Myth.

When losing weight what is most important is the total amount of calories you burn, not the type of calories you burn. During high-intensity exercise, you burn more calories than low-intensity exercise thereby creating a higher calorie deficit and promoting weight loss.  Exercising in the low heart-rate zone may end in frustration because the amount of weight and body fat loss will not be significant. When you engage in high-intensity exercises results are much faster. However, if training for a marathon or Iron Man, exercising at a lower intensity is better as you need to retain your glycogen storage in order to sustain long periods of exercise.