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What Is the Fat-Burning Zone?
When it comes to exercise, particularly cardio exercise, there are different heart-rate zones that equate to different levels of intensity. These levels actually determine which energy systems your body uses during exercise, and that often directly affects how many calories you burn.
There are four different training zones and each is a percentage of your maximum heart rate (MHR), the maximum number of times your heart can beat in a minute:
Low intensity, also known as the “fat-burning zone,” is 50% to 70% of your MHR. It’s usually considered light cardio or a warm-up level.
Moderate intensity is 70% to 80% of your MHR. At this level, you’ll be working out but still be able to talk. 
High intensity is 80% to 90% of your MHR. This usually puts you well out of your comfort zone and pushes you to your anaerobic threshold,  which is when your cardiovascular system can’t deliver oxygen to your muscles fast enough. 
Maximum effort is 90% to 100% of your MHR, something that even the most highly trained athletes can’t maintain for more than a few minutes.  
The Truth About Your Fat-Burning Zone
From these numbers, you can see that the fat-burning zone is the lowest intensity. So why is it called the fat-burning zone? Because the body relies on more stored fat (versus carbs) as its primary fuel source when you work at a lower intensity compared to a higher intensity.
Some people have translated this to mean that you actually burn more fat when you work at a lower intensity, but that’s a bit of a misconception. In reality, picking up the pace will torch more total calories—and ultimately more fat—in less time. And it’s the number of calories you burn overall that leads to the most weight (and fat) loss.
To give you an example, the chart below details both the total calories and the fat calories expended by a 130-pound woman during cardio exercise.
As you’ll see, the woman burns more total calories and more fat calories when working out at a higher intensity.
Low Intensity (60 percent to 65 percent MHR) High Intensity (80 percent to 85 percent MHR)
Total calories expended per minute 4.86 6.86
Fat calories expended per minute 2.43 2.7
Total calories expended in 30 minutes 146 206
Total fat calories expended in 30 minutes 73 82
Percentage of fat calories burned 50% 39.85%
While lower-intensity workouts are great for beginners and for building endurance, you need to work harder during some workouts if you really want to burn fat and lose weight.
The Case for Low-Intensity Exercise
Now, this isn’t to say that low-intensity exercise doesn’t have its place, especially if you’re just starting out and can’t sustain a faster pace. If you go slower, you may be able to exercise a lot longer, so you’ll end up burning more calories and fat that way. 
Even for more advanced exercisers, endurance workouts should be a staple of a complete fitness program along with shorter, higher-intensity workouts or interval workouts.  A type of training in which you alternate periods of high-intensity exercise with low-intensity recovery periods, intervals are proven to increase fitness and burn more calories than you would when doing the same thing during your whole workout.
Structuring Your Cardio Workouts
If you want to lose weight, a general cardio schedule would include workouts at a variety of intensities within your target heart rate zone.  If you’re doing five cardio workouts a week, you might have one high-intensity workout, one lower-intensity workout, and three somewhere in the middle.
Sample Cardio Workout Program for Beginners
A beginner cardio program lets you slowly build endurance while getting you a bit out of your comfort zone. That way you don’t have to spend an entire workout miserable, yet you’ll still challenge yourself, which will burn more calories.
Below is a sample program that will help get you started. 
Day Workout/Intensity Length
Monday Beginner-Interval Workout Up to 21 minutes
Tuesday Low-Intensity Walking 10 to 20 minutes
Wednesday Rest
Thursday Cardio-Endurance Workout Up to 35 minutes
Friday Rest
Saturday Interval Training Level 2 Up to 25 minutes
Sunday Low-Intensity Walking 10 to 20 minutes
The key is to start with what you can handle and slowly build from there. If you’re just getting started, don’t worry too much about how hard you’re working. Focus more on making exercise a habit you can manage on a regular basis.