The "fat-burning zone" and "cardio zone" became popular buzz phrases when equipment manufacturers began displaying the red and yellow graph on the consoles of treadmills, ellipticals and bikes. This phenomenon has led to the theory that you must exercise at low intensities in order to burn fat. As with many myths, there is some truth in this concept. However, it's more important to consider the amount of calories burned during your workout versus the amount of fat utilized.
The Truth in the Myth
The amount of calories you burn is directly related to exercise intensity. It is a fact that the body uses fat as the primary source of fuel during lower intensity exercise. About 60 percent of the calories burned come from fat. This fact has given birth to the fat burning zone notion. But, for overall fat loss, what matters most is the difference between the number of calories you expend and the number of calories you consume.
Heart Rate Zones
Both zones are designed to keep your heart rate within your target range. Your target heart rate ranges from 50 to 70 percent of your maximum heart rate states the American Heart Association. If you exercise in the fat burning zone, your heart rate stays in the lower end of the range, not exceeding 70 percent. To exercise in the cardio zone, the intensity of exercise must increase, resulting in an elevated heart rate.
Fat-Burning Zone Workouts
Exercises such as walking and cycling with little resistance prompt your heart rate to stay in the fat burning zone. Because the intensity of these types of workouts is low, you must exercise longer to burn the same amount of calories as you would compared to a higher intensity workout. Low-intensity workouts are recommended for those who are de-conditioned or significantly overweight. It's important to build an aerobic base before progressing to high intensity training.
The Cardio Zone
When you exercise at intensities greater than 70 percent of your max, your body uses carbohydrates as the primary source of fuel. Though you are not burning fat, you are burning lots of calories. Intervals are an excellent way to ramp up the intensity of your workout.
To try them, go hard for one minute at your cardio exercise of choice (jogging, running, elliptical), then recover for one minute and repeat. As you become proficient, increase the number of minutes spent on the intense portion and and lower the number of minutes on recovery. Interval training or HIIT burns more subcutaneous fat than endurance training according to ExRx.net.
Putting It All Together
Regardless of their names, exercising in both zones leads to weight loss. Ironically enough, the cardio zone, with its higher intensities will help you burn more calories in a shorter duration. For the purpose of losing weight, it matters little whether the calories burned during exercise come from fat or carbohydrates. Focus on a challenging workout and do the satisfying number crunching over your post-workout protein shake.