Debunking the Myth of Fasted Cardio

Aleisha Fetters
by Aleisha Fetters
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Debunking the Myth of Fasted Cardio

When vying for weight loss, a lot of people approach their goals with a calories in, calories out mindset. Burn more calories than you eat each day, and you’ll lose weight. Burn those calories from pure, unadulterated fat, and some say that you’re golden.

That’s what makes the promise of fasted cardio so attractive. On the surface, it also makes sense: Head into your sweat session first thing in the morning before breakfast, and, since there’s no food in your system for your body to use as fuel, your body will burn the fat you’ve stored for energy instead. But that isn’t the whole story.


“Burning more fat is much different in reality than it is in theory,” says Susan Kleiner, PhD, RD, author of “Power Eating” and a sports nutrition consultant to top NFL, NBA and Olympic athletes. She points out that, apart from burning fat in the form of triglycerides floating through your bloodstream (rather than as whatever’s hanging around in your so-called trouble spots), fasted cardio often burns fewer total calories — including calories from fat — than properly fueled exercise does.

That’s because, while fasted cardio does increase the proportion of calories burned from fat during exercise, it decreases the total number of calories burned at a given rate of perceived exertion (how hard you feel that you’re working), she says. It’s important to realize that when you exercise in a fasted state, you’ll actually feel like you are working out harder, even if you’re running, biking or swimming less intensely than you would with fuel in the tank.

It becomes physically painful to exercise in this state,” she says. “You can do it, but it’s not pretty and has never been proven to confer benefits in terms of exercise performance or intensity.”

Plus, if you’re going to get caught up in calories in versus out, it’s worth noting that research published in the International Journal of Sports Nutrition and Exercise Metabolism shows that when people eat a small snack before a cardio workout, they continue to burn significantly more calories and fat after their sweat sessions compared with those who work out while fasting.

As if that wasn’t reason enough to ditch fasted cardio for good, it’s important to remember that when you’re in a fasted state, your body doesn’t just turn to fat for extra energy. It also turns to protein. That’s right, muscle. In fact, research published in the Journal of Applied Physiology shows that an hourlong cardio workout, when performed on empty, results in up to 10% of all calories burned coming from protein — that’s about double what it would be otherwise.

YOUR PRE-CARDIO SNACK PLAN                                       

“Weight loss doesn’t occur with under-fueled training,” Kleiner says. “It happens when you give your body what it needs to perform its best.”

To get the most fat-burning benefits from your early morning exercise, she recommends eating a small snack that combines both carbs and fat, like whole-wheat toast and a hard-boiled egg, 30 minutes to an hour before exercise. The combo will increase your workout performance and, thus, calories burned, while also helping you to build more lean, metabolism-revving muscle.

If you’re getting into hard-core or long endurance workouts lasting for more than an hour, you need to think about fueling up not just before but also during your workouts, she says. Sports drinks, gels and even raisins — when consumed during long cardio workouts — have been shown to increase performance thanks to their blend of quick-acting carbs and electrolytes.

Remember, you get out of your workouts what you put in — and that requires proper fuel.

About the Author

Aleisha Fetters
Aleisha Fetters

Aleisha is a health and fitness writer, contributing to online and print publications including Men’s Health, Women’s Health, Runner’s World, TIME,, and She earned both her bachelor’s and master’s degrees in journalism from the Medill School of Journalism at Northwestern University, where she concentrated on health and science reporting. She is a certified strength and conditioning specialist through the NSCA. You can read more from Aleisha at, or follow her on Twitter @kafetters.


9 responses to “Debunking the Myth of Fasted Cardio”

  1. Avatar KstewKrew says:

    Good article. Thanks. Something to think about.

  2. Avatar Deborah Sprouse says:

    I disagree… I’ve been training in a fasted state for three years and I’ve never felt stronger, muscular or leaner. I follow a 16/8 fasting schedule. True, fasting may not be for everyone but for me I’m able to eat a higher amount of calories while maintaining my desired weight. I am able to consume around 2000 calories a day, maintain 127lb with a 16% body fat. Also, I’m 56, 5″8′ woman and a personal trainer.

  3. Avatar Dr. Bret Emery says:

    I also disagree with this article. While certain circumstances warrant the need for eating before exercise, there is also utility in completing fasted exercise sessions. Susan Kleiner’s referenced book is from 1998. In the nineteen years since that book was written there has been a wealth of new research released. If you are interested in learning more, take a look at intermittent fasting as one area. While the author states that it is painful to exercise in a fasted state, intermittent fasting research has demonstrated that humans become “fat adapted” and develop the ability to burn fat more easily as they spend time doing fasted exercise. I have had many patients who have told me they experienced an increase in overall fat loss after utilizing fasted state exercise sessions. The referenced article from the International Journal of Sports Nutrition only had eight subjects in the study and they were all males and did not use body fat as a measurement. The authors used VO2 and RER and did not measure body fat in any of the eight subjects. As for the referenced article in the Journal of Applied Physiology, it had only six subjects. It used serum urea as a marker for increased protein utilization. The authors found that in these six young (all under 30) subjects, exercise increases output of protein through sweat and urine. This result does not correlate with muscle loss, and the authors make no statement that it does. I would suggest that in exercise durations under 60 minutes and in conditions where fat loss is preferred over performance, that fasted state exercise be performed. In conditions where the time is greater than 60 minutes or the goal is performance (sporting events) the exercise be performed in a fueled state.

  4. Avatar Gabriel Ruiz says:

    I agree with what she has to say but she needs to have more evidence. I think that cardio in a fasted state will burn a good amount of fat if you go at a intense level. Yet we can’t do our best in a fasted state. Our performance decreases and that has to be a variable when it comes to the comparison you put into the play. So I would think we would burn much more calories when we consume food or drinks. You see that’s another factor. WHAT WE EAT!!! If you eat a energy bar that has more calories then taking a energy drink that has about 20 Calories versus the 160 energy bar. And with the energy drink you will even burn more calories with the energy your given. And I like her research on the muscle loss. That has to be an important factor as well. (10%) wow I did not know that! This is a good argument. And Although fasted Cardio can burn more calories some people say; you wont have enough energy to even go for another round so you can burn more calories if you chose to (.)

  5. Avatar Dharma Bum says:

    This article requires a very strong qualifier. I think the title, perhaps intended as click bait, is wrongly worded. Hour long “or more” workouts obviously will require more supplemtation. But no reason to ditch fasted cardio for a 30-45 min workout. Morning fasted cardio workouts can work wonders!

  6. Avatar Christin says:

    Disagree as well. This is starving yourself versus fasting after having consumed a couple thousand calories prior.
    I fast cardio as well, and have great results with more muscle growth. But mind you my day before was about a 2,500 intake, and after my cardio I bulk up on healthy eating – fat, carbs and protein. About 500 calories, and that’s just a snack for me.
    I’m also a woman, 33.
    This is different compared to someone who’s only eating 1,300 calories. They’re not getting enough nutrients in their bodies for this to work properly.

  7. Avatar geo2209 says:

    I disagree, just read up on Dr. Stoppani’s research on it. I do believe that you’ll lose muscle mass if you do cardio for an hour, but most people will a) gain it back quickly in a bulking phase and b) are usually using fasted cardio to cut. I do HIIT for 20-25 minutes and notice zero muscle loss while losing fat. So I wouldn’t recommend doing cardio for even an hour like this article suggests or you’re going to look like a marathon man (which I bet 90% of men don’t want to look like). I think for long term progress, I.F. is the way to go since fat loss has always been more about caloric intake than exercise.

  8. Avatar Jason Williamson says:

    Fat Dude chiming in…Fasted, or non-fasted doesn’t matter in my experience. I’ve lost over 150 pounds, doing cardio after I eat and late at night. Everyone is different. Do whatever works for you. The only advice I would give is…don’t do cardio then try to lift. Your workouts will suck A$$. I usually eat something, lift for 60-90 minutes then do cardio for 60min. So by the time I’m doing cardio I’m in a somewhat catabolic state. I’ve done both ways. Long low intense cardio is the weigh to go 🙂

  9. Avatar Jeffery Bradley says:

    I do high intensity interval Peloton spinning as my cardio 2x per week. I’m also practicing 12/12 intermittent fasting. An hour before morning workout I drink a cup of coffee and eat wheat toast with 1/2 banana and 1 tablespoon almond butter (240 calories total). About 30 minutes before workout I drink a pre-workout powder (Vintage Blast 0 calories). I easily burn 800-900 calories in the 45 minutes spin then I do a 30 minute walk burning another 250 calories. However, if I skip breakfast, I noticed my energy level drops. Even though I complete the 45 minutes workout, my intensity level drop half way in and I end up burning about 600-700 calories and about 150 walking. I also get similar result on my strength training days using TRX’s 45 minute workouts. This seems to work perfect for me. I’m losing weight, maintaining muscles, and have great energy throughout the day. I may substitute toast for a coffee infused smoothie with 25 grams of protein and keeping the total calories below 250. This may not work for everyone, but for me this article is spot on because I can actually prepare for the high intensity levels and increase my goals.

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