Exploring New and Old Health News Today – Fasted Cardio

Introduction

Fasted cardio is a debate amongst health practitioners that can ignite vigorous debate which can turn into gigantic arguments with abuse being hurled around. A consensus exists that if you do you cardio workouts before the morning you’ll burn more fat. On the flip side of that argument there is another consensus that if you fuel your body before you cardiovascular exercise you will be able to work harder increasing post oxygen workout consumption thereby increasing the amount of calories you expend.

What does the Science say?

Fat Oxidation in Fasted Cardio:

A decreased 24-h fat to carbohydrate ratio is one of the metabolic characteristics of subjects pre-disposed to obesity (Zurlo et al. 1990; Barwell et al. 2009). Glycogen (stored glucose ) has been shown to reach its lowest levels after an overnight fast and can be further decreased after an exercise bout.

In a recent study by Iwayama et al. (2015), exercise was performed at a workload of 50% VO2 max after breakfast, lunch, and dinner or without an exercise session (control). Carbohydrate depletion after the morning exercise session was equivalent to 18% of whole body glycogen storage (~2300kcal). Also, the lowest relative value of carbohydrate balance was found to be negatively correlated with fat oxidation (the use of fat as a fuel within the body). Interestingly, accumulated protein oxidation over 24-h were not different among each of the 4 trials.

This study represents the beneficial long-term effects of exercise in a carbohydrate-depleted state on glucose tolerance, insulin sensitivity, endurance capacity and fat oxidation have been reported (Van Proeyen et al. 2010; Hansen et al. 2005; Yeo et al. 2008). Also, although exercising whilst fasting, protein breakdown was no different between trials.

Fasted Cardio vs. Non-Fasted Cardio

In one study completed by Schoenfeld, B.J. et al (2014)  20 female students were assigned to a fasted cardio or non-fasted cardio group. Both groups were in a calorie deficit during the study which lasted four weeks. The groups trained 3 times per week consisting of 1 hour of running on a treadmill. It found that there was no significant difference between the fasted or non-fasted groups in body mass or BMI.

Conclusion

In the study by Iwayama et al. (2015), 24-h fat oxidation is increased by exercise only when performed before breakfast under energy balanced conditions.Fat oxidation does not always necessary mean you will lose weight, but if you are in a calorie deficit then you the weight loss most likely will come from fat and glycogen. In the study done by Schoenfeld, B.J. et al. (2014) There were no significant differences between the fasted and non-fasted groups.

Both studies have their limitations as all studies do. It’s up to you to read the studies and come up with your own conclusion. Studies that last longer and limit further the level of reporting errors need to be conducted in a repeated study before the debate to what is better, fasted or non-fasted cardio? For now if fasted cardio suits your training goals and lifestyle or non-fasted does then that’s what you should continue doing.

INTERESTINGLY fasted cardio did result in protein breakdown!

Updated 1st November 2019

References:

Iwayama K, Kurihara R, Nabekura Y, et al. Exercise Increases 24-h Fat Oxidation Only When It Is Performed Before Breakfast. EBioMedicine. 2015;2(12):2003–2009. Published 2015 Oct 30. doi:10.1016/j.ebiom.2015.10.029

Schoenfeld, B.J. et al. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr 11, 54 (2014) doi:10.1186/s12970-014-0054-7