The fat-burning zone, popular in fitness centers and in weight-loss arenas, is actually based on a misunderstanding of exercise physiology and some short-sightedness. It is true that at a low intensity, you dervive the greatest percent of your energy from fats over carbohydrate versus high-intensity exercise. But, that is just part of the story. High-intensity exercise may derive a lower percent of energy from fat while you are exercising because fat metabolism is inhibited, but high-intensity exercise will result in a greater energy deficit overall. While fat metabolism is inhibited during the session, it actually increases over the next 48 hours after the exercise session.
We have what is known as metabolic flexibility, we can shift back and forth between fat and carbohydrate metabolism. Certain situations shift our metabolism one way or another (see video below). But, the biggest factor in losing body fat is the energy balance. High-intensity exercise is more effective than low-intensity exercise in developing a large energy expenditure and therefore a greater net body fat loss (especially when combined with appropriate nutrition) in a given amount of time (most of us have limited amount of time to exercise). So, following the heart rate fat-burning zone is actually disadventagous. Instead, ditch the heart rate monitor and work as high of an intensity you can safely handle. Intermittant exercise works very well. Lastly, make sure you are always keeping your body strong and moving well, as this will help you better tolerate high-intensity exercise.
Here is a video I made last year about metabolic flexibility to help you better understand the basics of fat and carbohydrate metabolism:
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