Endurance athletes – use fat as fuel instead of carbs?

by | Sep 29, 2022 | Physical Wellness | 0 comments

For many years it has been believed that athletes need to carbo-load in order to be adequately fuelled for their training sessions or races. Also, athletes are well aware that if carbohydrate levels are not replenished in the right way during the event, training intensity will be reduced, fatigue will be experienced and recovery will be compromised (Gejl, 2014). This is why many athletes turn to sports drinks and energy gels to top up their carbohydrate stores and sustain them in this regard. The use of these artificial substances is not always ideal, however, and could even be harmful in the long run, as they tend to tax the liver and cause inflammation. Luckily, interesting recent research has shown that there is another way to fuel the body which might even be more effective performance-wise and also beneficial to long-term health.

 To explore this concept further, it is first important to understand that the human body can use two different sources of fuel or energy. One source of energy is glucose –  which we can get from carbohydrates and sugars (think potatoes, rice, fruits, sweets, sports drinks etc). If glucose is not available, however, and the stored carbohydrates in the body have been depleted, the body will switch to another energy source, fat. There seem to be many benefits when using fat as fuel instead of carbohydrates, especially for endurance athletes. Let’s look at a few reasons why you might consider training in a fat-adapted state instead of relying on carbohydrates:

  1. Fat provides us with almost double the amount of energy compared to carbohydrates. This is because the caloric value of fats per unit is much higher than carbohydrates (Wardlaw & Hampl, 2007). We, therefore, need less fat to fuel us in the same way.
  2. Fat can be stored in the body abundantly (in the liver, muscle and of course fat tissue), while carbohydrate storage is only limited to some areas in skeletal muscle and the liver. This means that we’ll have access to more energy and be able to use it as a consistent energy supply when using fat as fuel. An endurance athlete will, therefore, be supported in a much better way for a longer period.
  3. Using fat as fuel will not only alter how an athlete uses energy during exercise, but it also positively impacts how the muscle recovers post-workout as it reduces levels of inflammation  (Zinn et al, 2017), (Huang et al, 2018). Enzyme levels that measure muscle damage (including creatine kinase and lactate dehydrogenase) were reported to be much lower on a ketogenic diet (Zajac et al, 2014).
  4. The brain is a glucose-dependent organ, which means that it gets most of its energy from carbohydrates, but when glucose gets depleted during exercise, the energy supply to the brain will also be interrupted. This, however, will not be the case when using fat as energy, as it will provide a constant supply of ketones as energy to the brain (Volek et al, 2014), (Noakes, 2004). The brain of an athlete could become deprived if dependent upon glucose, which could put the athlete who is keto/ fat-adapted in an advanced position, especially when mental clarity, concentration, focus etc are at play.

Because we know that the brain can switch to ketone bodies (fat) as a source of energy when glucose is not available, ketones have even been applied in the treatment of neurodegenerative diseases (which are characterized by the brain not being able to metabolise glucose in the way that it should) (Jensen et al, 2020). Functional improvements have been seen in patients with both Alzheimer’s disease and patients with Parkinson’s disease, where cognitive benefits and neuroprotective functions were shown with ketogenic interventions (Jensen et al. 2020). A small study even showed that adhering to a ketogenic diet for 4 weeks led to a big improvement in the Unified Parkinson’s Disease Rating Score (Vanitallie et al, 2005).

Is this for me?

Although this approach might be highly beneficial to some, it is very important to remember that there is no one size fits all. Some athletes might be served well in a fat-adapted state, but others might experience that it impairs performance. For example, if we look into what type of fuel is needed for different types of events, fat-adaptation will most definitely be more relevant to endurance athletes (long-distance medium intensity exercise (marathons, Ironman’s, cycling races). When we are exercising at a 50% VO2 max (maximum oxygen uptake), only 35% of total energy requirements are met by carbs while 65% is met by fat (Maughan & Shirrefs, 2012). When energy intensity is increased to 75% VO2 max (sprinting, weight lifting, etc), however, the major fuel preference is carbohydrates (Maughan & Shirrefs, 2012). In these instances, it would be crucial to provide the body with enough carbohydrates to perform optimally.

As you can imagine, when it comes to sports performance, individualisation is key in order to perform at your best and keep on performing consistently. Experimenting with different approaches and observing closely how your body responds to different energy sources could bridge the gap between being good and doing your absolute best. 

Feel free to book a complimentary call with Zena to find out more about how you can improve consistent long-term performance for YOU in her personalised sports performance programs.


Gejl, K. D. et al. 2014. Muscle glycogen content modifies SR Ca2+ release rate in elite endurance athletes. Medicine and science in sports and exercise 46, 496-505, doi:10.1249/mss.0000000000000132 (2014).

Huang, Q., et al. 2018. An 8-week, low carbohydrate, high fat, ketogenic diet enhanced exhaustive exercise capacity in mice part 2: Effect on fatigue recovery, post-exercise biomarkers and anti-oxidation capacity. Nutrients, 10(10), 1339. DOI: 10.3390/nu10101339

Jensen NJ, et al. 2020. Effects of Ketone Bodies on Brain Metabolism and Function in Neurodegenerative Diseases. Int J Mol Sci. 2020 Nov 20;21(22):8767. doi: 10.3390/ijms21228767.

Noakes, T. 2004. Fat adaptation and prolonged exercise performance. Vol. 96

Vanitallie T.B., et al. 2005. Treatment of Parkinson’s disease with diet-induced hyperketonemia: A feasibility study. Neurology. 2005;64:728–730. doi: 10.1212/01.WNL.0000152046.11390.45.

Volek, J. S., Noakes, T. 2014. Rethinking fat as a fuel for endurance exercise. European journal of sport science, 1-8, doi:10.1080/17461391.2014.959564

Wardlaw, G. M. & Hampl, J. S. 2007. Perspectives in nutrition, 7th Edition. McGraw and Hill Companies, Inc. New York. ISBN-13 978-0-07-282- 750-7.

Zajac, A., et al (2014). The effects of a ketogenic diet on exercise metabolism and physical performance in off-road cyclists. Nutrients, 6(7), 2493-508. DOI: 10.3390/nu6072493

Zinn, C., et al (2017). Ketogenic diet benefits body composition and well-being but not performance in a pilot case study of New Zealand endurance athletes. J Int Soc Sports Nutr, DOI: 10.1186/s12970-017-0180-0

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