The rationale behind fasted cardio is to exercise early in the morning before breakfast, while your body has been fasting overnight. By exercising on low glycogen levels, your body will supposedly burn through fat for energy rather than carbohydrates. While bodybuilders and athletes have been known to swear by fasted cardio, science isn’t always behind this theory.
The science behind it
Fitness researchers such as Horowitz et al. (1997) found that when comparing 2 groups of fasted vs fed cyclists, a low- and medium-intensity fasted group was able to breakdown up to 22% and 25% more fat (released as fatty acids known as lipolysis), respectively, when compared to the fed group. That's 1 point for fasted cardio.
However, the bodies of the low-intensity fasted group didn't start to use up this fat as an energy source (known as fat oxidation) until the 80 - 90 minute mark of exercise. Fat oxidation of moderate-intensity fasted even remained the same the whole time they exercised. That's 1 point for breakfast.
I don’t know about you, but when I do cardio, whether fasted or not, I’m definitely not on the stair-master for 80-90 minutes... So currently the score is tied. But whats good to note from this study is that if you're going to do fasted cardio, it's best to go for a low intensity/pace such as steady state (i.e. jogging, stair-master), rather than a higher intensity such as HIIT.
If you're looking for a tie-breaker: Schoenfelt (2011) also pointed out that in even though more fat can be released through lipolysis during fasted cardio, the fat isn't guaranteed to be utilized as an energy source, and may end up being re-stored and negate any benefits anyway. So this might be dependent on our own body types, metabolism, hormone, etc. So it's also possible that the benefits of fasted cardio are individual-specific. That might also explain why we haven't been able to come to a consensus on the topic.
To eat or not to eat?
Other researchers have shown that by having something to eat before exercise allows bodies to have better post-exercise oxygen consumption compared to fasted cardio simply because of the thermogenic advantage it gives by providing energy for performance during a workout (Lee et al., 1999). Because let’s be honest, are you going to be able to go harder on a treadmill at 5 am when you dragged your tired ass out of bed but are still only half awake, or during your regular evening workout when you’ve been adequately fuelled throughout the day? So again, what it really comes down to, is your own workout style.
Once it's all over:
It's important to remember that if you’re doing fasted cardio, it can also break down protein faster than fed cardio. Lemon and Mullin (1980) found that during fasted cardio ~10% of the calories you burn come from protein - so if you want to maintain muscle mass, you’ll want to fill up on protein after your workout. Here are some morning meals that are higher in protein and fat and less carbs
Should YOU do fasted cardio?
So, should YOU do fasted cardio? Like everything else in fitness it comes down to your own personal goals and, of course, body type...
Fasted cardio is said to have greater benefits on ‘trouble areas’ for people of lower body fat percentages. Jim Stoppani, Ph.D, recommends fasted cardio moreso for men with 5 - 6% body fat and women with 13 - 14% for women.
People with higher body fat percentages are more likely to get more benefits by performing HIIT exercises rather than fasted cardio. High intensity workouts with short rest intervals allow the body to turn more fat than steady state cardio and continue to do so for up to two times longer post-workout.
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Horowitz JF, Mora-Rodriguez R, Byerley LO, and Coyle EF. Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise. Am J Physiol Endocrinol Metab 273: E768–E775, 1997
Lee YS, Ha MS, and Lee YJ. The effects of various intensities and durations of exercise with and without glucose in milk ingestion on postexercise oxygen consumption. J Sports Med Physical Fitness 39: 341–347, 1999.
Lemon PW and Mullin JP. Effect of initial muscle glycogen levels on protein catabolism during exercise. J Appl Physiol 48: 624–629, 1980
Schoenfeld, B. Does Cardio After an Overnight Fast Maximize Fat Loss? Strength and Conditioning Journal, 33: 23-25, 2001.