Why is the fat-burning zone a myth?

That "fat-burning zone" chart on exercise equipment might not be what you think it is. Keeping your heart rate within a certain range based on a chart is a bit misleading. The first thing to consider is that people's heart rates differ, which makes it difficult to generalize recommendations on a fixed chart. Second, during exercise your body uses both fats and carbohydrates, protein is an energy source too, but it's only used in very small amounts. However, the proportion of each macronutrient you need varies depending on the intensity of the exercise. The best way to burn more fat is to do more high-intensity exercises. You might not be exercising as long, but you'll burn more fat due to the higher overall energy, caloric, expenditure. Interval training is a great way to boost the intensity of your workout, and you get that afterburn effect. Fitness level also is a factor. Fitter people's bodies tend to utilize more fats than carbohydrates.