High Fat Diets and Ultra Endurance Performance


By Rebecca Dent, Uphill Athlete High Performance Dietitian 

This is the third article in a series on ‘Nutrition and Fat Adaptation’. Be sure to read the first, Nutrition and Fat Adaptation, as well as the second installment, Nutrition Strategies to Maximize Fat Adaptation.

This article discusses whether eating a low carbohydrate high fat diet further enhances ultra endurance performance beyond training compared to a diet containing carbohydrate. For our purposes, I define ultra endurance performance as any exercise lasting longer than four hours.

As we previously stated, due to the body having a limited storage capacity for carbohydrate but an abundant availability of fat to provide a steady supply of energy, the aim of every endurance athlete should be to train the body to increase it’s ability to use fat as a fuel source.

Regardless of dietary intake it has been shown that in well trained endurance athletes fat oxidation (fat used by the body as a fuel source) is greater compared to recreational / untrained participants. Well trained athletes are also able to oxidise fat at higher intensity exercise (1). This highlights it is the actual endurance training per se and the use of training strategies such as training in a fasted state, that are of the greatest importance when it comes to fat adaptation and ultra endurance performance (2).

The question this then raises is, could consuming a low carbohydrate high fat diet further enhance the availability of fat in the body to use as an energy source beyond the training effect?

Over the past few years there has been a resurgence in the consumption of a low carbohydrate high fat diet (LCHF) in ultra distance athletes, with anecdotal reports of this dietary method being superior for success in ultra endurance performance.

What is a Low Carbohydrate High Fat Diet? 

A LCHF diet that induces ‘ketosis’ (a metabolic response in the body when carbohydrate intake is minimal) is one of which >80%% of the daily energy intake is derived from fat, 15% energy contributed from proteins and <20g per day contribution of carbohydrate. In food terms for carbohydrate, this translates into no more than 1 thin slice of bread or a small banana per day. The rest of your daily energy intake comes from high fat foods including meats, fish, eggs, cheese, oils, nuts, seeds, butter, avocado (some nuts, seeds, fruit, vegetables and dairy products all contain a small amount of carbohydrate that has to be taken into account).

Studies have shown that well trained athletes who follow a LCHF diet have higher rates of fat oxidation and use a greater percentage of fat as a fuel source at low intensity exercise (4-7).


Does Eating a LCHF Diet Improve Performance in Ultra Endurance Athletes?

It has been proposed that a LCHF diet may enhance performance in ultra-endurance events by supporting a higher fat oxidation rate (providing more fat as a fuel source) and sparing muscle glycogen.

Despite this premise there is insufficient scientific evidence to support any claims of a superior performance eating a high fat diet vs. a high carbohydrate diet (5-7).

A recent study showed elite race walkers eating a LCHF diet impaired 50km race performance (7). In another study comparing world class ultra distance runners who consumed a LCHF to those who ate carbohydrates, higher rates of fat oxidation were shown in the LCHF runners but the study did not determine who were the better runners from the two groups (6). This perhaps suggests eating a LCHF is at least not detrimental to ultra endurance performance for some.

What is pertinent to highlight, it is not the high fat diet per se that enhances rates of fat oxidation but actually the low carbohydrate intake that augments fat oxidation (2,8).

It is also important to note that adapting to a high fat diet has been shown to impair your ability to perform high intensity exercise (e.g. speed or hill sessions, hard climbing efforts, strength training). In other words there is potentially a price to pay when consuming primarily fats in your diet. Your ability to use carbohydrate as a fuel source is impaired, you lose your top gear!

To support adaptive changes to endurance training perhaps it is not one or the other? A lower carbohydrate intake of 2g carbohydrate/kg of body weight per day, may be sufficient to encourage an increase use of fat as a fuel source during low intensity exercise. This then allows an intake of carbohydrate to be consumed around higher intensity training sessions.




  • Eating a LCHF diet has been shown to enhance fat oxidation in well trained athletes. However at present further research is required to determine if this provides additional benefits to ultra endurance performance.
  • It is not the high fat intake per se but the low carbohydrate intake that influences fat oxidation rates.
  • Endurance training ultimately makes us better at ultra endurance performance and it is the training effect which increases the body’s ability to use fat as a fuel source.
  • It is not a necessity and is not for everyone! There can be some health risks to following a LCHF diet over the longer term if the LCHF diet is not carefully considered and well planned to incorporate all nutrients.
  • It does not need to be one or the other when it comes to dietary choice of LCHF or including carbohydrates. Adjusting carbohydrate intake to suit the training required would be more beneficial.
  • There are some well trained ultra endurance athletes that report to eat and perform well on a high fat diet but equally some of the world’s best ultra endurance athletes consume higher intakes of carbohydrate.


Uphill Athlete High Performance Dietitian Rebecca Dent is available for both phone consultations about diet and she can create a Custom Performance Nutrition Plan for you.


References/ Further Reading


  1. Hetlelid KJ,Plews DJ, Herold E, et al. Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners. BMJ Open Sport Exerc Med 2015;0:e000047. doi:10.1136/bmjsem-2015- 000047.


  1. Jonathan D. Bartlett, John A. Hawley & James P. Morton (2015) Carbohydrate availability and exercise training adaptation: Too much of a good thing?, European Journal of Sport Science, 15:1, 3-12, DOI: 10.1080/17461391.2014.920926


  1. Jeffrey F Horowitz and Samuel Klein (2000) Lipid metabolism during endurance exercise. Am J Clin Nutr 2000;72(suppl):558S–63S.


  1. Phinney SD, Bistrian BR, Evans WJ, Gervino E & Blackburn GL (1983). The human metabolic response to chronic ketosis without caloric restrictions: Preservation of submaximal exercise capacity with reduced carbohydrate oxidation. Metabolism 32, 769–776.


  1. Burke et al (2016) Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol 595.9 (2017) pp 2785–2807.


  1. Volek JS, Freidenreich DJ, Saenz C, Kunces LJ, Creighton BC, Bartley JM, Davitt PM, Munoz CX, Anderson JM, Maresh CM, Lee EC, Schuenke MD, Aerni G, Kraemer WJ & Phinney SD (2016). Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism 65, 100–110.


  1. Louise M. Burke (2015) Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon? Sports Med (2015) 45 (Suppl 1):S33–S49.


  1. Hawley JA & Burke LM (2010). Carbohydrate availability and training adaptation: effects on cell metabolism. Exerc Sport Sci Rev 38, 152–160.


  1. Volek JS, Noakes T & Phinney SD (2015). Rethinking fat as a fuel for endurance exercise. Eur J Sport Sci 15, 13–20.




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