The Thinking Person's Guide to Fasted Cardio

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.

Fasted Cardio: The Pro Side

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.”

Fasted Cardio: The Con Side

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.

What Will YOU Do?

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.

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  1. 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.
  2. 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.

I know what I did that I thought produced the best results:
In the final few weeks before a show, I was into my most intense cardio to burn fat. I started (12 weeks out) with once a day in the AM before eating with 30 minutes of cardio, as I got closer I added to the cardio time. It was all low impact, fairly fast walking. As time got closer I would do cardio after dinner (before bed) and upon rising in the morning. Full on cardio for me was 45 minutes in the AM and 45 minutes in the PM.

The idea of the before breakfast AM cardio was I had much less muscle glycogen, and I might burn more fat. The late PM (after dinner) was to run out some muscle glycogen and set up the AM cardio with even less muscle glycogen.

Because I was running fairly high AAS, I had no concern about cortisol. All my androgen binding sites were full of steroids (anabolic, not catabolic.) The crotisol could just float around until they were eliminated.

Back in the good old days sleep was a “piece of cake” when doing cardio just prior to sleep. The health food store had GHB. If you sit still you can’t keep your eyes open. One scoop 20 minutes before you plan to lay down (and you don’t have much more time than that.) In almost exactly 4 hours you would wake up. I would take 1/2 a scoop and back to sleep for 4 more hours. Wake up feeling completely fresh, ready to pursue the next day, no groggy feeling at all.

I know this is not possible to do this today, as best I can tell.

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I almost addressed steroids in the article, but decided to keep it simple. For sure, enhanced lifters can get away with fasted cardio for the most part. As Thibaudeau said it (activate French Canadian accent):

“The exceedingly high levels of anabolic hormones in his body can counteract – to an extent – excess cortisol production.”

Sounds like you had it covered anyway by sticking to walking. Smart.

I’d add that TRT – real doctor-prescribed TRT for hypogonadal men – probably doesn’t protect naturals much from cortisol. They should still follow the cardio guidelines for natties.

I think part of the popularity of fasted cardio, besides the fact that it feels good (cortisol = adrenaline), is that pros and influencers promote it. But they’re on drugs, and most people aren’t.

Here’s Thibaudeau’s guidelines for natural lifters whose training is hypertrophy-focused:

Avoid the following types of cardio:

  • Fasted Intervals
  • Fasted Moderately Intense Steady-State Cardio
  • Long or High-Intensity Intervals
  • Intense Post-Lifting Cardio

These two types of cardio are your best bet:

  • Walking (the more, the better)
  • Lactate Sprints with Long Rests

I lost 20 pounds in 2 months doing fasted cardio (one mile swim at 6:30 am after coffee).

Nice work! Now, the questions we can ask are:

  1. Were the dietary changes you made (if any) the biggest impetus for that fat loss?

  2. Would fed cardio have gotten you the same results?

  3. Would the composition of that 20 pounds have been different with fed cardio (muscle:fat loss)?

If doing it fasted was motivating, fit well with your schedule, and you’ve kept the fat off, then who cares, right? But it’s an interesting topic to dig into.

  1. I made no dietary changes.
  2. I doubt it, as I’ve done fed cardio and never gotten those kinds of results.
  3. My main concern was fat loss; however, I didn’t notice any extreme muscle loss. Perhaps an inconsequential amount.

Great article. I read Christian’s too, and I think the proof is in the 1% more body fat study. That seems to be within statistical error. From personal experience, I do a sunrise to sunset fast a couple of days each year, and I continue to do my cardio workouts on those days. I have never noticed a body weight loss boost over the succeeding days, much to my disappointment :joy:.


Is fasted strength training not recommended due to the increase in cortisol?

It has become established that cardio training done in the morning when we are fasting will have the best results, as the body will derive energy from body fat and not, as after a meal, from delivered energy. However, there is no evidence that such fasted training is more beneficial than training that we have done after a meal

I have competed multiples times over the decades. My preference is fasted cardio (which I still perform) yet when had to perform cardio when I was fed, I still dropped weight (body fat, and since I was natural, likely some muscle). It made no difference to me, given the lower amount of calories I was consuming during the pre contest prep (which is confirmed by some of the studies). For the vast majority of people, the emphasis should be just getting their cardio in, whenever their can.


Agreed, and more so for long-term health instead of as a fat-loss tool.

Also, I’d add that fasted cardio has another aspect that many overlook: it’s usually morning cardio. That’s when energy and motivation are highest in most people. Wake up a little early; smash it before most people are awake. It’s satisfying. The fasted part is almost just a coincidence or time issue (and full-stomach cardio is unpleasant).

For those who like fasted cardio for the above reasons, I suggest getting a little liquid muscle-insurance down before starting, something like Mag-10 or half a serving of Surge Workout Fuel. I did that this week myself before hitting the Stairmill.


I hate getting my cardio in. My default, if you will, has been to split it in half and do it before and after weights. So I’ll do 15 minutes before I lift, drink SWF whilst I lift, and then do 15 minutes after. I am getting fasted and fed cardio done in the same session, therefore defeating science and becoming something more than human.

I do agree that whatever gets it done is the answer.

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