Heart rate is a measure of how hard your heart is working and thus helps people determine the intensity of a particular workout. When you work your body harder, your heart beats faster and your body turns to different fuel sources when working at various levels of intensity. It makes sense that people seeking to lose weight would want to maximize fat loss, so oftentimes they strive for a heart-rate level that falls within the “fat burning” zone.
To determine where your intensity lies, you must first figure out your maximal heart rate. The maximal heart rate is age dependent, for as we grow older, the heart beats a little more slowly. To find maximal heart rate, subtract your age from 220. The fat burning and cardio zones are defined as a percentage of this maximal heart rate.
A heart-rate zone that improves cardiovascular fitness is defined as between 55 and 80 percent of maximum heart rate, according to The American College of Exercise. A less-conditioned individual may see benefits to her heart health at the lower levels of this recommended zone while an athlete needs to work at a higher intensity to improve fitness. The fat-burning zone occurs at the lower end of the cardio zone—between 55 and 70 percent of maximum heart rate. When working at this intensity, a greater percentage of calories burned comes from stored fat.
Working anywhere within the cardio zone will improve cardio fitness and burn calories. The benefit of working in the fat-burning zone is that you target stored fat and thus encourage weight loss. But, keep in mind, that if you work in a higher zone, you burn a greater amount of calories overall—which is important to weight loss. Although a lower percentage of these calories will come from fat sources, the total number of fat calories burned will likely be the same or greater. Take, for example, a person who burns 100 calories in a 15-minute walk at a heart rate of 55 percent of maximum. Seventy-five percent of those calories, or 75 calories, may come from fat. However, a person who runs those same 15 minutes and burns 200 calories may burn only 50 percent of those calories from fat, but will burn a total of 100 fat calories in the same amount of time.
A study based on 36 relatively fit runners published in the "Journal of Strength and Conditioning Research" in October 2009 demonstrated that determination of a person's exact fat-burning zone may only be performed in a laboratory. In this study, significant overlap between the fat burning and cardio zones existed, indicating that a person working within 60 and 80 percent of maximal heart rate is likely to be maximizing fat oxidation.
Unless you are training for a figure or body building competition where working in the fat-burning zone exclusively helps you preserve every bit of lean body mass, it is best to focus on improving fitness and calorie burn with varying levels of intensity. If you choose to work in the lower range of the cardio zone—the fat-burning zone—be prepared to work out longer to burn enough calories to affect weight loss. Incorporating high cardio and lower cardio zone exercise into a workout routine will offer the most aerobic and weight loss benefit.