The Surprising Truth About Fasted Cardio

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The Surprising Truth About Fasted Cardio

Many athletes wonder whether they should work out on an empty stomach or whether it’s crucial to have a snack beforehand. As “fasted cardio” gains popularity, some are opting to skip the pre-workout snack altogether. But is this no-fuel strategy advised for athletes?

The short answer to that question is… (Are you ready for it?) it depends. There’s not a one-size-fits-all approach to nutrition and fueling for sports performance. There may be a time and a place for skipping that pre-workout meal or snack; however, whether or not you should do it depends on many factors.


While traditional sports nutrition guidelines promote carbohydrate-rich fueling strategies, there may be a benefit for some athletes to skip the snacks. Some endurance athletes take the approach of “training low,” or training on minimal to no glycogen stores for longer endurance runs. This is done in an effort to improve metabolic efficiency, or a greater utilization of fat for fuel.

Rikki Keen, MS, RD, a certified specialist in sports dietetics and certified strength and conditioning specialist says, “science has shown placing the muscle in a stressful state of low glycogen levels during selected aerobic training sessions can trigger a cascade of hormonal and gene signaling that further enhance training adaptations within the muscle cell.” Because stored carbohydrates are in limited supply and fats are plentiful, this metabolic shift could benefit endurance athletes, allowing them to go longer before crashing or bonking.

However, “fasted endurance training won’t show immediate performance gains,” says Scott Sehnert, MS, RD, a sports dietitian at Auburn University. “Instead it causes metabolic changes that may produce optimal performance later, when the athlete is well-fueled.”


If performance is the goal, there are clear benefits to eating shortly before exercise. For example, don’t skip a snack before a competitive event. Instead, consume a meal or snack that’s high in carbohydrates, low in fat and moderate in protein to keep energy levels high and fuel optimal performance. Eating before exercise is intended to delay fatigue, enhance endurance and support performance, promote mental clarity, and prevent low blood sugar and hunger.

If you’re looking for strength and muscle gains, foregoing the pre-workout snack is not the way to go. Consuming a meal with protein and carbohydrates before a workout will increase your body’s ability to burn carbs needed for energy to perform and utilize amino acids from protein to increase lean muscle mass.

And because high-intensity interval training relies on the anaerobic energy system, which requires carbohydrates to be burned as fuel, it’s not recommended to skip pre-workout nutrition. “Because HIIT is so reliant on carbs, and because most people doing HIIT (CrossFit and the like) want to see muscle gains, they need to be well-fueled for that and not break down their lean tissue for energy,” says Sehnert, a certified specialist in sports dietetics and certified strength and conditioning specialist.

Keep in mind these fueling recommendations are geared toward serious athletes with intense training regimens and performance goals. When exercising moderately for fitness benefits, it may not be necessary to follow the same guidelines.

“In general, if someone is going to do moderate cardio simply to burn calories or improve fitness, then I don’t think a snack prior is necessary, especially if they’ve eaten within the last 3-4 hours,” says Sehnert. If you’re doing a light early morning sweat sesh and skip a pre-workout snack, just make sure to fuel up after to recover quicker and reduce muscle soreness.


It’s important to understand that increased fat burning during exercise does not directly equate to bodyfat loss; calorie deficits still comes into play when aiming to lower body fat.

Fasted cardio “can be used a method to enhance fat oxidation during low-moderate exercise, assuming the individual still gets a quality workout in and controls for calories for the remainder of the day,” says Keen, the team sports dietitian for Orlando City Soccer Club. “It’s one tool that can be used to support weight management; however, it is not for everyone and could actually backfire with the person overeating at the next meal.”

Skipping meals may also lead to overeating later in the day, emotional eating and mood swings, and it can promote eating disorders. People susceptible to or experiencing these issues likely wouldn’t benefit from fasted exercise.

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  • Tim Green

    I rarely fuel up before my early-morning exercise – usually only when I’ll be doing fast intervals or running for over an hour. I can only tolerate simple sugars right before running, which is unnecessary for a slower run.

    After a run (of almost any length), I drink a protein shake to fuel recovery, and then manage the rest of my calories using MFP through the rest of the day. I don’t know if running helps with weight loss, but it certainly helps keep it off!

  • Robert L.

    I saw a program(I think it was on the BBC), and while I can’t remember the details of the trial, the conclusions suggested that Men were better off exercising on an empty stomach, and Women were better off after having eaten.
    I seem to remember half the Women ate before, and half didn’t. The same for the Men. The Men that didn’t eat performed better than those that ate, and the Women who ate performed better than those that didn’t eat. It was a small scale study, and as mentioned above, different types of exercise may have differing results. Interesting though, all the same.

    • Jack Callon

      The BBC program referred to was part of the series “Ask me, I’m a Doctor”.

  • Emma Richter

    Fasted cardio is good, but it’s important to refuel after workout. When lifting or working out to gain muscle, it’s good to eat moderate carbs and fat before, and more important to have
    substantial protein after so your muscles don’t burn out. In general, have something within two hours before a moderate workout is optimal. This way, your body will begin burning at a high rate, but quickly run out of immediate energy and turn to fat storage.

  • David

    Thanks for the blog. I’ve been pondering this very question for a long time. Due to a physical defect it’s desirable for me not to bend or lay down with very much in my stomach. The blog has cleared thing up for me – and the Comments have been very helpful too.

  • Jordan

    I really enjoyed reading this article. In the mornings I always do fasted cardio because it’s what my body is use to, but getting some of these answers questioned has been very beneficial. I also enjoy reading all the comments and input this article has started. Thanks!

  • MorCar

    Greetings – for my fellow workout folks who get to the gym right at 5am, could you give me some ideas on the kinds of snacks that you consume? I typically just go for a medium cup of coffee (light sweetener, light cream) and then head on out. Thanks!

    • Mmmmfit

      BCAAs 🙂

    • Kiesha Shelton

      I drink a protein shake before and my BCAAs during my workout.

  • Beth Lewis

    I’m actually trying to gain but I’ve been struggling all my life to gain weight and I’m 31 and barely pushing 94lbs. Any ideas or help? Not to mention I’m also a volunteer firewoman

  • Eliot

    Nice to see an article for once that doesn’t tell me “it’s this way or no way.” An article that starts with “it depends” is always going to attract my interest and increase my level of trust. The truth is, in almost everything, the is not one right answer. I had been exercising without eating first for a while. Not for any weight loss boost intent. I just felt better not eating first. I was told that was bad, so I ate. And I couldn’t work out as much and it didn’t feel good. Guess what the result was. Yeah, I stopped exercising. Now I’m going back to exercising without forcing a snack into me first.

  • Christopher

    My experience with bike rides seems to be that if I don’t eat during a three hour ride, I feel loggy and can’t produce as much power (measured by a power meter), but there are distinct phases. For the first hour, I am running on stored glycogen or what is in my system. It is not too bad. The second hour power goes down, I slow down, but I can keep going. In the third hour, I get stronger, but still don’t feel great. I assume that I am beginning to burn fat effectively, which is my major goal a this point. If I want to go fast or build strength, I have to eat before and during, but for fat loss, I think this is what works. Note that I ride a lot and have built up to my current level of endurance over a lot of miles. I am not sure it worked this way at the beginning. It is also true that I have lost more than 100 lbs and it is getting harder to get rid of the remainder.

  • Maegen Fariss

    For me, I can’t eat before I run, not even a small energy bar. If I do, it gets thrown back up after the first mile or so–and with my luck, it usually gets into my lungs when it happens.

  • Rachael Kvapil

    I was a die hard “fasted cardio” until this season when I got a swim coach and a bike coach. The intensities even in our “mellow” days is just too much to do without some kind of breakfast. I figure I kept a lower level of intensity in my previous training years and could get away with it.

  • Serena Tsang

    My rule is that if the morning run is <1 hour, I don't really need to eat anything, so that's about 6-7 miles. I like to eat something beforehand if I go above 7-8 miles, or if I'm doing strength or speed work.

  • Dharmish

    Fasted weight training and not too intense fasted cardio are beneficial for the most part, as shown in the intermittent fasting community and studies.

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  • BadKarmaGhost

    Reading this article was a huge waste of time.

  • Beaute’ Naturelle

    When articles contain it may or it can do this or that it amounts to a persuasive writing using logic and reasoning to persuade readers to adopt a certain point of view. You know your body better than any nutritionist. You live in it and your body let you know the pros and cons of food likes and dislikes, food intake and how it react to missing meals or eating a certain number of meals a day and what exercises it feels comfortable with. Every body does not like to run, swim, do intense workouts in the gym.

    When dieting and exercising do what is best for you and not what works for others. The most important factor when choosing a diet plan is patience and consistency. Also doing honest reflections on your behavior and progress when dieting is key to modify or make dietary changes and fitness changes until you reach your fitness goals.

    As Shameless Maya say, “Do you boo.” This is my view and I am sticking to it.

  • Bill Creed

    I get up at 4:30 every morning to workout before my kids wake up. I alternate between weights and running each morning. I never eat before exercising, but I do eat after (usually a smoothie with fresh fruit and protein powder). This works well for me I watch my calories throughout the day and go to jiu-jitsu 5 times a week after work, also not eating before class. For those trying to lose weight I suggest finding an activity that you love (jiu-jitsu for me) than you can do 2 hour workouts without feeling like you are working out.