of your efforts.

What we hope to accomplish with this series is a more in depth understanding of these common questions so that you can make informed decisions on how to apply research to your training so that you can accomplish your goals faster. Also, to help you learn to recognize the “spin” that the media places on research, and to view the broader application of individual studies.

If you have any myths or want to know whether something you’ve read or seen in the news is in fact true or false let us know and we’ll add it to an upcoming post.

We’ll look forward to seeing you in Training!

PS. Feel free to comment…

Schoenfeld, BJ, et al. Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition 2014, 11:54