Is low or high-intensity cardio better for burning fat?

High-intensity cardio is much better for burning fat. A decrease in body fat will take place anytime more energy is being burned than is being consumed. It is how many calories you burn that dictates body fat reduction. If your goal is fat loss then you will want to perform your cardio in a way where you will burn more calories. A great way to do this is by increasing the intensity of your workout. Performing exercise at a higher intensity will have your body working harder and therefore burning more calories. Interval training alternating high intensity periods with low to moderate intensity periods is a great way to exercise for maximal calorie burn. With this type of training you will continue to burn calories even after you have stopped exercising. Interval training is an advanced method of training so if you are a beginner start out slow and work toward increasing the duration of your cardio sessions first. Once you can hold a steady pace for 45 minutes a day, three to four days per week, then you can start working toward increasing the intensity of your workout. The more intense your workout the more calories and fat you will burn.
Wendy Batts

Often this question boils down to “what type of cardio is better?” The short answer is that the best type of cardio, whether low or high intensity, is the one you will do consistently over time.

Now, with that being said, if we were to compare only intensity, the optimal goal is to start at lower intensity if you are new to cardio and slowly work your way to higher intensities as your endurance and cardio respiratory work capacity improve. This is because you will just flat-out burn more calories during the workout at higher intensity. And remember losing fat is about burning more calories than we consume over time. The other advantage to higher intensity cardio is that the “after-burn”, or the calories we expend afterwards to adjust to the workout, is actually higher. So not only are you burning more calories during, but after the cardio as well. Both of which will contribute to hitting your weight loss goals faster. Remember to progress slowly with baby steps though. Doing too much too soon can lead to injury or burn out, which can take you out of the game.

Dr. Mike Clark, DPT
Ultimately, weight loss comes down to eating less and moving more. With that said, it’s important to understand that different exercise intensities require your body to use different amounts of fat for energy. Typically during low-intensity cardio a higher percentage of the calories you burn will come from fat; however, because it is not as strenuous, your total calorie burn will be lower than with higher intensity activities. High-intensity cardio is effective for fat-loss because you burn more calories per minute while doing it– when compared to low-intensity cardio, as well as during the time it takes your body to recover from the strenuous workout. Therefore, for successful weight loss to occur it is best to incorporate a mix of both low and high-intensity cardio, not only to maximize your body’s ability to use and burn fat, but also burn more total calories overall.
Dr. Michael Roizen, MD
Internal Medicine

Technically, the charts you see at the gym and on exercise machines are right: With a low heart rate, your body likes to fuel activity by using fat. When you work harder and your heart rate climbs, your body prefers the quicker energy hit of any sugar lying around in your system.

But that doesn't mean that keeping your heart rate low for 30 minutes will make you slimmer than if you had pumped things up for 30. Quite the opposite! You incinerate more calories per minute -- that's what melts pounds -- when your heart rate is high.

A lower intensity cardio will but 50 percent of the calories from fat, while at higher intensities it may only burn 35 percent. But at higher intensities you burn way more total calories and more fat calories overall than you do at lower intensities.

The answer is Yes and No.  It is true that if you exercise at a higher rate of intensity, you burn a little less fat per calorie, and going slower you can burn a little more fat per calorie. 

BUT, what is more important is... overall amount of total calories burned.

Cranking up the pace means you burn more calories because the harder the workout, the more oxygen you use, and this translates to burning more calories. So when it comes to weight loss, it will benefit you to burn more calories overall at a higher intensity workout. 
If you are a new exerciser, try interval training. Warm-up at a low intensity, and alternate one minute of high intensity with a one minute low (or you can call it recovery) intensity.
As you progress, you can then start to either increase the intensity or duration of the high intensity part and decrease the duration of the low intensity part.

Your ability to burn fat depends upon your fitness level, capabilities, and preference. The myth that surrounds this stems from information posted on most cardio machines and out-dated posters in gyms and fitness facilities. For weight loss to occur, one must simply burn more calories than they consume. In a deficit, the missing calories will come ultimately come from fat. Just because you burn a lot of fat in 45 minutes of low-intensity cardio, there is no guarantee that weight loss will occur. If you eat more calories throughout the day than you burned, then you will actually gain fat, even though you were in your fat burning zone during the workout. The premise is that at lower intensities you burn a higher percentage of fat per minute than you do at a higher intensity when you burn more carbohydrates (in the form of glycogen). As a percentage of nutrients burned this may be true, but at a higher workout intensity you will burn more calories and more fat per minute overall when compared to a lower intensity level. Imagine how this might work using the demonstation-only numbers below. At a low intensity, you may burn 70% of calories from fat while at a high intensity you burn 50%. In 20 minutes of cardio exercise you may burn 100 calories at a low intensity and 200 at a higher intensity. So, would you rather have 70% of 100 or 50% of 200? 70 versus 100? The higher intensity cardio activity will burn more calories and more fat per unit of time. If however you do not have the conditioning, or other factors prevent you from working at a higher intensity, then it is perfectly acceptable to train at a lower intensity for a longer period of time to achieve the same fat burning results.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.