Fasted cardio is exactly what it sounds like– doing a cardio workout in a fasted, or unfed, state. Usually this is done after waking up before consuming ANYTHING prior to training– no food or drink other than water–this includes
BCAAS and preworkouts. The theory behind fasted cardio is: you are completing physical activity when your body has received no fuel from food–so the fuel for your workouts will come from what your body already has stored. What does your body have stored? Fat, oh that glorious fat!
How your body uses that fat? While in a fasted state your insulin level is low– insulin decreases the breakdown of fatty acids, therefore when your insulin levels are low, your body use more fat for energy¹. So, on the other hand, in fed cardio, after you eat a meal your insulin levels rise, this shuts down the fat breakdown and uses the energy from the food consumed for your workout.
Physiologically this all sounds good, fasted exercise must be the way to go, right?! So I did some research. The two research articles I found have the same result.
The two credible research studies found no difference when comparing the end result of fed vs. fasted cardio.
Article 1 studied the difference in overall body composition, both groups (fed and fasted) lost significant amount of weight and fat mass–but no significant difference between the two groups was noticed³.
Article 2 measured the effect of lipid utilization (bodys ability to utilize stored fats/lipids for energy) in fasting vs fed cardio –the authors concluded that lipid utilization is NOT enhanced with fasting before physical activity².
The reason why there may be no difference between fasted and fed exercise is because the fed cardio uses the energy that is readily available–thus leading to an optimal workout. With fasted cardio, you may be more sluggish and tired– you may not get the most out of the workout4
Therefore, in fasted your body may be utilizing your stored energy/fat, but in fed your burning more calories, and calories burned=fat lost.
Also, in my personal opinion, what happens when I get to the point of extreme hunger? I binge eat! Therefore, all of the work I did during the fasted cardio session would be null and void as soon as I walked into my kitchen.
All in all, based on the research I’ve read, I am not inclined to perform cardio in a fasted state, nor do I recommend it. I’ll take my morning oatmeal over hunger and misery any day! 😛
¹Matthews, Michael. “Why and How I Use Fasted Cardio to Lose Fat as Quickly as Possible.” Muscle For Life. Web. 8 July 2015.
²Paoli A, Marcolin G, Zonin F, Neri M, Sivieri A, Pacelli QF. Exercising
fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio
and excess postexercise oxygen consumption after a bout of endurance training.
Int J Sport Nutr Exerc Metab. 2011 Feb;21(1):48-54.
³Schoenfeld, Brad Jon et al. “Body Composition Changes Associated with Fasted versus Non-Fasted Aerobic Exercise.” Journal of the International Society of Sports Nutrition 11 (2014): 54. PMC. Web. 8 July 2015.
4Simmons, Amber. “The Scientific Evidence Surrounding Intermittent Fasting.” Intermittent Fasting Scientific Evidence | EAS Nutrition Research. EAS Academy. Web. 8 July 2015.