Does It Burn Fat Faster or Not?
Fasted cardio is a divisive topic. Does it really burn fat faster? Here are both sides of the argument. You decide.
Does fasted cardio lead to faster fat loss? Oh man, I wish I could give you a firm answer.
The problem is, this question is like one of those debate team prompts. In schools, competitive speakers are often given a hot-button social topic to research, then they debate it head-to-head with another persuasive speaker. The catch? They’re not told which side they’re taking. Right before the debate, they learn if they’re “pro” or “con.” A master debater (had to say it!) could win either way.
I could do that right now. Using science and big words, I could convince you to adopt fasted cardio. I could also take the other side and make you scared to death of it. Instead, I’ll present both sides, peek into some studies, and let you decide for yourself, like a grown-up.
When you wake up in the morning from an overnight fast, you’re primed to expel body fat to fuel a cardio workout. Metabolically, your body is optimized for lipolysis (breakdown of stored fat) and oxidation (the “burning” of that fat).
Fasted, your insulin levels and liver glycogen stores are low, and epinephrine and cortisol levels are high: the perfect hormonal environment for ripping fat off your body. But, if you eat something before the workout, especially carbs, you put the kibosh on all of that. Your body uses the food/carbs to fuel the workout, not the extra belly fat.
In one 6-week study, overweight men consumed the same low-calorie diet and did either fasted or fed cardio. (1) For the workout, both groups walked briskly on a treadmill for 30 minutes. The fasted group did it an hour before eating breakfast; the fed group did it an hour after eating breakfast.
The fasted group lost 1% more body fat than the fed group. Not a huge difference, but if you’re going to do cardio anyway, you might as well do it at a time that leads to more fat loss. That adds up over time.
As a bonus, another study using male runners showed that fasted cardio leads to decreased energy/calorie consumption at night. (2) The researchers concluded:
“The reduced 24-hour energy intake on fasting days was not only due to the fact that breakfast was skipped but also due to a decreased energy intake at night. This finding suggests that fasting prior to exercise may suppress energy intake over an extended period of time.”
If you gather up a bunch of fasted cardio studies and summarize their results, you’ll conclude that fasted cardio isn’t superior to fed cardio. That’s exactly what a 2017 meta-analysis concluded. The research said, in a nutshell, that fasted cardio isn’t superior and doesn’t lead to faster fat loss than fed cardio, but it’s probably not harmful either.
Bill Campbell, PhD, summarized the topic like this:
“Overall, reduction of body fat is a result of an overall calorie deficit, whether exercise is completed on an empty stomach or not.”
Remember that most fasted cardio studies use overweight, sedentary subjects, not weight-lifting men and women carrying around 10-40 pounds of bonus muscle. What about muscle retention? Does the “mobilization hormone” – cortisol – mobilize lean muscle tissue as well as fat? And what does fasted cardio do to your resting metabolic expenditure for the rest of the day?
Christian Thibaudeau, citing several studies, concluded:
“Fasted cardio leads to a lower resting energy expenditure (fewer calories burned at rest) as well as less total fat utilized over a 24-hour period.”
He points out a study (Paoli et al.) showing that while subjects did burn a tiny bit more fat during a fasted cardio workout, they burned less for 24 hours after the fasted session. Their counterparts in the study, the fed group, had higher fat utilization rates during the 24 hours after the workout.
The risk of muscle loss is also higher with fasted cardio, at least with experienced lifters. Cortisol mobilizes stored energy, which is good for fat loss. To use body fat for energy, more cortisol is released. The more cortisol released, the longer it’ll take for it to return to normal levels after the workout. In other words, you’re in a catabolic state.
Also, keep in mind that cortisol increases adrenaline levels by increasing the conversion of noradrenaline to adrenaline. That amps us up and makes fasted cardio feel good, at least when it’s over. But remember, that “feel good” feeling is caused, in part, by cortisol release, which may be excessive.
These studies are also narrow. They ask, “Is more body fat broken down when fasted?” Sometimes the answer is yes, but lipolysis (breakdown) doesn’t always result in oxidation (burning). And, to further complicate things, Dr. Campbell adds,
“Increased fat oxidation during exercise doesn’t necessarily mean increased loss of body fat.”
Fasted cardio looks good under the microscope and on paper. The real-life benefits? Sometimes minimal, but usually nonexistent.
Considering all the pros and cons and fancy words, I know exactly what I’m going to do when it comes to fasted cardio. Do you? Let me know in the comments below.
Make any workout work better. Fuel it.
- Liu, et al. “The Effects of Six Weeks of Fasted Aerobic Exercise on Body Shape and Blood Biochemical Index in Overweight and Obese Young Adult Males,” J Exerc Sci Fit. 2023 Jan;21(1):95-103. doi: 10.1016/j.jesf.2022.11.003. Epub 2022 Nov 11.
- Bachman, et al. “Exercising in the Fasted State Reduced 24-Hour Energy Intake in Active Male Adults,” J Nutr Metab. 2016;2016:1984198. doi: 10.1155/2016/1984198. Epub 2016 Sep 21.