Fat Combustion…The Bottleneck Of Endurance Performance

Fat burning has long been a common term within endurance athlete circles. What is “fat burning”? Is it important, and, if so, how can it best be achieved?

     In simplified terms, “fat burning”, or fat combustion, is the process by which the body metabolizes fat stores for energy production. The greater the body’s ability to oxidize these fat stores, the more carbohydrate is spared as fuel for the creation of energy.

     It is well known that highly-trained endurance athletes have an increased capacity to oxidize fatty acids during exercise compared to recreational athletes. As a result, these elite-level athlete’s improved fat utilization decreases carbohydrate consumption across all sub-threshold intensities, which is especially useful when glycogen stores are limited. By comparison, a recreational athlete is typically limited in their ability to combust fat as fuel, thus placing a premium on carbohydrate and glycogen consumption. It is not uncommon for a recreational endurance athlete to metabolize a maximum of no more than 200 calories from fat per hour, while a top level professional will oxidize 700+ calories from fat and do so at a much higher intensity.

Factors affecting fat oxidation

  • Exercise Intensity – One of the easiest means of managing the amount of fat combustion is through the intensity of your efforts. As exercise intensity increases, carbohydrate oxidation increases proportionally. However, fat oxidation initially rises to a maximal point and then begins to fall until reaching zero at the athlete’s Anaerobic Threshold. We reference the exercise intensity at which fat combustion is greatest as Fatmax. Each athlete’s Fatmax varies greatly and is ultimately determined by the individual’s metabolic profile. The only way to pinpoint an athlete’s Fatmax, in watts for cycling and pace for running, is to perform sport specific metabolic testing (more info can be found on our website: VO2max/VLamax).

  • Dietary effects – Another important factor that impacts fat metabolism during exercise is diet. Studies have shown that high carbohydrate diets subdue fat oxidation rates, while low carbohydrate, higher fat & protein daily diets result in greater fat combustion rates. A raise in insulin levels from consuming carbohydrate prior to a workout will suppress the body’s fat metabolism. As a consequence, the highest rates of fat oxidation typically occur after an overnight fasting and starting a workout on an empty stomach with low glycogen and blood glucose levels. However, as we mentioned in the first bullet point, exercise intensity plays an even bigger roll. If the session requires high intensity, then pre-workout carbohydrate can be very beneficial. To maximize the benefits to your fat metabolism from a fasted workout, the effort must remain aerobic and, ideally, at your individual Fatmax intensity.

  • Aerobic fitness level – An individual’s aerobic capacity is directly linked to their body’s ability to burn fat. A higher VO2max means that a particular athlete will uptake more oxygen per minute, resulting in a greater quantity of fat being contributed as a fuel source. Since fat can only be combusted in the aerobic metabolism, improvements in oxygen utilization rates greatly increases the body’s fat combustion capabilities. As an added bonus, the more fat that the body can consume in a given hour of exercise, the less carbohydrate is required (blood glucose and/or glycogen).

Exercise training

  • Routine physical activity at the proper intensities is the only proven means of increasing fat combustion rates. Each athlete’s specific metabolic profile determines how they will respond to various training overload & intensities. Following a consistent, well-planned training program will increase overall blood volume and the oxygen carrying capacity of the blood, improve capillary density and mitochondrial mass & efficiency, and improve the cellular response of the enzymes responsible for the regulation of the fat oxidative pathways. Along with a host of other positive physiological adaptations that regular exercise provides, higher fat combustion rates are one of the most beneficial to endurance athletes.


  • Fat oxidation rates are highly individual.

  • Significant differences have been identified between athletes and their response to specific training stimulus and intensities.

  • To maximize endurance performance gains it is important to understand your fat combustion profile. Knowing your personal Fatmax Zone will enable you to train fat combustion most efficiently.

  • Seek nutritional advise that can accompany your training program in order to raise Fatmax even higher.

Assess Fatmax where it matters: on the road (or in the lab)!

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