Busting the “fat-burning zone” myth

You’ve probably seen those colorful charts on exercise machines at the gym, showing your ideal heart rate zone for optimal fat burn. Is this “zone” the best way to burn fat?

The concept of the “fat-burning zone” might not be entirely true. Many people assume that in order to burn fat, they must keep their heart rate within the defined range. This can be misleading for a few reasons. First, people’s heart rates are very different, making it difficult to generalize recommendations from a fixed chart. Second, your body burns two main sources of energy during exercise: fats and carbohydrates. (Protein is an energy source, but it’s only used in very small amounts.) For any given heart rate, your body will burn both carbohydrates and fats; however, the proportion of each will vary. Low-intensity exercises (lower heart rate) with a longer duration (30 minutes or more) mostly rely on fat for energy. So, there’s a zone in which a higher proportion of fat is being used for energy, but that doesn’t necessarily mean more fat is being burned. Your body actually gets most of its energy from fat during rest. In theory, your ultimate “fat-burning zone” is in your living room: you lying on your couch, binge-watching your favorite new series.

So, how do you burn more fat? High-intensity exercises actually burn the most 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.

If you’re training for endurance activities, the “fat-burning zone” on the exercise machines might be the “right zone” for you. To burn even more fat, you ultimately need to burn more overall calories. High-intensity workouts are a challenging and efficient way to help reach your goal.

References

Coyle, E. F. (1995). Substrate utilization during exercise in active people. The American Journal of Clinical Nutrition, 61(4), 968S–979S.

Hetlelid, K. J., Plews, D. J., Herold, E., Laursen, P. B., & Seiler, S. (2015). Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners. BMJ Open Sport & Exercise Medicine, 1(1), e000047. doi:10.1136/bmjsem-2015-000047

Horowitz, J. F., & Klein, S. (2000). Lipid metabolism during endurance exercise1,2,3. American Society for Clinical Nutrition, 72(2), 558s–563s.

Santiworakul, A., Chuaychoo, B., Kriengsinyos, W., & Jalayondeja, W. (2014). Substrate utilization during and after high intensity exercise in healthy lean and obese men. Journal of the Medical Association of Thailand, 97(Suppl 7), S50–S54.