Fat Burning Zone – Fact or Fallacy?
If you were to look on most pieces of gym-based cardiovascular equipment, you could bet your last pound that the heart rate training zone chart or illustration printed on the machines console includes a ‘fat burning zone’, and that this zone represents the lowest of the suggested exercise intensities. In this article we will explain why this zone originated but more importantly, we will answer that all important question, is this best intensity to exercise at if you want to rid the body of excess fat stores?
Most fitness professionals will agree that while they promote the health benefits of cardiovascular exercise, the most common motive for engaging in exercise is to lose weight, or more candidly, lose fat! The assumption that fat can only be burned over prolonged periods of time, and that the exercise intensity must be low enough to allow oxygen saturation is widespread, and factually incorrect.
The simple law of thermodynamics states that energy storage (which includes fat), can only be reduced when more energy is expended from the body than is consumed. Thus to reduce body fat, more calories must be used and converted to energy, and ultimately heat, than are consumed from food. When energy expenditure exceeds intake, the body will draw upon on its stores (fat, glycogen and protein) to make up the difference. Thus, the greater the difference between energy intake and expenditure, the greater the fat loss.
When one considers the blend or mix of fuels used at different levels of exercise and physical activity, it is clear that as a percentage of the gross energy expended, a greater reliance on fat is demonstrated. This is largely brought about because fat can only be burned in the presence of oxygen, or aerobically, thus when larger quantities of oxygen are delivered to the bodily tissues, the easier it is for the body to oxidise or burn the fat. Conversely, higher intensity workloads rely more on stored carbohydrate sources, namely glucose and glycogen, and less on fat. This principle is what the fat burning myth is built on and originated from, and does not consider total energy expenditure.
Lower intensity exercises typically result in less energy or calories being burned. While these workloads may indeed use ‘as a percentage of total energy expenditure’ more fat, the number of actual fat calories expended is still fewer than that consumed during higher intensity cardiovascular exercises. For example, consider the following energy expenditure for an 82 kg male:
45 minutes of brisk walking at approximately 4 mph= 315 kilocalories
Approximately 60% of energy burned from fat*
Total fat calories= 189
30 minutes jogging/running at approximately 7.5mph= 503 kilocalories
Approximately 40% of energy burned from fat*
Total fat calories= 201
It is important to note from the above data that not only is the total number of calories expended higher, the gross fat calories consumed is also greater; thus more body fat would be burned.
The health and fitness benefits associated with higher intensity exercise are also likely to be superior to those achieved from lower intensity workloads, making the training more effective across the board.
Exercise adherence may also be positively influenced because individuals are able to exercise for a shorter period of time, making it easier to fit the programme into their busy schedule.
Higher intensity exercise modalities also have a greater effect on oxygen consumption post exercise (Excess Post-exercise Oxygen Consumption-EPOC), increasing the recovery and energy demands further. When the intensity of the exercise is greater, the body’s metabolic processes will be elevated for longer, further increasing the energy expenditure of the training session, and making it even more effective for burning body fat.
In summary, it is not the percentage of fat burned that will dictate how effective the exercise modality is for fat burning, it is the gross number of calories that will determine the effectiveness of the programme. Thus for maximum fat loss, exercise professionals should seek to prescribe exercises that significantly increase energy expenditure, and within the physical and technical capabilities of their clients.
*the precise energy mix and percentage of energy derived from fat will vary from one individual to the next. These figures therefore only represent mean values and are provided only to illustrate the concept.