Fasted cardio is exactly what it sounds like: aerobic exercise in a fasted state.
For example, if you often wake up and go for a run or spin before breakfast, you engage in fasted cardio. Whether or not you do it intentionally, anytime you perform aerobic exercise with an empty stomach, you’re experiencing fasted cardio.
While some say you can engage in fasted cardio after not eating for 4-6 hours, most research on the subject is done with a 10 to 14 hour period of abstaining from food.
Lots of athletes like to exercise before breakfast. Some like to sleep in, others don’t like to exercise with a full stomach, while others just don’t like to eat early in the day.
But some athletes are waiting to eat breakfast until after their workout to try and burn more fat.
So, does it work? The quick answer is not really, but it’s important to understand why it isn’t very effective. Especially because at first glance, the research seems to support that it is the bestway to reduce fat.
At face value, fasted cardio absolutely burns fat during exercise. This is well established in the research.
A 2016 meta-analysis of 27 studies compared fasted and fed states on fat and carbohydrate metabolism in adults. Researchers concluded that that was a significant increase in fat oxidation during fasted cardio compared to fed cardio. (Vieira, et al.)
So, what’s happening to your body during a fasted state?
After a night with an empty stomach, your glycogen stores, or your stored energy in the form of glucose, will be slightly depleted. Without any carbs in sight, your body will look for alternative sources of fuel: protein and fat.
However, these results aren’t as meaningful as you might think.
The real question here isn’t whether you’ll burn fat during fasted cardio. You don’t care about burning fat during exercise--you care about decreasing visible fat and increasing lean muscle in a way that will improve your overall performance and appearance.
A more discerning question is this: “Will fasted cardio improve my body composition?” Short answer: probably not.
Your body’s systems are dynamic and complex. To judge fasted cardio’s efficacy based solely on whether fat is burned during 30 minutes of cardio is only a snapshot of a much bigger picture.
It’s pretty near-sighted to only look at whether fasted cardio burns fat during an exercise session. That’s because your body compensates for and responds to exercise throughout the entire day.
Which fuel source (carbs, fat, or protein) your body uses and in what proportion depends on a bunch of factors, like exercise intensity, duration and type, energy expended, and individual fitness level.
And it’s all regulated by systems like hormonal responses and enzyme activity that can change moment to moment, working throughout the day. (Hansen) (Schoenfeld) (Sonko)
All this to say that fat loss should be evaluated over a 24-hour period, not during an individual session. (Hansen)
While substrate utilization is a complicated process, it does follow a general rule: if you burn more carbs during a workout, you’ll burn more fat after. And if you burn more fat during a workout, you’ll burn more carbs after. (Schoenfeld)
And when we consider the effects of fasted cardio over the course of the day (or in the case of the following study, three weeks), the difference between fasted and fed exercise just isn’t noticeable.
A 2014 study of 20 healthy women compared the body composition after three weeks of exercise. One group tried fasted training,\ while the other engaged in fed training. Both groups lost a significant amount of weight and fat from baseline, but there was no meaningful difference between the two groups. (Schoenfeld)
What’s more, when researchers further examine how the results of fasted cardio differ between trained and untrained people, the fasted cardio argument becomes even thinner.
As Schoenfeld points out in his research, both training status and aerobic exercise intensity mitigate the effects of a pre-exercise meal on fat oxidation.
In one study, 6 moderately trained participants cycled for two hours at different intensities in a fasted versus fed state for a total of four trials.
A different study compared the effect of pre-exercise and during exercise carbohydrate consumption on fat oxidation.
So, what does it mean if more fat is broken down in a fasted state, yet no meaningful differences occur between fasted and fed participants long term?
Schoenfeld hypothesizes that these two studies suggest that during moderate to high-intensity exercise, and in trained athletes, that during a fasted state more fat is broken down than the body can use.
The free fatty acids that aren’t oxidized simply return to adipose tissue throughout the day. (Schoenfeld)
The location of the fat burned during fasted exercise is also worth considering.
When we think of fat burning, intuitively we imagine cellulite or beer bellies.
However, research suggests that for untrained individuals, up to 50% of the fat oxidized during exercise is from intramuscular triglycerides (IMTG). (van Loon)
IMTGs are subcutaneous lipid droplets stored in the mitochondria of muscles. Their location and function mean that they make no difference on appearance or health.
And the more you train, the more IMTGs you store (with some researchers estimating that trained athletes have twice as many IMTGs as untrained individuals). (Boesch, et al.) (Goodpaster, et al.)
Just because fat is broken down during a fasted state doesn’t mean your body will use the fat you want it to.
If you enjoy exercise on an empty stomach, by all means, continue to train in a fasted state. However, there is no convincing research to suggest that you’ll burn more fat by doing so. Actually, you could be hindering fat loss.
That’s because one of the single best ways to reduce fat is high intensity interval training (HIIT).
HIIT is characterized by short bouts of high-intensity exercise, followed by a resting period. Multiple studies show that HIIT is superior to moderate, steady-state training for fat loss.
This may be surprising, considering the findings of the study above by Horowitz that shows less fat oxidation during high-intensity training.
But despite initial low levels of fat oxidation, athletes who use HIIT burn more fat throughout the day, with some studies suggesting up to 10% greater fat loss. (Zhang, et al.)
You may be thinking, “I can perform HIIT in a fasted state!” But to reach the level of intensity needed for HIIT to be effective, it’s best for athletes to have access to carbs.
In one study, nine well-trained cyclists exercised at 70%maxVO2 until exhaustion.
There is no obvious harm in exercising in a fasted state. However, the research does not suggest that it will increase fat loss. On the contrary, exercising without proper fuel may make it difficult to train at an adequate level for fat loss.
Matt Mosman (MS, CISSN, CSCS) is a research scientist, endurance athlete, and the founder and Chief Endurance Officer at EndurElite. Matt holds his B.S. in Exercise Science from Creighton University and his M.S. in Exercise Physiology from the University of California. Matt and his family reside in Spearfish South Dakota, where they enjoy running, mountain biking, camping, and all the outdoor adventures Spearfish has to offer.