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Does Cardio On An Empty Stomach Burn More Fat?

Whether cardio on an empty stomach can speed up fat loss is a commonly debated fitness topic. Some believe fasted cardio is more effective at burning fat while others believe it can decrease performance and limit fat loss.

Who is right? The following will inform you of what research has to say about the topic and offer compelling reasons why fueled exercise leads to greater overall fat burn.

Cardio on an Empty Stomach Burns More Fat DURING Exercise

Two fuel sources—carbs & fats—are used to generate energy for muscle contraction during exercise. For endurance exercise performed at a moderate intensity, you obtain 50–60% of energy needed from glycogen (which is stored energy from carbs) and the rest from fats.

When you deplete glycogen stores by fasting overnight, or going several hours without refueling, fatty acids break down in the mitochondria to be used as a secondary energy source. As workout intensity increases, your reliance on carbohydrates increases as well.

In one study that tested the fat burning effect cardio on an empty stomach, six healthy men cycled for 60 minutes at a low to moderate intensity:1

Group 1– Fasted overnight before the bike ride.

Group 2– Performed the bike ride after ingesting 0.8g/kg of glucose or fructose to replenish glycogen levels 1 hour prior to the workout.

Results: After 20-30 minutes of exercise, the rate of fat burn was higher in the fasted group than in the glucose or fructose group. This trend continued throughout 50-60 minutes of exercise. There was also a higher quantity of FFAs (Free flowing fatty acids) available in the blood in a fasted state throughout the exercise.

The Take Away: This particular study suggests that more fat was burned by the group that performed MODERATE activity on an empty stomach… DURING THE EXERCISE ITSELF.

But Empty Stomach Cardio Does Not Burn More TOTAL Fat

Not so fast. Notice how “moderate” exercise is emphasized in the example above? Research shows that people who burn fat during their workouts actually burn less fat the rest of the day. Overtime, fat burning is not an immediate process, rather, it occurs over the course of, not a few hours, but a few days.

As you burn more carbohydrates during your workout, the body will burn more fat post exercise. This “afterburn effect” where your metabolism is elevated for several hours or days following your workout is critical when debating the benefits of fasted cardio.

While you may burn more fat during your workout on an empty stomach, your overall workout intensity may decline.2 Your body’s ability to burn fat post-exercise is compromised. Consider the whole 24 hour period and cardio on an empty stomach is less effective.3

Evidence supporting fueled exercise

Researchers from Italy investigated the contrasting reports on whether training in a fasting condition enhances weight loss. There were 8 healthy young men who performed early morning slow cardio under 2 conditions:Adaptations to skeletal muscle with endurance exercise training in the acutely fed versus overnight-fasted state.

1. Empty stomach

2. After eating

Eating increased both oxygen consumption (VO2) and respiratory exchange ratio (RER) significantly, 12 hours after the cardio, VO2 was still higher for the group who had eaten, although RER was significantly lower in the FED test, indicating greater fat burn.

The group that ate before the cardio session continued to burn significantly more calories up to 24 hours after the exercise bout. The authors concluded that “when moderate endurance exercise is done to lose body fat, fasting before exercise does not enhance lipid utilization (fat loss); rather, physical activity after a light meal is advisable.” 4 Check out this article for more on pre-workout meal ideas.

High Intensity Cardio on an Empty Stomach Can Burn You Out

During intense exercise that approaches your maximum effort, most of your energy comes from glycogen. If you deplete your glycogen stores, you compromise your energy output. As glycogen stores in the muscles and liver are depleted, and the blood glucose level begins to fall, fatigue, lack of coordination, light-headedness and lack of concentration can occur. Commonly known as “hitting the wall” or “bonking,” fat simply can’t be metabolized fast enough to support the higher pace, so you slow down or even stop.

While research and studies are still ongoing, there are a few certainties. Steady state fasted cardio might burn more fat during your workout, but your post-workout fat burn is compromised. When performing high intensity cardio and exercises, glycogen levels need to be restored to achieve optimal performance and results. Doing cardio on an empty stomach if you aren’t able to sustain adequate energy levels, your workout will suffer.

Always choose energy and sustainability over anything else! Even early in the morning, grab a protein shake, piece of fruit, or handful of trail mix, to help your body to use efficient energy sources to power your workout.

Show 4 References

  1. Zoladz JA, Konturek SJ, Duda K, Majerczak J, Sliwowski Z, Grandys M, Bielanski W. Effect of moderate incremental exercise, performed in fed and fasted state on cardio-respiratory variables and leptin and ghrelin concentrations in young healthy men . J Physiol Pharmacol. 2005 Mar;56(1):63-85.
  2. The TRUTH About Fasted Cardio . MensHealth. 2011.
  3. Onunkwo, D. Fasted Cardio For Fast Loss…Does It Work? . Health & Fitness. 2011.
  4. Paoli A, Marcolin G, Zonin F, Neri M, Sivieri A, Pacelli QF. Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training. . Exp Biol Med (Maywood). Int J Sport Nutr Exerc Metab. 2011;21(1):48-54


  • Kevin Deeth says:

    @Scott, I generally do not recommend high intensity intervals in a fasted state for the sole fact that this usually compromises your work output and energy levels to perform at your peak. If you are the type of person that feels like a pre-workout meal has no substantial influence on your energy or work output than high intensity intervals might be permissible for you. Just remember, your body will also burn through muscle as well as fat if glycogen stores are depleted.

  • Kevin Deeth says:

    @Anthony, I would like to echo what Marc said but also like to add sometimes water based fruits or fruits with less sugar are easier on people's stomachs early in the morning. If you have a sensitive stomach try a less ripe banana as they have less sugar or half of a grapefruit.

  • Lauren says:

    I've found a bunch of articles on this topic, but this is the best, most comprehensive one I've ever read! Thank you - I feel like this question has finally been answered!

    • Marc Perry, CSCS, CPT says:

      That's great to hear, Lauren. Thanks for sharing! Kevin did A LOT of research for this article.

  • Wajisa says:

    Is it okay if I do HIIT in the morning after taking a protein shake??

    • Marc Perry, CSCS, CPT says:

      Hi Wajia - As long as you don't have a very light stomach, give yourself 30 minutes, and drink a light protein shake that you make yourself with whey protein (i.e. banana, crushed ice/water, chocolate whey protein) the short answer is you should be ok. I suggest trying different combinations and foods to see what works for you.

  • Paden says:

    I have found that not eating before any type of exercise or activity directly affects the performance of that task in a negative way. At a minimum, it takes me 15 to 30 minutes longer to warm up physically (this probably also accounts for the mental fatigue as well).

    My question is this: What is the best preworkout meal? Is a protein shake the best thing to have, or is something like 2 or 3 eggs better to eat? Additionally, how many carbs are you looking to have in that meal?

  • Eve says:

    Okay sorry if I've missed something here....it seems these studies are conflicting?

    I have recently been told to do cardio (low intensity for one hour) each morn on an empty stomach and trying to decide whether it's worth the extra effort I need to put in to do that.

    I eat 6 meals a day, a meal every 3 hours. The last two meals have carbs only in nuts and my breakfast has carbs in the form of oats and lts of protein.

    Would it be beneficial for me, in terms of fat burn, to go ahead and eat my carb / protein breakfast after a fasted cardio session and then go onto weights once I've eaten and sufficiently digested?

    • Marc Perry, CSCS, CPT says:

      @Eve - Kevin can chime in, but it sounds like you are getting hung up on the minutiae while losing sight of the big picture. If you want to maximize fat burn, doing some form of interval training is vastly superior to hanging out on a piece of cardio equipment for an hour. Doing a full body strength training workout for 20-30 minutes followed by some interval training would be a very effective workout, and it's along the lines of how I've structured the BuiltLean Program. I highly recommend checking out my free Get Lean Guide to learn the basics. Whether or not you should eat before hand has more to do with your energy levels. That's really the entire conclusion of the article; it's not about fat loss, it's about energy levels.

  • Kush says:

    This is a brilliant article! My question is ...i do high intensity jogging for 45 to 55 minutes during late evenings rather than mornings...is it ok? Or should i switch to early mornings?...i just want to know..what's the PERFECT time for high intensity cardio?? Ur help would be appericiated!!

    • Marc Perry, CSCS, CPT says:

      @Kush - Whatever you can sustain and whatever time works best for your schedule is the best time to workout.

  • kael tan says:

    may i ask?i am 26age 177cm weight 70kg and now my body fat% is 16% so i plan to loss my fat % i wish to to 10%....so i just do a cardio exercise to loss fat without doin muscle building exercise. So do i still need to consume protein source such as eggs,milk,etc....???? actually i want my abs to get ripped....Hope to get some advice from you.thank you. =]

    • Marc Perry, CSCS, CPT says:

      @Kael - please check out my Get Lean Guide to learn the basics. It's a good starting point for you.

  • Deepak Hari says:

    HI, I would like to know if drinking coffee like 4 times a day interrupts my weight loss program? Thanks in advance. Extra info: i drink 1/2 a litre of coffee each day.

    Thanks in advance.

    • Marc Perry, CSCS, CPT says:

      @Deepak Hari - drinking coffee should not interrupt your weight loss program, as long as you it's not affecting your sleep. Generally speaking, drinking coffee much after 2-3pm may affect your sleep because caffeine can linger in your system for several hours. A quality sleep is very important because it helps you control your calorie intake.

  • Jeremy Monaco says:

    Also to comment on the last 2 posters, coffee wont hurt you at all as long as your drinking it as is. Coffee actually has natural caffine in it, so it will actually probably help you lose weight slightly. Now if your adding creamer and sugar etc, its really bad for you, about as bad as drinking the same ammount of soda.

    With regards to protein intake, yes!!!! Protein is the most important building block your body needs to repair itself, and grow. You should aim for a high protein diet, but you also want to get a good macro setup with fats and carbs. this isnt ideal for everyone, but I do 40% protein 40% carbs 20% fat.