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Aside from politics or religion, few topics are as divisive as fasted cardio, and everyone lands squarely into one of two camps: It’s the best thing since sliced bread, or it’s the worst idea ever. But which philosophy is right? Here, two trainers offer differing opinions on the subject.
Fasted Cardio Cons
Brooke Erickson, NASM-CPT, nutrition specialist
Research has shown that there is no difference in body-fat loss between fasted and fed cardio. Fasted cardio may burn more fat during the time of the actual workout but less the rest of the day, whereas non-fasted cardio may burn less fat during the workout but more throughout the day. In the end, the difference in fat burned is insignificant. Considering what I have read, I choose to do cardio when it works best for my schedule because I know I will most likely be consistent.
I have done fasted cardio in the past during the summer months when I was trying to beat the heat, but about halfway into it, I felt shaky, drained and unfocused, and I was unable to push myself. It also made me feel even more hungry after I was done, and often I would overeat to quell that feeling.
Furthermore, I usually train in the afternoon, so doing fasted cardio is not conducive to a productive workout. My performance and ability to focus is much better when I have eaten before training, and since I do most of my cardio on my mountain bike — often riding 20-plus miles outdoors — I would not make it very far without fuel. I also make sure to consume protein before any kind of training because research has shown that protein aids in muscle retention without inhibiting fat loss and metabolism.
The bottom line is to do what works best for you and what you know you will stick with long term. The significance of doing one over the other really comes down to personal preference rather than efficacy.
Fasted Cardio Pros
Lexi Berriman, NASM-CPT, NFPT-CPT, ACSM-CPT, CF-L1, CF-L2
I’ve been a trainer and a coach for more than 20 years, and I have done cardio pretty much any and every way possible using myself as a guinea pig. What I have found is that since I follow a ketogenic diet, fasted cardio has a lot of benefits. The point of doing fasted workouts is to turn body fat into ketones to use for energy, and a ketogenic diet primes the pump for the body to get even more out of fasted cardio. It also increases autophagy — in which your body cleans out toxins and replaces old cells with new ones — increases mental acuity and initiates the fight-or-flight mode in your sympathetic nervous system, triggering glycolysis and fat oxidation. And plenty of research has shown that fasted cardio causes a natural increase in human growth hormone production.
I do, however, like to put branched-chain amino acids in my water for my fasted cardio versus eating solid food. Even though I am dropping weight, I don’t feel like my muscles are wasting away, and I can maintain focus and energy.
I’m also a huge fan of intermittent fasting, which also lends itself easily to a fasted cardio protocol. If I eat dinner late and have enough food at that meal, I don’t need to eat anything in the morning before cardio. But outside of that 12-hour window, there is a fine line for me, and if I happen to eat dinner early the night before or if I sleep in, I must have some calories before my workout.