The fat-burning zone is a widely misconstrued concept concerning weight loss and fat loss. In the past, endurance athletes have focused a large amount of their training at approximately 60% of their max heart rate and at relatively low exertion. The idea behind this is that training the body to metabolize stored fat efficiently is the only key to performance and weight loss. While the oxidation of fat is a critical component of metabolic efficiency and performance, training purely in a specific zone is not a one-stop answer for weight loss and fat loss.
The oxidation of fat by the mitochondria is the primary source of energy when the intensity of exercise is low (Randell et al., 2019). At rest, fat comprises as much as 85 percent of calories burned (Margolis et al., 2019). As an athlete’s pace increases, a higher percentage of energy is derived from carbohydrates, as does the overall calorie expenditure. Often, as an athlete’s intensity approaches and surpasses the anaerobic threshold (AT), metabolic testing has revealed that little or no fat calories are used and energy is derived purely from carbohydrates. According to the above analogy, one might surmise that the only way to “burn fat” and lose weight is to train exclusively below the AT, but this analogy does not take into account calorie expenditure.
An athlete's weight loss should be aligned with physiological goals and objectives for specific training cycles. Weight loss is commonly associated with the base cycle in an athlete’s performance schedule and is not generally recommended during specific race preparation or competition cycle. To effectively lose weight, an athlete must increase his or her caloric expenditure. Running at or above AT requires a more substantial caloric expenditure than running below the aerobic threshold (AeT). While less fat is used as a source of energy, more calories are expended and the lack of oxygen available above the anaerobic threshold produces excess post-exercise oxygen consumption (EPOC). EPOC aids in both an individual’s metabolism remaining elevated following a workout and in a higher caloric expenditure overall. An endurance athlete cannot train primarily above the AT. When an athlete trains too often above his or her AT, there can be repercussions both in health and long-term aerobic development.
Ideally, an athlete who is looking to lose weight should incorporate a variety of workouts both at or below AeT and at or above AT. A combination of these workouts will keep the body guessing and increase calorie expenditure.
- Margolis, L. M., Wilson, M. A., Whitney, C. C., Carrigan, C. T., Murphy, N. E., Hatch, A. M., ... Pasiakos, S. M. (2019). Exercising with low muscle glycogen content increases fat oxidation and decreases endogenous, but not exogenous carbohydrate oxidation. Endocrine and Metabolic Science, 97, 1-8. https://doi.org/10.1016/j.metabol.2019.05.003
- Randell, R. K., Carter, J. M., Jeukendrup, A. E., Lizarraga, M. A., Yanguas, J. I., & Rollo, I. (2019). Fat oxidation rates in professional soccer players. Medicine and Science in Sports and Exercise, 51(8), 1677-1683. https://doi.org/10.1249/mss.0000000000001973