Exercise physiologists have long recommended that to lose bodyfat, it best to train at a lower intensity level. The higher the level of training intensity, as determined by maximum heart rate, the more the body relies on carbohydrate reserves, such as glycogen and blood glucose.
This appears to be true because carbohydrate is more readily available for quick energy purposes and doesn't require oxygen for metabolism. Fat, on the other hand, does require oxygen for proper combustion, or burning.
Hence, the oft repeated advice that to burn fat, you must exercise at a lower level of intensity that permits maximum oxygen uptake. Canadian researchers from Laval University in Quebec recently challenged this long-held dogma about exercise and fat-burning, however, they put two
groups of men and women on two different workout regimes. One group, which consisted of eight men and nine women, engaged in a 20-week endurance-exercise program that involved cycling four to five times a week for 30 to 45 minutes per session. They used a level of intensity
corresponding to 65 to 85 percent of maximum heart rate, which is typical for an aerobic workout.
The second group consisted of five men and five women who used a higher-intensity program for 15 weeks that featured endurance training combined with high intensity intervals. The results showed that while the high- intensity group burned fewer calories overall, those subjects
showed a fat loss that was nine times greater than the endurance groups. Analysis revealed that the high-intensity group also showed greater activity of muscle enzymes involved in both sugar and fat burning.
The researchers believe that the higher-intensity group lost more fat for several reasons. To begin with, the subjects continued to burn fat after the exercise ended, an indication of higher metabolic rates. In addition, higher-intensity exercise exerts a post exercise loss of
appetite that favors a lower calorie intake. The mechanism at work appears to be increased release of a stress hormone from the hypothalamus called corticotropin-releasing Factor. While it seems that higher-intensity exercise causes greater fat loss compared to the usual
lower-intensity mode, its best not to jump into such exercise until you've developed a sufficient aerobic base, and lower-intensity work is the way to accomplish that.
As you develop more endurance, you can spike your aerobics with high-intensity intervals, where you increase the speed for short periods of two to three minutes. This will produce the effects on fat loss noted in the study described above.