What exactly is the Fat Burning Zone?
The Fat Burning Zone is claimed to be the optimal training intensity for burning fat: an intensity between 60 and 75 percent of your predicted maximum heart rate (208 minus Age (0.7)). The theoretical reason behind this argument is based on a principal whereby, the lower the exercise intensity, the greater the percentage of energy requirements is met by fat. Inversely, in the Cardio Training Zone (between 70 and 85 percent, depending on references), the main energy supply comes from carbohydrates. The reason for the shift is due to difficulty in obtaining, transporting and utilizing sufficient oxygen at higher intensities to metabolize fat (which has a higher hydrogen component than carbohydrates), leading to the use of carbohydrates as a preferred fuel source.
This is the portion of information most often used to justify low intensity Fat Burning Zone training. Consider this: When sleeping, over 90 percent of energy used comes from fat stores. With this in mind, why train at all? Herein lies the flaw in the "fat burning zone" argument - the disregard for the total amount of calories consumed.
Let us say that Subject A ran or walked at 65 percent (in his Fat Burning Zone) for 30 minutes and burned 400 calories; 60 percent of these calories are made up from fat, which means that 240 calories of the overall 400 calories burned are from fats.
However, if this same person trained in his Cardio Zone at 75 percent and burned an estimated 700 calories, the now lower 40 percent of fat calories burned equals 280 calories with the overall total being 700 calories burned.
This shows that not only are more fat calories consumed but so too are the total amount of calories. Remember: a key concept behind fat loss is in creating a calorie deficit (more energy out than in).
There are benefits of low intensity training, however. If you were to train for longer in the Fat Burning Zone (compared to Cardio Zone), a greater percentage of fat would be burned from higher total values. For example, if Subject A were to instead run or walk at 65 percent for an hour, the total calories burned would equal 800, with fat making up 480 calories. Furthermore, injury or a poor fitness base may mean that high intensity training is inappropriate at a given stage in a training program. Conversely, higher intensity training has benefits in terms of a proposed increase in metabolic rate and hunger suppression following high intensity sessions.
So which to use? As with any training program, the purpose of the training and history of the client are paramount considerations. Applying the basic concept of periodization to fat loss, low intensity exercise of long duration can be used to develop an initial training base (and anatomic adaptation) with gradual progression to higher intensity, shorter duration sessions. If already fit, periodized cycles can be shorter (e.g., two weeks low intensity, one week medium intensity, one week very high intensity) or even interspersed in a single session (e.g., 10 minute run at 80 percent followed by 30 minute cycle at 60 to 70 percent). So forget the hype, stick to the facts. Fat loss is simple: input versus output.